407A.F. AbuRahma, D.F. Bandyk (eds.), Noninvasive Vascular Diagnosis,DOI 10.1007/978-1-4471-4005-4_33, © Springer-Verlag London 2013

Introduction

This chapter addresses the application of noninvasive vascu-lar diagnostic laboratory (VDL) testing and comments on complementary imaging techniques, in the evaluation of vas-cular anomalies containing arteriovenous fi stulas (AVFs). In

applying the methods described, it helps to have a concept of the hemodynamic or physiologic changes that characterize vascular anomalies (VA) ( Note : the commonly used term congenital vascular malformations (CVMs) is no longer in use, being redundant, and has been replaced by vascular anomalies). This chapter will focus on VAs containing arte-riovenous fi stulas (AVFs). Vascular anomalies can take sev-eral forms: ranging from diffuse micro fi stulas to macro fi stulas that involve major artery distributions, and more mature maturational defects, which tend to involve a single artery. The anatomy of the VA are characterized in two

Abstract

The main focus of this chapter will be on diagnostic approaches that are available in most vascular diagnostic laboratories (VDLs). The application of noninvasive VDL tests in diag-nosing vascular anomalies containing arteriovenous fi stulas (AVFs) use basically the same methods and instrumentation used for assessing peripheral arterial occlusive disease, most of which are qualitative, not quantitative. They gauge the pressure, volume, or velocity changes associated with the contained AVFs but do not visualize the AVFs themselves, as magnetic resonance imaging can do. The latter applications are therefore also mentioned. These diagnostic methods are covered elsewhere in this book and are not described at length here. Rather, the utility of these diagnostic tests in this setting is discussed, in terms of the instrumentation used, the interpretation or analysis of the test results, and appropriate clini-cal applications, as well as diagnostic limitations. These “physiologic” VDL tests can pro-vide much useful clinical decision-making information regarding AVFs occurring in peripheral congenital vascular anomalies. Their different application in diagnosing AVFs is described along with a brief description of the hemodynamic characteristics of arterio-venous fi stulas, as needed to perform and properly interpret these tests. Their main limita-tions are that most can be applied only to peripheral or extremity AVFs and arteriovenous malformations (AVMs). But when the diagnostic goals are simply to determine the pres-ence or absence of AVFs in an extremity with congenital vascular anomalies and the relative magnitude of their peripheral hemodynamic effects, these methods serve well.

Keywords

Congenital vascular malformations • Noninvasive diagnosis • Segmental limb pressure measurements • Segmental plethysmography • Velocity waveform analysis • Magnetic resonance imaging

Noninvasive Evaluation for Congenital Arteriovenous Fistulas and Malformations

Robert B. Rutherford

33

R. B. Rutherford , M.D. Department of Surgery , University of Colorado Medical Center , 14337 Dorsal Street , Corpus Christi , TX 78418 , USA e-mail: rbruth@aol.com

408 R.B. Rutherford

categories using the Hamburg classi fi cation [ 1 ] : (1) predomi-nantly AVFs (estimated to constitute just over one-third of all VAs) and (2) AVFs constituting part of more complex mixed anomalies, e.g., those present in VAs that may even be pre-dominantly venous [ 2 ] . The latter are the most dif fi cult to identify with noninvasive test and imaging studies. Nevertheless, noninvasive methods available in the vascular laboratory can provide useful clinical decision-making infor-mation regarding such AVFs, with one important proviso; they must be located in the extremities – an important quali fi cation. Fortunately the majority of these vascular con-ditions are.

Clinicians using vascular testing methods described in this chapter should have knowledge about clinical diagnosis, obtained by history and physical examination in addition to testing modalities available in the noninvasive vascular labo-ratory. In general, vascular testing should be considered an adjunct to clinical fi ndings and the anatomy depicted by magnetic or computed tomography imaging. Clinically signi fi cant AVMs will typically present well before adult life because a vascular “birthmark,” localized skin color change, overlying varicose veins, or other prominent blood vessels, or occasionally, a distinct vascular mass or tumor or enlarge-ment of the limb (increase in the length or girth) has gained the attention of parent or child. Changes in limb dimension are unusual in very early childhood and in the absence of signi fi cant AVFs, but minor differences may be overlooked early on. Characteristically, these occur as the result of long-standing AVFs present during the growth period, but it should also be noted such changes in girth or length also have been observed with pure venous anomalies, in the absence of AVFs, as in the von Klippel-Trenaunay syndrome. Whether or not the reported absence of AVFs relates to failure to detect micro fi stulous AVFs in predominantly venous anom-alies, described in an earlier era before noninvasive tests (NITs) were employed, is debatable, for it is true that such small AV fi stulas are often missed by angiography, the imag-ing technique most commonly applied in the past [ 3 ] .

One other sometimes confusing aspect should be men-tioned here. The majority of “birthmarks” – often the signal to parents that something is “wrong” with their child – may indeed be a signal of a vascular anomaly, and some “birth-marks that appear in childhood may indeed be sentinel lesions.” However, birthmarks in childhood can also repre-sent true hemangiomatous lesions and others can be super fi cial VAs, i.e., cutaneous capillary or super fi cial venous malformations, which are still commonly referred to as “cav-ernous malformations.” Differentiating between these two entities and true sentinel lesions is extremely important in early childhood and usually can be done on clinical grounds alone, based upon their time of appearance and their growth rate, or lack of growth with time. Importantly, juvenile hemangiomas are true tumors, with a rapid endothelial

turnover, which characteristically are discovered shortly after birth, undergo rapid early growth then spontaneously invo-lute, usually between 2 and 8 years of age, whereas true vas-cular malformations, including cavernous malformations, and sentinel lesions overlying an arteriovenous malforma-tion (AVM) are present at birth and characteristically main-tain the same size relative to the growing child. Juvenile hemangiomas may leave an inelastic scar after involuting, but this can be dealt with later, as necessary. The point here is that intervention is unnecessary with the former lesions, although a number of treatment claim success (dry ice appli-cations or super fi cial salves). Signi fi cantly, juvenile heman-giomas may be found to have high fl ow characteristics on ultrasound study.

The diagnostic methods described here are all aimed at detecting AVFs in VAs and can even be used, albeit with some differences, in diagnosing acquired AVFs, i.e., those due to iatrogenic and other penetrating trauma in an extremity.

The instrumentation employed is basically the same as used in diagnosing peripheral arterial occlusive disease: seg-mental limb pressures and plethysmography, velocity wave-form analysis, and duplex scanning [ 4 ] . The former three have been called “physiologic testing” by those who stress imaging methods, e.g., radiology-run vascular laboratories, but it is the hemodynamic or physiologic attributes of AVFs that distinguish them, so these methods will be described here.

A number of considerations govern how these diagnostic methods can be applied in diagnosing AVFs. First, a basic understanding of the hemodynamic characteristics of arterio-venous fi stulas is needed in order to perform and properly interpret these tests. (This has already been discussed above.) Second, the diagnostic capabilities and limitations of the dif-ferent tests to be described must be understood in applying them. “Physiologic” VDL tests simply gauge the pressure, volume, or velocity changes associated with peripherally located AVFs but do not visualize the AVFs, as duplex ultra-sound imaging can do. Third, most of these tests have limita-tions, e.g., (a) they are primarily qualitative and not quantitative, and (b) they can only be applied to peripheral or extremity AVFs and AVMs. Fourth, congenital and acquired AVFs differ from each other signi fi cantly in terms of their anatomic localization. Congenital AVFs are rarely isolated lesions; they more commonly occur in clusters within major arterial distributions, but may be even more diffuse in loca-tion, involving an entire limb segment. As a result, they can only be localized to a particular limb segment by these so-called physiologic tests. They may be visualized, and their characteristics identi fi ed, by Doppler ultrasonography, but even this modality usually will usually not completely encompass them. Finally, the diagnostic goals may vary con-siderably in different clinical settings and this signi fi cantly

40933 Noninvasive Evaluation for Congenital Arteriovenous Fistulas and Malformations

affects the application of the tests. In this regard, appropri-ately sized smaller cuffs will be needed for the very young who are brought in by their parents for evaluation.

The simplest diagnostic goal may be to determine the presence or absence of an AVF or AVFs, but beyond their presence, which may be clinically obvious, the relative mag-nitude of the AVF’s peripheral hemodynamic effects should be gauged, or the lesion(s) overall effect on the peripheral circulation determined. For example, the presence or magni-tude of a distal “steal” or, in other words, the severity of the associated distal ischemia can be determined.

An additional word of caution is appropriate at the outset. The AGA (Always Get an Angiogram) approach, commonly applied by practicing physicians faced with these problems must be actively resisted. Very few of these conditions con-stitute true emergencies and the “urge” to obtain an angio-gram prior to referral unfortunately opens the door to those who can’t seem to resist intervening, with embolo- or sclero-therapy, which may be ill-advised and untimely in the very young patient. Pointedly, even in young children, any inter-vention for AVFs is not only not an emergency, but is better done when the patients is old enough to be “cooperative.”

The main focus of this chapter will be on diagnostic approaches that should be available in most VDLs. The basic diagnostic methods and the instrumentation behind each of these diagnostic methods will be covered elsewhere in this book, and will not be described at length here. Rather their utility in this setting will be discussed, in terms of the instru-mentation used, the interpretation or analysis of the test results, and appropriate clinical applications as well as diag-nostic limitations.

Clinical Evaluation

Localized warmth and vascular-based color changes (birth-marks), prominent veins, compressibility of vascular masses, the presence of a thrill or bruit, and inequalities in the dimen-sions of the limb (as measured from the anterior iliac spines) all should be noted. However, while the triad of birthmark, varicose veins, and limb enlargement is well known, and usually sought, a limb presenting with these may or may not harbor major AVFs. In fact, the presence of AVFs in such limbs has been the basis for the traditional distinction between the Parkes Weber and von Klippel-Trenaunay syn-dromes, the former being associated with AVFs and the latter not. (This might re fl ect the diagnostic limitations of the day, it being claimed that the latter are associated with micro AVFs.) The point is that these are not of clinical signi fi cance. The limbs harboring these anomalies may appear quite simi-lar, i.e., their outward physical fi ndings are similar, regard-less of the presence of AVFs. In addition, this triad is not consistently found in congenital AVMs. In Sziylagyi’s

classic study of 82 cases of angiographically proven congen-ital AVFs, this classic triad was present in only 57% [ 3 ] .

Diagnostic Studies for Congenital AVFs or AVMs

The vascular diagnostic techniques described below, if used knowingly, can be valuable in ruling in or out the presence of AVFs in this setting, in patients presenting with atypical (location, age of onset) varicose veins and/or a birthmark, with or without limb enlargement [ 5 ] . Depending on the location and relative magnitude of the component AVFs, the same simple “physiologic tests” used in diagnosing periph-eral arterial occlusive disease can also be employed in diag-nosing AVFs or AVMs, and they can do it relatively inexpensively, avoiding the need for angiography, which is particularly important since many of the presenting patients are young children. Although qualitative in nature, the degree of abnormality observed in these tests in association with congenital AVFs should also give the clinician a rough impression of their relative magnitude, and thus be suf fi cient for clinical decision making and parental counseling. Increasingly, the current workhorse of the VDL, the duplex scan, has found useful application in evaluating AVFs. It adds important additional perspective; therefore its use will also be described here.

Characteristic Hemodynamic Changes Associated with AVFs: Diagnostic Implications

Arteriovenous fi stulas can be considered a “short-circuit” between the high-pressure arterial and the low-pressure venous systems. If the AVFs present in vascular anomalies are hemodynamically signi fi cant enough, they will result in (1) an arterial pressure drop/decrease (“steal”) distally, due to a signi fi cant diversion of fl ow into the venous system rather than through the peripheral microcirculation, (2) increased pulsatility (which can be measured as volume change), and (3) an increase in velocity, often with turbu-lence (seen best with a duplex ultrasound probe).

The mean arterial blood pressure distal to an arteriovenous fi stula is nearly always reduced to some degree, although minor degrees of change may be within the accuracy range of these methods. This lower distal pressure is the result of blood being shunted away from the distal arterial tree into proximal low-resistance pathways offered by the arterio-venous communication(s). The reduction in mean pressure is greatest when the fi stula is large and the arterial collaterals are small or poorly developed. On the other hand, when the fi stula is small and the collaterals are well developed, there may be little or no perceptible pressure effects away from the

410 R.B. Rutherford

fi stula itself. AV malformations (AVMs), usually containing a number of AVFs may, in combination, have the same hemo-dynamic effects of a single large AVF. Thus, the magnitude of the pressure drop across an AVM, or the limb segment containing congenital AVFs, can provide a fair assessment of their hemodynamic signi fi cance. If the pressure drop and fl ow diversion are signi fi cant enough, there may even be a measurable degree of distal ischemia, one that can be detected by standard VDL tests. If overall fi stula fl ow is great enough, there will also be associated venous hypertension. The latter is not readily measured noninvasively but increased fl ow velocity in the major vein draining the affected limb, when compared to its normal contralateral counterpart, can and should be assessed, as it will add to the diagnostic impres-sion. A large AVF or AVM can produce signi fi cant pressure swings, locally perceived as increased pulsatility, which are re fl ected in associated volume changes in the involved limb segment, and these can be detected plethysmographically . Finally, AVFs are associated with signi fi cant velocity changes that are greatest closest to the AVF. To appreciate the basic nature of these velocity changes, and how best to detect them, one must fi rst appreciate that a pattern of low fl ow and high resistance characterizes the normal resting extremity circulation. This is in contrast to the high- fl ow and low-resis-tance pattern associated with exercise. The velocity patterns associated with peripherally located AVFs are similar to those associated with exercise, and readily distinguished from than those observed in the normal resting limb by studying arterial velocity in the VDL. Thus, by understand-ing the characteristic underlying hemodynamic changes associated with AVFs, and thus knowing what to look for, congenital lesions containing AVFs can be detected and their relative severity assessed by studying the arterial pressure, volume, and velocity changes they produce, using VDL tests designed to gauge these same hemodynamic parameters.

Diagnostic Tests: Descriptions and Applications

The focus of this section will be on diagnostic approaches still available in most VDLs, which can be applied to the diagnosis of peripheral AVFs, whether they be single, or multiple, as characteristic of AVMs. The same noninvasive “physiologic” tests used for many decades in the diagnosis of peripheral arterial occlusive disease can be applied here. More recently, duplex scanning has greatly augmented these noninvasive tests. The basic diagnostic methods and the instrumentation behind these tests have been covered earlier in this book (Chaps. 21 and 22 ), and will not be described at length here, but their utility in this setting will be discussed, as will their interpretation, appropriate clinical applications as well as their limitations. It should be emphasized at the

outset that while the pressure, fl ow volume, and velocity changes associated with AVFs located in an extremity can be readily assessed in the VDL that, particularly with the nonin-vasive physiologic tests, this application assumes that the fi ndings in the abnormal extremity can be compared to those of the normal contralateral extremity.

Segmental limb pressure measurements are a standard technique described in detail in Chap. 21 . Noninvasive methods of measuring segmental systolic blood pressures are reasonably accurate and reproducible and are painless and simple in application. Brie fl y, a pneumatic cuff is placed around the limb segment at the chosen level and in fl ated to above systolic pressure. As the cuff is de fl ated, the systolic pressure at which blood fl ow returns distal to the cuff is noted by some distally placed fl ow sensor (e.g., Doppler probe, mercury-in-silastic strain-gauge, photopl-ethysmograph, pulse volume recorder, or any other sensor capable of detecting fl ow distal to the cuff). This is analo-gous to taking a patient’s pressure in the arm using an aner-oid or mercury manometer. In the upper extremity, pressure measurements can be made using cuffs placed at the upper arm, forearm, wrist, or fi nger levels; in the lower extremity, segmental pressure measurements are usually made at the high or low thigh, and the calf, ankle, foot, or toe. Although ankle pressures are primarily used in screening for occlu-sive disease (as in the ankle-brachial index [ABI]), multiple segmental cuffs can be used to detect and localize AVFs, particularly AVMs. Importantly, cuffs should be applied bilaterally to allow comparison with the normal contralat-eral limb.

Interpretation of Findings

A hemodynamically signi fi cant arteriovenous fi stula will reduce mean arterial pressure in the limb near and distal to the fi stula. But these cuffs measure systolic pressure, and even though mean pressure is reduced in the arterial tree as one approaches an AVM, the pressure swings between sys-tolic and diastolic (i.e., the pulse pressure) are increased, so that systolic pressure may well be measured as being ele-vated proximal to, or at the level of a fi stula or group of AVFs (e.g., an AVM). Again, the systolic pressure can be detected as being elevated only by comparison with that of the oppo-site limb at the same level [ 5 ] . It will also be elevated if the pressure cuff has been placed directly over an AVM or its afferent branches. Compared to the contralateral extremity, cuffs at or above a hemodynamically signi fi cant fi stula or group of fi stulas (AVM) will usually record a higher systolic pressure, and those below the fi stula will record a normal or lower systolic pressure, with major fi stulas being associated with a detectable degree of distal steal or pressure decrease. Such pressure differences between equivalent limb segments

41133 Noninvasive Evaluation for Congenital Arteriovenous Fistulas and Malformations

or levels, greater than measurement variability, indicate a signi fi cant AVM or group of AVFs.

Segmental Plethysmography

Segmental plethysmography is also as standard technology described in Chap. 22 , which uses cuffs of precise dimen-sions applied at various levels/locations along an extremity, much as for measuring segmental limb pressures. In this diagnostic setting, plethysmographic recordings are extremely helpful. Air- fi lled cuffs are normally used, but in some VDLs mercury-in-silastic “cuffs” are employed. The contour of the resulting tracing is generally assessed in terms of magnitude and shape. When the pulse-sensing cuff is placed over the fi stula(s) or just proximal to an AVM, the pulse volume may be observed to be increased, re fl ecting increased pulsitility [ 5, 6 ] . In a limb with signi fi cant con-genital AVFs, the increased pulsations observed plethysmo-graphically are diagnostic in themselves (Fig. 33.1 ). Although the pulse contour (PVR) may be normal (or nearly so) in a limb distal to an AVF or AVM, it is frequently reduced, par-ticularly in the presence of a “steal” (reduced pressure distal

to an AVF) (Fig. 33.2 ) [ 7 ] . As in the case of segmental limb pressure measurements, the reduction in pulse-volume recordings (PVRs) distally depends on the magnitude of the fi stula(s) and the adequacy of the collateral development. Therefore, very much as described for segmental limb pres-sures above, plethysmography tracings are increased in mag-nitude above or at the level of an AV fi stula, or group of AV fi stulas (AVM) and, depending on the degree of fi stula fl ow and therefore the degree of distal “steal,” the tracings below the fi stula will be reduced or, at best, normal in magnitude. A study of the tracings compared with the contralateral extrem-ity will not only allow detection of AVFs in an extremity but allow their segmental location or level to be identi fi ed. The degree of change in these cuff tracings re fl ects the underly-ing lesion’s hemodynamic signi fi cance and may help with counseling regarding prognosis and timing of intervention.

Velocity Waveform Analysis

Velocity tracings can be recorded over any extremity artery by a Doppler probe connected to the DC recorder and strip chart, or more commonly nowadays, by the velocity readout of a duplex scanner. This is increasingly being done by the latter technique nowadays, not only because the duplex scan has become the “workhouse” of most VDLs but also because it offers other valuable information in this setting (see below). The strip chart recording of velocity waveforms generated by a Doppler probe is uncommonly performed today as a sepa-rate test. Nevertheless, the characteristic fi ndings are the same with either instrument and will be described here. In evaluating for AVFs, the velocity is recorded over the major proximal in fl ow artery, e.g., femoral or axillary. The reason for selecting this location, rather than directly over the sus-pected fi stula(s) will become apparent later, in describing

Thigh

Calf

Toe

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% Δ

V0.

05%

ΔV

Right Left

Fig. 33.1 Plethysmographic (PVR) tracings at thigh, calf, and toe lev-els in a 4-year-old girl with multiple congenital AV fi stulas involving the entire left leg (Reprinted from Rutherford [ 5 ] . With permission from Elsevier)

Right Left

Fig. 33.2 Toe plethysmographic tracings from a patient with a congenital AVM involving the left calf. The reduced PVRs re fl ect a distal steal from this, but not to critical ischemic levels for some pulsitility remains. The left ankle pressures was 55 mmHg (Reprinted from Rutherford [ 5 ] . With permission from Elsevier)

412 R.B. Rutherford

duplex scan fi ndings. A high-velocity fl ow pattern in an artery leading to the area of suspicion is good evidence that the artery is serving as the in fl ow for AVFs [ 7, 8 ] . For many if not most clinical purposes, a qualitative estimate of fl ow velocity and the contour of the analog velocity tracings or “waveforms” obtained in this manner with a directional Doppler velocity detector, or duplex scanner, provides suf fi cient information for diagnosing the presence of AVF(s), and the magnitude of the changes provides some indication of the magnitude of fi stulous fl ow.

To recognize a tracing diagnostic of AVFs, one must real-ize that, in contrast, the velocity tracings of a resting normal extremity is characterized by end-systolic reversal following peak systolic fl ow, followed by low fl ow in early diastole and negligible fl ow in late diastole. Such a low- fl ow, high-resis-tance pattern is most pronounced in the lower extremity. In the upper extremity, there may be little end-systolic reversal. However, high- fl ow low-resistance arterial velocity pattern, while seen in a number of high fl ow visceral arteries (e.g., the renal, carotid, celiac arteries), in the extremities, such a high- fl ow pattern is seen after exercise, and after sympathetic blockade, but, importantly, also in association with AVFs. In these settings, peak systolic velocity may be quite high but, of more diagnostic signi fi cance, there is continuous fl ow throughout diastole and a the “dip” in the tracing between systole and diastole does not approach the zero velocity base-line, let alone show the typical end-systolic reversal of the normal resting extremity. The characteristic arterial pattern associated with arteriovenous fi stulas, as shown in Fig. 33.3 , thus consists of (1) an elimination of end-systolic reversal and (2) a marked increase in diastolic velocity, which appears to “elevate” the entire tracing above the zero-velocity baseline. The degree of elevation in end-diastolic velocity correlates directly with the fl ow increase caused by the arteriovenous

fi stula [ 5, 6 ] . By using these characteristic Doppler velocity signals as a guide, one can detect and localize congenital arte-riovenous communications that otherwise might escape detection [ 9, 10 ] . Peripheral AVFs constituting 5% of extrem-ity fl ow or more can be readily detected by this means alone. This test is more sensitive than segmental pressures and plethysmography, and can be detected by a duplex scan.

Although these changes are diagnostic enough that com-parison with the other extremity would not seem necessary, this comparison still holds value, to rule out hyperemia and similar velocity signals there. Hyperdynamic fl ow is associ-ated with conditions such as beriberi or thyrotoxicosis but since, in these conditions, the effect is generalized, it would affect all extremities, emphasizing the importance of com-paring with the contralateral normal extremity. False-positives can occur in other hyperemic settings, e.g., in fl ammation associated with super fi cial thrombophlebitis, lymphangitis, bacterial infection, and thermal or mechanical trauma. Other causes of hyperemia isolated to an individual vessel or limb (e.g., exercise or reactive hyperemia following a period of ischemia) are transient. Externally applied heat, local infection (e.g., cellulitis or abscess), or sympathetic blockade (permanent or transient, as in epidural anesthesia) can also increase fl ow velocity and give this pattern, but none of these should create any signi fi cant confusion in the usual patient referred to the VDL for evaluation of congenital AVFs, and some (e.g., throtoxicosis, beri-beri, high fever) should produce bilaterally equal changes.

Evaluation for Congenital Arteriovenous Fistulas Using These Three Physiologic Tests in Combination

These three tests are preferably done in combination for their fi ndings and reinforce each other, and they share the advan-tages that they are inexpensive, easily applied, and require only basic operator or interpretive skill. The instrumentation is simple and used on an everyday basis in most VDLs. Some limitations to the above tests must be noted, however. They include the following: (1) These tests give qualitative rather than quantitative information. (2) They can be applied only to arteriovenous fi stulas located in the extremity proper (i.e., at or below the highest cuff or point of Doppler probe inter-rogation). Thus, pelvic AVMs could not be studied by these methods. (3) It should again be noted that in children appro-priately smaller cuffs are required. (4) These tests may not detect diffuse congenital micro fi stulas or overall fi stula fl ow constituting less than 5% of total extremity fl ow. (5) Single limb studies may not be diagnostic unless compared with a normal contralateral extremity. Nevertheless, in combina-tion, these tests can be very useful for screening for congeni-tal AVFs in the extremities of patients presenting with suggestive signs (e.g., a vascular “birthmark,” atypical (early

RightV

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Left

Fig. 33.3 Velocity tracing from the femoral arteries in a 4-year-old girl with a large AVM involving the left thigh. Note that the tracing on the left, compared to the normal right tracing, has higher peak and mean ( dashed line ) systolic velocity, and there is no end systolic reversal. Rather, there is high fl ow continuing throughout diastole, as a result of which the tracing does not drop back to the zero baseline at the end of diastole, but is elevated well above the zero baseline (Reprinted from Rutherford [ 5 ] . With permission from Elsevier)

41333 Noninvasive Evaluation for Congenital Arteriovenous Fistulas and Malformations

onset/unusual location) varicose veins, or limb enlargement), and for detecting, roughly localizing, and assessing the rela-tive magnitude of such congenital lesions. With anatomically localized lesions, these tests, with or without duplex scan-ning (see below) suf fi ce for most clinical decision-making.

Duplex Scanning

It is recognized that duplex ultrasound scanning plays an increasing role in today’s VDL. The basic duplex scanner combines an ultrasound image with a focused directional Doppler probe. In modern instruments, the velocity signal is color-coded so that red represents arterial fl ow and blue rep-resents venous fl ow (going in opposite directions). The velocities are also displayed on the screen, as needed, for speci fi c applications (e.g., bypass graft surveillance, carotid artery interrogation).

Because the duplex scanner provides velocity informa-tion, it can serve as a practical means of performing velocity waveform analysis, the observed patterns serving as a simple yet sensitive means of diagnosing an AVF. Because of the other additional information obtainable from duplex scan-ning, it has mostly replaced using a simple Doppler probe connected to a DC recorder and strip chart for this purpose. High-peak mean velocity readings recorded over the main in fl ow artery of the involved extremity, compared with those at the same location of the contralateral normal extremity, will often con fi rm the presence of an arteriovenous fi stula in that limb. As pointed out above, the characteristic pattern of the velocity tracing, with its elevated end-diastolic fl ow, and its extended persistence over the in fl ow artery should distin-guish this high-velocity reading from the more focused high-velocity reading observed in association with a arterial or bypass graft stenosis.

The software of some of today’s duplex scanners also allows a rough estimation of volume fl ow, with diameter measurements being used to estimate cross-sectional area and the velocity signals and the angle of incidence of the probe allowing the Doppler equation to be applied ( fl ow = velocity (frequency shift) × cosine theta (angle of incidence of the ultrasound beam) × cross-sectional area, divided by C (velocity of sound in tissue, a constant)). However, a problem in using the duplex scan to obtain accu-rate velocity or fl ow measurements directly over an AVF is the presence of turbulence and multidirectional fl ows associ-ated with aliasing. On the other hand, the fl ashes of yellow representing turbulent fi stula fl ow will be seen, and these along with higher than normal arterial velocities upstream are, in themselves, diagnostic of AV shunt fl ow. So the diag-nosis is readily made by this approach, but quanti fi cation is not possible at the fi stula site. Congenital arteriovenous fi stulas are more complex, especially when part of an AVM, but their high- fl ow patterns are readily recognized and, by

moving the scan head, the nature and extent of the more localized super fi cial lesions can sometimes be delineated. This in itself can be diagnostic, and is particularly useful when applied to mass lesions, which often present with a network of varicosities near the surface of the skin. Higher than normal fl ows in these veins will also betray an underly-ing AVM. The diagnostic dilemma here is that these vari-cosities may either be part of a venous malformation or be associated with an underlying arteriovenous malformation, but this be resolved by their fl ow characteristics.

The problem not being able to directly measure fi stula fl ow can be accomplished indirectly by comparing velocity readings obtained proximally over the major in fl ow artery of the involved limb with those from the contralateral normal extremity recorded at the same level. The latter approach is recommended when quantitative measurements of fi stula fl ow are desired for decision-making purposes. Subtracting the contralateral normal limb fl ow from that of the involved limb will provide a fair estimate of AV shunt fl ow in the involved limb as long as the interrogation sites and monitor-ing techniques are the same for both limbs.

Duplex scanners offer the advantage that they are in everyday use in today’s VDLs for many other applications, so the necessary instrumentation and the operator skills are available. The duplex scan is rather versatile in evaluating AVFs, used either to interrogate a penetrating wound or groin hematoma following a catheterization procedure, or the limb of a young patient suspected of harboring congenital AVFs. It may directly visualize and interrogate AVFs as well as pro-vide velocity evidence of their existence, e.g., high fl ow in the artery leading to suspected AVFs. On the other hand, in some congenital anomalies, where multiple AVFs may be spread out over a larger area, the duplex scanner may still be useful, even though not being able to directly visualize all the fi stulous mass, and in larger mass lesions, it may not be able to delineate the full anatomic extent of an AVM. Like the previously described “physiologic” tests, it can be applied to extremity lesions but not to central lesions, e.g., in the trunk or pelvis. Much of its application is qualitative not quantitative, although rough fl ow estimates are possible using the technique described. So, it can detect and generally localize AVMs and guide or monitor thrombotic or embolic therapy in those congenital lesions that are relatively super fi cially located and reasonably well localized.

Competing and Complementary Diagnostic Studies

Arteriography

Arteriography was the gold standard in the past, before non-invasive tests and imaging became available. Unfortunately, many if not most primary care physicians presented with such

414 R.B. Rutherford

patients today perpetuate this primary reliance on angiogra-phy due to lack of awareness of the value of noninvasive tests and imaging. This misguided “AGA” approach is particularly unfortunate because angiography is only required if the need for therapeutic intervention for congenital arteriovenous fi stulas has been determined and will be undertaken soon, in which case it can be obtained at the same time as embolo-therapy. The presence or absence of congenital arteriovenous fi stulas, and their relative severity (the latter determining prognosis), can be determined by noninvasive methods in most cases, assuming comparison with normal limb and the additional use of duplex scanning as described. This overall approach allows management decisions to be made, and the parents advised, without the use of arteriography as a diag-nostic tool. Furthermore, arteriography may fail to demon-strate the fi stula or fi stulas either because they are too small or because the fl ow is too rapid. Furthermore, arteriography is invasive, associated with certain risks (contrast allergy, idio-syncratic reaction, renal toxicity), expensive, and uncomfort-able and a major consideration in infants and young children is that it risks injury to their smaller arteries such that it usu-ally requires either general anesthesia or heavy sedation and analgesia, and therefore hospital admission.

However, while this addiction to contrast angiography deserves opposition, there are a number of noninvasive or mini-invasive imaging approaches that have emerged in recent years that deserve mention here in that they offer signi fi cant additional perspectives over that which can be achieved by the “physiologic” VDL diagnostic methods described above, particularly in the evaluation of congenital AVFs. These diagnostic modalities include: (1) radionuclide quanti fi cation of AV shunting, (2) computed tomography (CT), and (3) magnetic resonance imaging (MRI). Additional discussion of these is included here to provide the reader with suf fi cient knowledge of their capabilities and clinical applications, as additional diagnostic options that must be considered in this setting. On the other hand, it must be real-ized that these new imaging methods are considerably more expensive and time-consuming than VDL testing, and require a considerable degree of patient cooperation (dif fi cult in the very young or the claustrophobic) so that if the additional perspective they offer is not required for decision-making, their immediate use may be inappropriate or at least may be delayed until later, a particular advantage in dealing with very young children.

Radionuclide AV Shunt Quanti fi cation

Radionuclide-labeled albumen microspheres can be used to diagnose and more or less quantitative arteriovenous shunting. Unfortunately, this method, one that is within the

capabilities of any modern hospital’s nuclear medicine labo-ratory, has been largely overlooked. The basic principle behind the study is simple: radionuclide-labeled albumen microspheres too large to pass through capillaries are injected into the in fl ow artery proximal to the suspected AVM. Those passing through arteriovenous communications are trapped in the next vascular bed, in the lung, and may be quanti fi ed by counting the increased radioactivity in the lungs, with a gamma camera, or a discrete sample of it, by maintaining a rectilinear scintillation scanner in a fi xed position over a lim-ited pulmonary fi eld [ 6, 11 ] . The fraction of microspheres reaching the lungs is determined by comparing these counts with the lung counts following another injection of micro-spheres introduced into any peripheral vein, 100% of which should lodge in the lungs. (This latter injection is usually reduced to one-fourth, or less, than the arterial injectate to allow relatively equal counting ef fi ciencies.) The agent com-monly used consists of a suspension of 35- m m human albu-min microspheres labeled with technetium-99 m (similar to that commonly used in lung scans) but other radionuclide labels have been used.

The basic formula is: % AV shunt = the ratio of the lung counts after the arterial injection (lung counts A) to the amount of radioactivity injected (injectate A) divided by the ratio of the counts over fi xed lung fi eld after venous injection (lung counts V) to the amount of radioactivity injected (injec-tate V). ( Note : “Injectate” in this formula is determined by counting of the injecting syringes before and after the injection.)

The study is minimally invasive, relatively simple to per-form, causes little discomfort, and carries a negligible risk. It roughly quanti fi es the degree of AV shunting, something none of the other tests do. Because shunt fl ow can be quanti fi ed, the results have prognostic value [ 5, 6 ] . One can, in this way, better estimate the hemodynamic signi fi cance of an AVF(s) or AVM and thus be better able to predict the need for and timing of intervention. Serial measurements can also be used to gauge the success of interventions designed to eliminate or control AVMs. The radionuclide-labeled albu-men microspheres commonly used in lung scans can be used here, and so this approach is both practical and useful for studying patients with suspected congenital arteriovenous fi stulae [ 2, 6 ] . In patients with diffuse or extensive congenital vascular malformations presenting with a vascular “birth-mark,” varicose veins, and/or limb overgrowth, it may be dif fi cult to distinguish clinically between patients with mul-tiple AVFs (so-called Parkes Weber syndrome), some so small they cannot be visualized arteriographically, and those with the same triad but with predominantly venous malfor-mations (e.g., von Klippel-Trenaunay syndrome). The labeled microsphere study solves this dilemma. Importantly, the success of surgical or endovascular interventions in

41533 Noninvasive Evaluation for Congenital Arteriovenous Fistulas and Malformations

eliminating or controlling AVMs can be adequately gauged by pre- and post-intervention studies. Finally, serial mea-surements will indicate whether the fi stula is following a stable or a progressive course and whether previously dor-mant arteriovenous communications have begun to open up or “grow,” as has been claimed to occur with the approach of puberty. In short, while it has a number of values, unfortu-nately, it is currently underutilized.

Although naturally occurring “physiologic” arteriovenous shunts are present in normal human extremities, less than 3% of the total blood fl ow (and usually much less) is diverted through these communications, so they normally do not pro-duce an interpretive error [ 11 ] . However, measurements made during anesthesia are not accurate because anesthesia, both general and regional, signi fi cantly increases shunting through these naturally occurring arteriovenous communications. The examiner must be also be aware that the percentage of blood shunted through arteriovenous communications can be quite signi fi cant, in the range of 20–40%, in the limbs of patients soon after sympathetic denervation, and in patients with cir-rhosis, or those with hypertrophic pulmonary osteopathy [ 12 ] . Finally, this study shares the limitation of the physiologic studies previously described in that it does not readily local-ize the lesion. However, several injections can be made at key locations in the arterial tree at the time of arteriography, if a gamma camera is present, and these can quanti fi ed against a later venous injection, to give localizing information.

Magnetic Resonance Imaging and CT Scan

The previously described VDL studies cannot properly assess the anatomic extent of large or deep vascular malfor-mations, and even angiographic studies tend to underesti-mate their full anatomic extent. Computed tomography (CT) will usually demonstrate the location and extent of the lesion and even the involvement of speci fi c muscle groups and bone [ 13, 14 ] . Offsetting these desirable features of CT are the need for contrast, the lack of an optimum protocol for its administration, and the practical limitation of having to use multiple transverse images to reconstruct the anatomy of the lesion. A 3D reconstruction of CT angiography data over-comes some of these limitations, but subtracting away mus-cle, skin, and bone, as performed in most vascular applications, prevents the true anatomic extent of AVMs from being accu-rately determined.

Magnetic resonance imaging (MRI) possesses a number of distinct advantages over CT in evaluating vascular malfor-mations. There is no need for contrast (gadolinium, but not contrast in the usual sense), the anatomic extent is more clearly demonstrated, longitudinal as well as transverse sec-tions may be obtained, and the fl ow patterns in the congenital malformation can be characterized. An example is shown in Fig. 33.4a–c . As a result, MRI has become the pivotal diag-nostic study in the evaluation of most vascular malforma-tions presenting with mass lesions.

a b c

Fig. 33.4 ( a ) Arteriogram prior to referral of a 23-year-old male with a “localized” AVM, “suitable for surgical excision,” ( b ) MRI (cross-sectional view) of AVM lesion, showing involvement of entire anterior muscle compartment, and ( c ) MRI (longitudinal view) of the same

AVM. MRI views indicate resection not possible without including nerves that would leave patient unable to raise leg. He was therefore treated with embolotherapy (Reprinted from Pearce et al. [ 14 ] . With permission from Elsevier)

416 R.B. Rutherford

Overall Diagnostic Strategy and Clinical Correlation

Although predominantly venous malformations are more common than arteriovenous malformations (roughly half vs. one-third of all vascular anomalies), determining whether or not a vascular anomaly or malformation contains AVFs is the usual starting point, even in presumed venous lesions, and particularly in presumed von Klippel-Trenaunay syndrome. The VDL can provide much useful information in this regard, using segmental limb pressures and plethysmography, veloc-ity wave form analysis, and duplex scanning, and most vas-cular malformations containing AVFs can be evaluated adequately enough with these basic VDL tests for clinical decision-making. A radionuclide-labeled microsphere shunt study can added if it is important to quantify the AV shunt-ing, and magnetic resonance imaging (MRI) is used in mass lesions to determine their anatomic extent, particularly the involvement of adjacent muscle, bone and nerves, which in turn determines respectability of mass lesions. It also dem-onstrates the lesion’s fl ow characteristics (e.g., distinguish-ing venous from arteriovenous malformations).

After utilizing the above diagnostic tests, without the use of contrast angiography, one should be able to categorize the lesion as either one of the following: a localized AVF, an extensive malformation with macro fi stulous AVFs (an AVM) fed by speci fi c named vessels, diffusely scattered micro fi stulous AVFs (which may or not be associated with venous malformations), venous angiomas (an extratruncular venous malformation consisting of multiple venous lakes located to the side of major veins), a congenital defect of the deep veins, or an arterial anomaly. In most cases, duplex scanning will aid in sorting these out if the noninvasive “physiologic” tests are not de fi nitive. Angiography should be rarely used initially, being saved to guide interventions once they have deemed necessary. The need for surgical intervention is limited to the more localized AV malforma-tions that may be resectable (less than 10%). Larger AVMs, composed of macro AVFs can be “controlled” but rarely cured by modern embolotherapy, which is more effective for venous or lymphatic mass lesions. Diffuse micro AVFs and extensive or diffuse venous malformations usually require no treatment other than conservative management of

the associated venous hypertension (e.g., by elastic stock-ings and intermittent elevation). Thus, the noninvasive stud-ies featured in the today’s VDL, and described above, can and should play a pivotal role in the diagnosis of AVMs, and in ruling in or out AVFs as signi fi cant components of other congenital vascular anomalies.

References

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