planning for success

farabi strabismus ward

Planning for successNaeini MDFarabi eye hospital


Important question "Why are we operating?“


"Why are we operating?“

Restoration of Binocular Vision• Chavasse challenged Worth's theory.• Data supporting Worth's theory were obtained at a

time when strabismus surgery was rarely performed prior to the age of 2 years.(Until the 1960s)

• Chavasse lived before surgery was commonly performed to treat strabismus, support for this theory was limited until Ing and co-workers began to report favorable binocular vision in some infants who underwent strabismus surgery between the ages of 6 months and 2 years.



Subnormal binocular vision• the level of binocular function obtained in patients with

idiopathic infantile esotropia is almost uniformly subnormal.

• Subnormal binocular vision as described by von Noorden or the monofixation syndrome, as described by Parks, is considered the optimal result in children with idiopathic infantile esotropia by most pediatric ophthalmologists

• Patients who develop this level of binocular vision are more likely to maintain normal ocular alignment throughout their life



Abnormal sensory adaptations• Older children with normal binocular vision,

but who remain immature from the standpoint of cortical visual development, are still at risk for the development of abnormal adaptations in their binocular system if strabismus develops.

• Suppression and abnormal retinal correspondence are frequently seen in disorders such as intermittent exotropia.



Cont…Abnormal sensory adaptations

• These adaptations may allow an intermittent deviation to become manifest more frequently or even to become constant.

• Once these adaptations develop, they may place a child at higher risk for the recurrence of strabismus later in life, even after successful surgical realignment.



Cont…Abnormal sensory adaptations

• In patients with intermittent exotropia, for example, strabismus surgery is often recommended when the deviation increases in frequency and there is evidence that suppression has become more ingrained.

• Surgery before these abnormal adaptations are well developed may be helpful in providing long-term stable ocular alignment.



Diplopia• If the patient has a history of normal ocular

alignment and develops a deviation in later life, the patient will experience diplopia when the eyes are not aligned.

• Patients with this history who do not report diplopia are generally ignoring the extra image and are not aware of its presence.



Cont….Diplopia

• The degree of visual disturbance experienced is dependent upon several factors.

• The frequency of the deviation is often the most important factor in determining the patient's tolerance of symptoms.

• Interestingly,deviations that are large are often less bothersome to the diplopic patient than smaller deviations.



Cont….Diplopia

• The position(s) of gaze where the double vision occurs is also an important consideration when evaluating diplopic patients and when discussing the potential benefit of treatment.

• Deviations that occur in primary position, down gaze, or in the reading position tend to be the most troublesome.

• Diplopia that occurs only in up gaze tends to be the least bothersome and often well tolerated without treatment.



Cont….Diplopia

• If the deviation is small and relatively comitant, prism added to spectacle lenses may offer significant relief from symptoms.

• If the deviation is larger, prism will often produce unwanted image distortion and the lenses themselves are often too heavy and uncomfortable for reasonable wear.

• Though there are exceptions, patients rarely tolerate prism correction of a deviation greater than 8-10 prism



Cont….Diplopia

• Surgery will eliminate bothersome diplopia in most patients, though the addition of a small amount of prism in the patient's spectacle lenses is required for some patients with a small residual deviation producing symptomatic diplopia after surgery.

• Patients who otherwise do not require spectacle correction or who prefer wearing contact lenses often request surgery even for small deviations to avoid the need for prism.

• This may seem an unreasonable request on the surface.



Cont….Diplopia

• However, the need for prism glasses to achieve fusion and single vision is not comparable to the need for glasses to correct a refractive error.

• Even patients with significant uncorrected refractive errors are usually able to function relatively well in many activities of daily living without significant difficulty.

• In contrast,patients with diplopia are rarely able to comfortably participate in most activities without the need to constantly close one eye to eliminate diplopia.

• Thus, patients who find the use of prism glasses undesirable may be excellent surgical candidates.



Incomitant Strabismus• A significantly incomitant deviation with symptomatic diplopia

is only occasionally successfully treated with prism and thus surgery is usually the primary treatment option.

• A surgical plan that takes into account the etiology of the incomitant strabismus can usually provide a significantly expanded field of single vision compared with prism in such patients.

• Superior oblique paresis is a typical example.• Prism correction may provide single vision in primary position

but most patients continue to experience bothersome diplopia in other important fields of gaze despite the use of prisms.



Asthenopia

• Even if the patient's deviation remains latent most of the time,it may give rise to bothersome symptoms of asthenopia, in the absence of diplopia.

• Symptoms may include eyestrain, reading difficulties, headaches,vague symptoms of fatigue or other symptoms with prolonged eye use.

• Asthenopia is usually not present with small horizontal phorias but may be experienced with medium to large horizontal phorias especially those that approach the relatively large horizontal vergence amplitudes.



Cont… Asthenopia

• Small vertical phorias, on the other hand, often produce asthenopia due to the smaller vertical fusional vergence amplitudes possessed by most normal people

• Surgery to eliminate a symptomatic latent deviation can provide great comfort to the patient.

• If there is a question as to whether or not a strabismic deviation is responsible for a patient's visual complaints, a trial of prism correction or monocular occlusion is often of great diagnostic value.

• Resolution or marked reduction of symptoms with either of these diagnostic trials is good evidence that the latent strabismus is contributing to the problem.



Asymptomatic Patients• Treatment of strabismus in patients who are not

experiencing diplopia or other symptoms, but who have an angle of deviation too large for the development of binocular vision, may still be justified, though not all ophthalmologists agree.

• Some ophthalmologists believe that any deviation greater than 8 prism diopters warrants surgical correction in order to reduce the deviation and provide a chance for the patient to develop some degree of fusion.

• This goal can be achieved often, even in older patients with long -standing uncorrected deviations


Cont…"Why are we operating?“

Asymptomatic Patients

• Morris and co-workers operated on 24 adult patients with longstanding strabismus, 7 of whom had congenital esotropia or exotropia.

• All patients developed some degree of fusion after surgery and 50% achieved stereopsis.

• These ophthalmologists believe that even peripheral fusion is beneficial to patients and that the prognosis for maintaining ocular alignment long term will be enhancedif even rudimentary fusion is restored.

• Other ophthalmologists believe that surgery should be performed if the deviation is significant as determined by the patient and/ or family.



Compensatory Head Posture• Some patients are able to achieve single vision

only in eccentric gaze. Such patients have an incomitant deviation and will typically develop a compensatory head posture to take advantage of the fusion that they can achieve in the eccentric gaze position

• Paralytic, restrictive and A-pattern and V-pattern strabismus are frequent causes of incomitant deviations that are often associated with a compensatory head posture.



Miscellaneous Surgical Indications• Patients with Duane syndrome who have

pronounced upshoots, downshoots, or globe retraction due to co-contraction may benefit from strabismus surgery to blunt these conditions, with a reasonable risk to benefit profile

• Extraocular muscle surgery may also be of value in some patients with nystagmus, including patients with and without a compensatory head posture.


"Why are we operating?“ Expansion of the Field of Vision in Patients with Esotropia

• Kushner described a series of patients who underwent surgery for the treatment of esotropia . Each patient achieved a significant expansion of their binocular visual field consistent with the degree to which their eyes were surgically straightened.

• This study demonstrated a benefit to strabismus surgery unrelated to the patient's level of binocular vision


"Why are we operating?“ Psychosocial and Vocational Indications

• After successful strabismus surgery, many patients relate improvement in selfimage, interpersonal relationships, and school and work performance.

• In addition, operated patients often feel that others view them more positively after their strabismus is rendered less obvious with surgery.

• Use of the term cosmetic in the treatment of affected patients is inaccurate.(surgery should be referred to as "reconstructive”)


Urgent surgery

• reestablish binocular fusion in a child with an esophoria that has recently broken down to a tropia.


Fusion potential

• Esotropic patients with fusion potential generally require large amounts of surgery, more than the standard surgical numbers

• A plan based on standard surgery in these patients routinely results in undercorrection.

• Esotropic patients without binocular fusion potential, however, are ill served by planning for "more" surgery as a consecutive exotropia will inevitably increase over time and an exotropia is a poor cosmetic outcome.


• Monocular recession-resection surgery produces incomitance which is not optimal in a fusing patient

• incomitance can cause diplopia in eccentric positions of gaze

• Monocular surgery on the blind eye is, however, the procedure of choice for sensory strabismus to protect the only seeing good eye.


Signs of Binocular Fusion Potential


Specific strabismus diagnosis

• In most cases the strabismus can be classified into a type, such as partially accommodative esotropia, intermittent exotropia, Duane's syndrome type 1

• At times, it may be difficult to determine the exact etiology of the strabismus.

• In these cases an MRI of the head and orbit may be indicated.

• then it is appropriate to operate for the strabismus pattern taking into account the ductions, versions, and the presence of incomitance.


Paradoxical Diplopia

• Planning strabismus surgery for adult patients with childhood strabismus offers a special challenge as they may have anomalous retinal correspondence(ARC) and develop postoperative paradoxical diplopia.

• When the strabismus is corrected the PF is now out of alignment

• Paradoxical diplopia is usually not as bothersome as diplopia associated with NRC and patients know which is the "real" image.


• In most cases paradoxical diplopia will resolve spontaneously over several days to months.

• Rarely, however, patients may have persistent diplopia requiring prisms, or even additional strabismus surgery, to reverse the correction and re-create the original strabismus.

• Neutralize the deviation with a prism and ask the patient if they see double.


• Test for diplopia in free view, then repeat prism neutralization with a red filter over one eye and use a hand light as a fixation target.

• If the patient sees double with the deviation neutralized the patient should be advised that they will probably see double after surgery.

• It is a good rule to inform all adult patients that postoperative diplopia is a possibility.


FACTORS INFLUENCING RESPONSE TO STRABISMUS SURGERY

•Testing Factors•Patient Factors•Anatomic Factors•Surgical Factors


Testing FactorFACTORS INFLUENCING RESPONSE TO STRABISMUS SURGERY

• TYPE OF TARGET FOR FIXATION• The target should have sufficient detail, sustain the

patient's interest, and be larger than the patient's threshold acuity.

• a patient with 20/50 Snellen acuity should be presented with a 20/70 letter as a fixation target.

• The patient should wear the maximum hyperopic or least myopic spectacle correction so that no accommodative effort is required at the 20ft testing distance.


Testing Factor…DISTANCE BETWEEN PATIENT AND TARGET

• The standard 20ft test distance is designed to eliminate any meaningful accommodative effort

• only 0.13 D of accommodation is required at this distance in the emmetropic patient.

• Kushner confirmed the frequent increase in the size of the exotropic deviation when fixing on an accommodative target at distances greater than 20ft.


Cont…DISTANCE BETWEEN PATIENT AND TARGET

• Frequent cause of undercorrection after surgery for intermittent exotropia, because the target angle for surgery may be underestimated.

• Esotropic patients with a high (AC/ A) ratio often exhibited higher distance deviation in 10-ft lanes than in 20-ft lanes.


METHOD OF TESTING USED TO OBTAIN MEASUREMENTS

• The most effective ways to suspend fusion are the alternate cover test and prolonged monocular occlusion.

• In performing the alternate cover test,the patient is never allowed to regain fusion while the cover is transferred from one eye to another.


Cont…. METHOD OF TESTING USED TO OBTAIN MEASUREMENTS

• Prolonged monocular occlusion was developed by Marlow to uncover the full amount of heterophoria. This is needed to completely eliminate tenacious fusion in patients who have developed strong fusional vergence.

• 45 minutes have been effective,especially in patients with exotropia.

• Many patients with distance-near disparity will have equalization of the distance and near deviations after occlusion


Cont…. METHOD OF TESTING USED TO OBTAIN MEASUREMENTS

• Light Reflex Tests• Some patients are too immature to cooperate

with alternate cover testing. The deviation then may be estimated using corneal reflex tests such as the Hirschberg and Krimsky methods.

• When performing the test, The examiner must be in front of the deviated eye to avoid parallax errors in observation.


Cont…Light Reflex Tests

• Hirschberg found that each millimeter of decentration of the corneal reflex corresponds to 7 degrees of deviation of the visual axis. Whereas the true relationship between degrees and prism diopters is trigonometric. For angles less than 100 PD. every 2 PD is approximately equal to I degree. Later studies using photographic calibration established a conversion factor of 21 PD/mm of displacement.


Cont…Light Reflex Tests

• For incomitant strabismus,however,dissimilar measurements of the magnitude of deviation are obtained with the fixing eye in the primary and secondary positions. To allow measurement with the fixing eye in primary gaze, the prisms can be placed over the deviating eye


We recommendLight Reflex Tests

• We recommend measuring angle kappa first when using corneal reflection tests in immature patients. An increased prevalence of positive angle kappa is found in esotropia. and an increased prevalence of negative angle kappa is found in exotropia.

• We also suggest coupling an accommodative target to the fixation light.

• For Hirschberg measurements,we correlate each millimeter of decentration with 21 PD of misalignment.

• When using the Krimsky method, we prefer to place the prisms over the deviated eye.


PRISM PLACEMENT

• Plastic prisms should be held in the frontal plane rather than the Prentice position

• The measurement error introduced by using the Prentice position increases as prisms greater than 20 PD are used.

• When deviations exceed the amount of the largest available prisms. the examiner should not stack prisms.


PRISM PLACEMENT….Three common positions for ophthalmic prisms.


cont….. PRISM PLACEMENT

• Prisms may be stacked,however,if they are used to simultaneously measure vertical and horizontal misalignment. which are independent of each other.

• It is inaccurate to split the angle of misalignment between the two eyes by holding prisms up in front of each eye.

• It is obvious that substantial measurement errors occur when a more than 20PD prism is added in front of each eye.



• The prisms power is also influenced by the distance prisms are placed in front of the eye. The prisms power needed to neutralize a deviation with near fixation will increase as the prisms are held farther from the cornea.


Testing factor ……..TORSIONAL DEVIATION

• Prisms are unable to correct misalignments around the torsional Y axis.

• Torsional alignment can be assessed using both subjective and objective techniques.

• Commonly used subjective measurement methods include the Maddox rod and Bagolini striated lenses.‘

• Objective measurements can be obtained by indirect ophthalmoscopy and fundus photography


Patient Factors FACTORS INFLUENCING RESPONSE TO STRABISMUS SURGERY

• ACCOMMODATION• accommodation must be controlled by uncovering

and correcting full hyperopic refractive errors in patients with strabismus.

• The full correction of hypermetropia can have varying results in exodeviations. Classically,correcting hyperopia will worsen an exodeviation.If there is significant blurring of retinal images, however, correcting hypermetropia will improve fusional convergence and lessen the amount of exodeviation.


Cont…..patient factors

• AC/A RATIO• The AC/A ratio refers to the amount of

accommodative convergence exerted per unit of accommodation.

• Gradient method:AC/A=(+L)-(-L)/D• Heterophoria method:AC/A=IPD+(N-D)/D • Graphic technique • Fixation disparity method


Cont….AC/A, PATIENT FACTORS

• The heterophoria method can result in a falsely elevated AC/A ratio owing to convergence at near and is dependent on the IPD.

• The term clinically high AC/A ratio describes patients who show a greater amount of esodeviation at near than at distance.


Cont….AC/A ,patient factors

misleading "clinically high AC/A ratio."

• V-pattern esotropia• partially accommodative esotropia with greater

near than distance deviation who prism-adapt to the amount of esodeviation at near.

• Patients with intermittent exotropia at distance may be orthotropic at near. This lack of exodeviation at near satisfies the criteria for a "clinically high AC/A ratio" and can be misleading.(tenacious proximal fusion)


Cont….patient factors

HIGH REFRACTIVE ERRORS• The peripheral prismatic effects of corrective spectacles

introduce an artifact when measuring strabismus.• Plus lenses decrease,whereas minus lenses increase,

the measured deviation• This effect is clinically significant with corrective lenses

of more than 5.00 D.• This artifact from the peripheral lens of spectacles may

also be reduced by using lenses in trial frames and moving them until they are centered in front of the visual axis of the deviating eye.


Cont…..patient factors , HIGH REFRACTIVE ERRORS

Hyperopic spectacle power


Cont….patient factors , HIGH REFRACTIVE ERRORS

Myopic spectacle power



ECCENTRIC FIXATION

• The results of tests that require fixation by an eccentrically fixing eye such as the cover test and alternate cover test will underestimate or overestimate the true strabismic angle by an amount equal to the magnitude of eccentric fixation



NEUROLOGIC OR ORBITAL DISEASE

• There is a higher than normal frequency of strabismus in patients with cerebral palsy. ranging from 15% to 62%

• Developmentally delayed children have an increased effect from the same amount of surgery than do normal children.

• This happens despite the common use of Krimsky measurements that usually underestimates the deviation compared with prism and cover testing.


Cont…..patient factors, NEUROLOGIC OR ORBITAL DISEASE

• Bilateral medial rectus recession for esotropia resulted in satisfactory alignment in only 39% of developmentally delayed children. compared with 73% normal children after a mean follow-up of 24 months.

• Dyskinetic or variable strabismus is seen in 30% of strabismic patients with cerebral palsy.

• This type of strabismus does not usually evolve to a more constant deviation over time.

• Patients with dyskinetic strabismus are not appropriate candidates for surgical correction.


Cont…..patient factors, NEUROLOGIC OR ORBITAL DISEASE

• Myasthenia gravis and thyroid orbitopathy can result in variable angles of misalignment. A careful search for associated ocular and systemic abnormalities usually allows the clinician to determine the underlying disease causing variable strabismus in these patients.



CHILDHOOD STRABISMUS

• there are some types of strabismus that preclude the development of bifoveal fixation after strabismus surgery, Except for anecdotal case reports, the goal of restoring bifoveal fixation in children with infantile esotropia has proved elusive

• A subnormal degree of binocularity can nonetheless be obtained in the majority of patients with infantile esotropia


Cont…..patient factors ,CHILDHOOD STRABISMUS

• Fortunately, children with acquired esotropia or intermittent exotropia have the potential to completely regain bifoveal fixation.



AMBLYOPIA• Patients with amblyopia have a greater chance of

postoperative overcorrection and undercorrection after strabismus surgery.

• In one study no difference was found in surgical outcome between patients having surgery after amblyopia therapy was completed and others operated on during amblyopia therapy (as long as treatment continued after surgery)

• we continue to recommend strabismus surgery only after amblyopia has been fully treated.


ANATOMIC FACTORS FACTORS INFLUENCING RESPONSE TO STRABISMUS SURGERY

• Axial Length and Globe Size• The smaller size of neonatal eyes has been invoked as an

explanation for poor predictability of surgical results in these cases.

• Most ocular growth occurs in the postequatorial sclera, half of it during the first 6 months of life. Although the distance from the insertions of the extraocular muscles to the limbus is 80% of that in adults, the insertions are closer to the equator

• Three- to 4-mm recession of the horizontal rectus muscles may place the muscles posterior to the equator in a patient younger than 6 months.


Cont…. ANATOMIC FACTORS Axial Length and Globe Size

• Axial length measurements correlate closely with the surgical dose-response relation.

• Larger eyes have a lesser response to the same amount of surgery than do smaller eyes.

• estimate the location of the equator based on axial eye length and corneal diameter measurements.



• Mismatch between the curvature of the curved ruler and the curvature of the globe is a potentially important source of measurement error as well

• The curved ruler should directly overlay the sclera to allow precise measurement of arc length.


calipers

Scott curved ruler prototype

small

curved

ruler


Cont….ANATOMIC FACTORS

Muscle Length-Tension Properties • Beisner' theorized that the effect of rectus muscle recessions

is caused by alteration of the Iength-tension curve.• Passive length-tension studies of the horizontal rectus

muscles found no relationship between preoperative deviation and the response to surgery in patients with exotropia and infantile esotropia

• A relationship was,However, found between Iength-tension properties, the preoperative deviation and the response to surgery in acquired esotropia.

• Future understanding of Iength-tension relationships in the awake, alert patient should lead to more accurate surgery.


Surgical Factors FACTORS INFLUENCING RESPONSE TO STRABISMUS SURGERY

• TIMING• Patients who achieve satisfactory alignment

before age 2 years have a better binocular outcome than those whose eyes are aligned after this time.

• It should be noted, however, that even adults with infantile esotropia who have never undergone previous surgical correction can develop subnormal binocularity after surgery.


Cont…..surgical factors, timing

• In patients with accommodative esotropia, a positive correlation was found between the development of normal binocularity and prompt correction of strabismus soon after its onset.

• Surgery should not be delayed in a child or adult who is medically stable and has a reproducible angle of deviation.


cont…..surgical factors

Artifacts Introduced in Surgery

• The anterior extent of the medial rectus insertion retracts toward the corneoscleral limbus after disinsertion . This can result in more than a millimeter of anterior displacement of the insertion site.

• Many surgeons measure the distance of the anterior aspect of the insertion before disinserting the muscle from the globe. The muscle can then be recessed a predetermined amount posterior to the original site of the insertion before anterior displacement (by measuring recession from the limbus).


cont…..surgical factors, Artifacts Introduced in Surgery

• Surgeons may also inadvertently distort the insertion site by applying traction on the globe during suture placement.

• This produces a V-shaped deformity,which results in anterior displacement of the insertion.

• If the amount of recession is measured from the anteriorly displaced V deformity, less recession will be performed than was planned



V-shaped deformity



• Sutures should be placed no more than a millimeter from the end of the insertion. Placing the sutures too far posterior will impose a resection effect on a recessed muscle.


• The strongest determinants of the response to surgery

• type of strabismus • preoperative deviation• interval between the onset of strabismus and

initiation of treatment

planning for success

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