Nonaccommodative Convergence Excess

Gunter K. von Noorden, M.D., and Cynthia Waters Avilla

Nonaccommodative convergence excess is acondition in which a patient has orthotropia ora small-angle esophoria or esotropia at dis-tance and a large-angle esotropia at near, notsignificantly reduced by the addition of spher-ical plus lenses. The AC/A ratio, determinedwith the gradient method, is normal or sub-normal. Tonic convergence is suspected ofcausing the convergence excess in these pa-tients. Nonaccommodative convergence excessmust be distinguished from esotropia with ahigh ACI A ratio and from hypoaccommodativeesotropia. In 24 patients treated with recessionof both medial recti muscles with and withoutposterior fixation or by posterior fixationalone, the mean correction of esotropia was 7.4prism diopters at distance and 17 prism diop-ters at near.

A LARGE-ANGLE ESOTROPIA at near fixation inpresence of orthotropia or a small-angle eso-phoria or esotropia at distance fixation is re-ferred to as convergence excess' or an abnormaldistance-near relationship.! A common causeof this form of strabismus is an excess of accom-modative convergence associated with eachunit of accommodation, that is, a high AC/Aratio. Indeed, Parks" stated that convergenceexcess is invariably caused by a high AC/Aratio. We differ with this widely held opinionand contend that factors other than excessiveaccommodative convergence may cause a sig-nificant increase in esotropia at near fixation.

In 1978 we suggested the term "nonaccom-modative convergence excess" for this type ofstrabismus." We define it as a condition inwhich a patient with a full refractive correction

Accepted for publication Oct. 2, 1985.From the Cullen Eye Institute, Baylor College of Medi-

cine and the Ophthalmology Service, Texas Children'sHospital, Houston, Texas. This study was supported inpart by grants EY 07001 and EY 02520 from the NationalInstitutes of Health and by the Houston Delta GammaFoundation. This study was presented in part at theAnnual Meeting of the European Strabismological Asso-ciation, Lausanne, Switzerland, Sept. 2, 1985.

Reprint requests to Gunter K. von Noorden, M.D.,Ophthalmology Service, Texas Children's Hospital, Box20269, Houston, TX 77225.

has orthotropia or a small-angle esophoria oresotropia at distance fixation (6 meters) andwho at near fixation (33 em) has an esotropia of15 prism diopters or more than at distancefixation. However, unlike someone with a highAC/A ratio in whom the near deviation decreas-es dramatically when the accommodative re-quirement is reduced with additional sphericalplus lenses, the patient with nonaccommoda-tive convergence excess shows no such re-sponse. The AC/A ratio, determined with thegradient method," is normal or abnormally lowand, consequently, relaxation of accommoda-tion causes no significant decrease in the angleof strabismus.

SUbjects and Methods

The study included 24 consecutive patientsobserved between 1977 and 1985 who had non-accommodative convergence excess and whounderwent surgery in our department. Table 1summarizes some clinical characteristics ofthese patients. In most patients the esotropiawas acquired in childhood and was precededby a period of intermittency of the deviation.No patient had had previous surgery. Measure-ments of the deviation were obtained with theprism cover test, with the refractive error (ifpresent) fully corrected while the patient wasreading a 20/20 line or, if not literate, whilewatching an animated cartoon at a fixationdistance of 6 meters. The measurements wererepeated while the patient was reading a re-duced Snellen chart (or watching small pic-tures) at a distance of 33 em. None of thepatients had manifest or manifest-latent nys-tagmus.

Two patients had amblyopia (visual acuitieswere 20/30 and 20/100); both patients respond-ed well to therapy and visual acuitry norma-lized. The others had standard visual acuitiesin each eye. Only two patients were myopic; allothers had hypermetropia ranging from mild(plano to +2.00 diopters) in 14, to moderate(+2.25 to +5.00 diopters) in five, to severe

70 ©AMERICAN JOURNAL OF OPHTHALMOLOGY 101:7Q-.73, JANUARY, 1986

Vol. 101, No. 1 Nonaccommodative Convergence Excess 71

TABLE 1CLINICAL CHARACTERISTICS OF THE 24 PATIENTS Results

(+5.25 diopters or more) in three. To distin-guish between convergence excess ca~sed byhigh AC/A ratio and nonaccommodative con-vergence excess, the near deviation was re-measured in all patients while they fixated anaccommodative target at a distance of 33 emthrough +3.00-diopter spherical lenses. Themean reduction of the deviation observedthereby amounted to 9.9 prism diopters, indi-cating an average AC/A ratio of 3.3 prism diop-ters per 1 diopter of accommodation, a valuewithin the normal range.' The Figure comparesthe AC/A ratio obtained by the gradient meth-od over the range of 6 diopters in a typicalpatient with nonaccommodative convergenceexcess with those of a normal subject and apatient with an abnormally high AC/A.rati~.

Eleven of 24 patients were treated with bifo-cals preoperatively for different periods. Al-though the reduction of the deviation throughthe addition of plus lenses was small because ofa normal or subnormal AC/A ratio, it was suffi-cient to permit fusion of the residual esotropia.However, surgery eventually became neces-sary in all patients because of bifocal intoler-ance, failure to fuse at near when lookingthrough bifocals, in the presence of a cosmeti-cally significant esotropia, or loss of fusionalcontrol after an initially good response to bifo-cal therapy.

Surgery consisted of recession of both medialrecti muscles (4 to 5 mm) in 13 patients, reces-sion of both medial recti muscles (4 to 5 mm)with posterior fixation sutures 13 to 15 mmbehind the anatomic insertions in eight pa-tients, and posterior fixation of both medialrecti muscles (12 to 15 mm) without recession infive patients. No statistical significance existedwith regard to the size of the preoperativedeviation between these surgical groups. Twopatients underwent two operations (lysis ofposterior fixation sutures followed by recessionor posterior fixation of previously recessedmuscles) to eliminate undercorrection. One pa-tient underwent a third operation to correct anovercorrection.

TABLE 2FUNCTIONAL RESULTS IN THE 24 PATIENTS

TABLE 3PREOPERATIVE AND POSTOPERATIVE

DEVIATIONS IN 24 PATIENTS

A satisfactory functional result may be de-fined as the presence of motor fusion (negativecover test) at near and distance fixation ornormal stereoscopic visual acuity (60 secondsof arc or better) at near fixation, or both. Table 2shows an increase in the number of patientswho had motor fusion and normal stereopsisafter surgery.

Table 3 shows the difference between preop-erative and postoperative deviation for all pa-tients, regardless of the type of surgery em-ployed. Gross motor alignment, defined by acosmetically satisfactory eye position, occurredin all patients. Statistical comparison of theresults by an analysis of variance showed noadvantage of one surgical method over theother. However, the statistics obscured the un-predictability of the surgical effect in the indi-vidual patient with nonaccommodative conver-gence excess, a finding also noted by U.Meltzer and M. Romem (unpublished data).Several patients showed no significant im-

44

2015

RANGE

POSTOPERATIVE

PRISM DIOPTERS

22

NO. OF PATIENTS

177

MEAN

6.7 ET Orthotropic to 25 ET0.7 XT 23 XT to 12 ET7.4

25.5 ET 12 ET to 40 ET9.9 ET 5 to 16 ET8.4 ET 14 XT to 27 ET

17.0

PREOPERATIVE

DEVIATIONS

Peripheral fusionDistanceNear

Stereoscopic visual acuityDistanceNear

FUNCTIONAL RESULTS

Distance fixationPreoperativePostoperativeMean correction

Near fixationPreoperativeReduction with +3.00 spherePostoperativeMean correction

RANGE

Birth to 15.51 to 25

1.25 to 251.5 to 94

MEAN

3.06.367.14

23.6

Age at onset of ET (yrs)Age at 1st visit (yrs)Age at surgery (yrs)Postoperative follow-up (mas)

CLINICAL CHARACTERISTICS

72 AMERICAN JOURNAL OF OPHTHALMOLOGY

en"- 50Q).....0-00 40 Normal Subjecl-O-E Normal AC/A---en High AC/A~...a. 30c:c:0 20....(1l

>Q)

"0 100enW

c: 100.....(1l

> 20Q)"00X

W 30

January, 1986

+3 +2 +1 o -1 -2 -3 -4 -5 -6

Diopters of Accomodative StimulusFigure (von Noorden and Avilla). ACiA ratio in a normal subject" and in accommodative and nonaccommoda-

tive convergence excess. Note the similarity of slopes in the normal subject and in nonaccommodativeconvergence excess and the steepness of slope in high ACiA ratio.

provement in near deviation despite maximalsurgery; in others the effect of surgery wastemporary and the near deviation recurred andeven increased beyond the preoperative devia-tion several months after the operation.

In one such case, a 51J2-year-old girl had hadintermittent esotropia for three years. Her un-corrected visual acuity was R.E.: 20/40 andL.E.: 20/25; cycloplegic refraction was R.E.:+ 1.75 sph and L.E.: +2.00 sph. Prism cover testshowed 12 prism diopters of intermittent eso-tropia at distance and 40 prism diopters ofesotropia at near; this was reduced to 30 prismdiopters with +3.00 spherical lenses (low AC/Aratio). The patient underwent a 4.5-mm reces-sion of both medial recti muscles. The devia-tion six months after surgery was 10 prismdiopters of esophoria at distance and 40 prismdiopters of esotropia at near. An overaction ofthe left inferior oblique muscle had developedwith a V-pattern. One year after the first opera-tion the patient underwent myectomy of theleft inferior oblique muscle combined with ad-

ditional recessions of both medial recti mus-cles, reinserting these muscles a total of 13.5mm behind the corneosclerallimbus. A consec-utive exotropia of 25 prism diopters of thedivergence excess type (orthotropia at near)developed but responded well to additionalsurgery, consisting of a 5-mm recession of bothlateral recti muscles. At the last examination,three years after the last operation, the patientwas orthotropic at near and distance.

Discussion

These data supported our contention that anabnormal distance-near relationship in esotro-pia is not invariably associated with a highAC/A ratio but may be associated with normalor low AC/A ratios. In such instances, excessinnervation must flow to the medial recti mus-cles at near fixation from sources other thanaccommodative convergence.

Vol. 101, No. 1 Nonaccommodative Convergence Excess 73

It may be argued that an office measurementwith additional plus lenses does not sufficient-ly relax accommodation and that bifocals needto be worn for some time before their full effecton the near deviation can be assessed. To clari-fy this possible factor in obscuring a high AC/Aratio, we determined in 20 patients that therewere no differences between the near measure-ment taken during the first examinationthrough +3.00 spherical lenses and those ob-tained after bifocals had been worn for five to15 weeks."

With the gradient method, the stimulusAC/A ratio is measured, the fixation distance isnot altered, and the changes in accommodationare induced by variation in the power of plus orminus spherical lenses added to the refractivecorrection for distance. Thus, the stimuli fortonic and proximal convergence remain con-stant during the measurement. 5 It is possible,therefore, that proximal or tonic, or both, com-ponents of convergence are excessive and con-tribute to the development of nonaccomrnoda-tive convergence excess. It appears unlikely,however, that the awareness of nearness of afixation object (proximal convergence) is capa-ble of producing much additional esotropia, ashas been suggested by Albert and Lederman! toexplain an abnormal distance-near relation-ship, because the values of proximal conver-gence established for normal observers have amean of only 2.25 prism diopters with a rangeof -3.12 to 7.25 prism diopters." In the absenceof other known convergence mechanisms,tonic convergence apparently plays a majorrole in the development of nonaccommodativeconvergence excess.

The possibility that patients with an abnor-mal distance-near relationship have hypoac-commodative esotropia must also be consid-ered. Costenbader? described this form ofstrabismus and explained the increase of thenear deviation on the basis of a remote nearpoint of accommodation. This "juvenile pres-byopia" causes a patient to exert excessiveaccommodative effort at near, thus generatingexcessive convergence. Not only was the nearpoint of accommodation normal in our patientswhenever tested, but the small reduction in thenear deviation when looking through +3.00lenses (Table 1) was not compatible with thistheory.

The average effect of weakening the action ofthe medial recti muscles on the near deviationin our patients was similar to that reported byRosenbaum, [ampolsky, and Scott" in patients

with an abnormal near-distance relationshiptreated with bilateral medial rectus muscle re-cessions although they did not distinguish be-tween high AC/A ratio and nonaccommodativeconvergence excess."

Because a posterior fixation of the medialrecti muscles alone or combined with recessionof these muscles did not increase the effect ofthe operation on the near deviation (see alsoHiles, Gigean, and Shuckett"), we no longer useposterior fixation for nonaccommodative con-vergence excess.

Finally, we agree with Breinin," who lament-ed the widespread practice of attempting toobtain the AC/A ratio by comparing the dis-tance and near measurements. Breinin pointedout that this technique suffers from the inaccur-acy of the proximal factor plus unknown ele-ments of the vergence mechanism. Such ele-ments may, as shown in this study, cause anabnormal distance-near relationship unrelatedto a high AC/A ratio.

References

1. Duane, A.: A new classification of the anoma-lies of the eye, based upon physiological principlestogether with their symptoms. Diagnosis and treat-ment. Ann. Ophthalmol. 5:969, 1896, and 6:84 and6:267, 1897.

2. Albert, D. C.; and Lederman, M. E.: Abnormaldistance-near esotropia. Doc. Ophthalmol. 34:27,1973.

3. Parks, M. M.: Ocular motility and Strabismus.Hagerstown, Harper and Row, 1975, p. 60.

4. von Noorden, G. K., Morris, J., and Edelman,P.: Efficacy of bifocals in the treatment of accommo-dative esotropia. Am. J.. Ophthalmol. 85:830, 1978.

5. Sloan, 1. 1., Sears, M. 1., and Jablonski, M. D.:Convergence -accornrnodation relationships. Arch.Ophthalmol. 63:283, 1960.

6. Ogle, K. N., Martens, T. G., and Dryer, J. A.:Oculomotor Imbalance in Binocular Vision and Fixa-tion Disparity. Philadelphia, Lea and Febiger, 1967,p. 131.

7. Costenbader, F. c.: Clinical course and man-agement of esotropia. In Allen, J. H. (ed.): Strabis-mus Ophthalmic Symposium II. St. Louis, C. V.Mosby, 1958, p. 325.

8. Rosenbaum, A. 1., [ampolsky, A., and Scott,A. B.: Bimedial recession in high AC/A esotropia.Arch. Ophthalmol. 91:251, 1974.

9. Hiles, D. A., Bigean, A. W., and Shuckett,E. P.: Posterior fixation suture operation (Faden-operation). Ophthalmic 5urg. 12:578, 1981.

10. Breinin, G. M.: Accommodative strabismusand the ACiA ratio. Am. J. Ophthalmol. 71:303, 1971.