BILATERAL AMETROPIC AMBLYOPIA RESULTING FROM BILATERAL CORNEA PLANA

STANLEY R. SHORB, M.D.

San Francisco, California

A patient whose primary complaint was an inability to tolerate her contact lenses was found to have bilateral cornea plana and sec­ondary bilateral amblyopia.

CASE REPORT

The patient, a 47-year-old German woman, first noted severe bilateral reduction of vision at the age of six years. Convex lenses were prescribed which she did not wear faithfully. She received contact lenses soon thereafter and she was eventually fitted with contact lenses by Mueller Welt in the late 1930s with some improvement in vision.

Her father had angle-closure glaucoma and one brother has congenital glaucoma.

Visual acuity was 20/60 in each eye and could not be improved. A small exophoria was present at near. There was scleralization of the corneoscleral limbus. The corneas showed obvious flattening. Each cornea measured 10 mm in diameter. Biomi-croscopy of the anterior segments was normal ex­cept for the guttata centrally behind the flattest area. Ultrasonography showed each eye was 25 mm in length. Keratometry yielded values of 31 diopters in each eye. The fundus examination was normal.

DISCUSSION

Cornea plana is a congenital corneal ab­normality in which the radius of curvature of the cornea and that of the sciera are similar and sometimes identical. Some authors1 state that the condition is of autosomal dominant inheritance, although it has been considered of recessive inheritance.2

The hallmark of the condition is the flat cornea, usually of less than 32 diopters power. The developmental defect in cornea plana probably occurs sometime after the fourth intrauterine month. Until that time, embryologically, the cornea and sciera have the same radii of curvature, after which the

From the Ophthalmology Service, Department of Surgery, Letterman General Hospital, Presidio of San Francisco, California.

Reprint requests to Major Stanley R. Shorb, MC, USA, 130th Station Hospital, A P O New York 09102.

cornea becomes more convex. In pure cornea plana, the cornea is usually normal histologi-cally, but occasionally this condition is asso­ciated with other ocular anomalies, such as colobomas and ectopic lens. Scleralization of the superior and inferior limbi gives the cor­nea a horizontal appearance. Occasionally the anterior chamber is shallow so that an­gle-closure glaucoma results. A mild pseudo-blepharoptosis may be produced secondary to flat corneas. The globe often has normal dimensions or is elongated so that the total refraction can vary from the highly hyper-opic to highly myopic.

Bilateral ametropic amblyopia is probably more common than has been realized and re­ported. This condition is generally seen in high refractive errors, particularly high hy-peropia. Linksz4 stated that one will never, or hardly ever, find hypermetropia with bi­lateral ambyopia, and if one does, the am-byopia is surely caused by other pathology, usually congenital. Several studies by others contradict this statement. In 1964, Abraham,5

reported 12 cases of the syndrome. His pa­tients had hyperopia greater than 5 diopters, astigmatism greater than 1.25 diopters, or both. In 1950, Southgate" reported two cases of bilateral amblyopia secondary to high hy­peropia. Best corrected visual acuity was 20/ 50. Pratt-Johnson7 reported five cases of bi­lateral ametropic amblyopia. The refractive errors were of different types, usually with high cylinder. None of his patients was younger than seven years of age when he re­ceived his prescription. Retinal correspon­dence was normal in all, and only one demon­strated a relative scotoma in both eyes. Pratt-Johnson noted that an absolute scotoma was not seen in any of the five patients, which is in marked contradistinction to the patients with strabismic amblyopia he tested. Also, he noted that good stereo-acuity (40 sees) is

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664 AMERICAN JOURNAL OF OPHTHALMOLOGY MAY, 1972

compatible with bilateral ametropic amblyo­pia.7

Wiesel and Hubel's8 kitten studies provide us with anatomic and physiologic bases upon which to discuss amblyopia. They found that the unilateral loss of form vision in these kittens was associated with loss of cell mass in the lateral geniculate body and a marked decrease in the number of occipital cortical cells capable of being driven by light stimu­lation of the previously occluded eye. It seemed as if pathways from the good eye could preempt the cortical synapses of the occluded eye. Total occlusion was not neces­sary, but merely loss of form vision, such as obtained by a translucent semi-opaque con­tact lens. Bilateral occlusion caused a de­crease of cell size in the lateral geniculate bodies and decreased occipital cortical re­sponse to retinal stimulation, but the changes were more marked in the unilaterally oc­cluded animals. The concept of one eye's preempting cortical synapses from the other cannot be used to explain bilateral amblyo­pia, but decreased cortical responsiveness and lateral geniculate cellular loss may ex­plain some of the loss of visual acuity. Since these changes were noted in kittens, compa­rable changes and conclusions may not be possible in humans.

In bilateral high refractive errors such as a cornea plana, there is a marked loss of form vision with blurring of the border of the images. The mammalian visual system is designed to respond to sharp borders with increasingly complex requirements as one ascends the order of mammals and the com­plexity of their visual system. Hill and Ikeda" recently demonstrated that even on the reti­nal ganglion cell level, responses are mark­edly dampened by refractive blurring of the image. It thus is becoming easier to under­stand how refractive blurring, even when bi­lateral and without accompanying squint, might affect the developing mammalian vis­ual system and limit its final resolving power. This may produce bilateral amblyo­pia.

The patient with high myopia or aniso-myopia usually will not develop bilateral am­blyopia since a near point exists for each eye that enables a clear retinal image to be per­ceived without accommodation. The highly hyperopic patient gives up trying to compen­sate for his refractive error by accommodat­ing and hence never attains a clear image. Since there is no excessive stimulation of ac­commodative convergence, these patients usu­ally do not develop esotropia. Also, when such patients are corrected optically at a later age, it is often found that they fail to develop a normal accommodative amplitude.10

SUMMARY

In a 47-year-old woman with secondary bi­lateral amblyopia, cornea plana was detected during routine ophthalmic examination, as was poor bilateral visual acuity that was felt to be secondary to the corneal abnormality. Ametropic amblyopia may be considered a mild form of form vision deprivation am­blyopia.

ACKNOWLEDGMENT I thank Dr. Alexander R. Irvine, Major, MC, for

his helpful suggestions in the preparation of this manuscript.

REFERENCES

1. Barkan, H., and Bodey, W. E. : Familial cor­neal plana complicated by cataracta nigra and glau­coma. Am. J. Ophth. 19:307, 1936.

2. Thomas, C. I. : The Cornea. Springfield, 111., Thomas, 1955, p. 233.

3. Mann, I. : Development of the Human Eye. New York, Grune and Stratton, 1950, p. 243.

4. Linksz, A. : Theory of pleoptics. Int. Ophth. Clin. 1:749, 1964.

5. Abraham, S. : Bilateral amblyopia. J. Pediat. Ophth. 1:57, 1964.

6. Southgate, P. : High hypermetropia. Report of two cases. Am. J. Ophth. 33:466, 1950.

7. Pratt-Johnson, J. A. : The significance and characteristics of ametropia amblyopia. Tr. Pac. Coast Oto-ophth. Soc. 49:231, 1968.

8. Wiesel, T. N., and Hubel, D. H. : Comparison of the affects of unilateral and bilateral eye closure on cortical unit responses in kittens. J. Neurophys-iol. 28:1029,1965.

9. Hill, R. M., and Ikeda, H. : Refracting a sin­gle retinal ganglion cell. Arch. Ophth. 85 :592, 1971.

10. Scott, A. : Personal communication.