Megalocornea is a non-progressive, uniformly bilateral, congenital enlargement of the anterior segment in the absence of episodic or constant intraocular pressure elevation. The clear corneas are of normal thickness with well-denned limbi and horizontal diameters ranging from 12.5 to 18.0 mm.1-2 Megalocornea is often familial, in which case it is almost always sex-linked recessive,1-4 although a few autosomal dominant and autosomal recessive pedigrees have been described.1-3,5-8 Other less common ocular and systemic abnormalities have been reported with megalocornea.1'8,15
Megalocornea denotes anterior segment enlargement only; the posterior segment is of normal size.1,2 Pathologic examination of a sporadic case confirmed this concept.9 To our knowledge, there is only one echographic evaluation to corroborate this isolated pathologic report. This was performed on a patient with megalocornea and the Marchesani syndrome. Only the anterior segments were enlarged; the posterior segments were normal.12
FIGURE 1: Pedigree of family affected with megalocorriea.
We present the ophthalmologic and ocular echographic findings in three members of a family with isolated (no associated systemic disorders such as the Marchesani or Marfan syndromes) megalocornea, who represent three consecutive generations. The pedigree (Figure 1) is consistent with an autosomal dominant disorder. To our knowledge, this is the first echographic evaluation of what appears to be autosomal dominant, isolated megalocornea.
The proband !Figure 1, HI-2) is a female who was first seen in the University of Iowa ffediatric Ophthalmology Clinic at age one year because of "large" eyes. There was no history of photophobia, blepharospasm, or epiphora. Her developmental milestones were normal. The fixation pattern was central, steady, and maintained OU. There was no strabismus and the pupils reacted normally. Horizontal corneal diameters were 14.5 mm and in the intraocular pressures by MacKay-Marg tonometry were 16 mm OU. Annual examinations were recommended.
Case 1 next returned at age four years. The visual acuity was 20/25 OU. The horizontal corneal diameters were unchanged !Figure «!Ai. Pachometry measurements were normal. There were no striae or other evidence of keratopathy by slit lamp examination. Applanalion tonometry was normal. Gonioscopy showed no pigmentation, iris processes, or evidence of anterior chamber angle dysgenesis ( Table It. There was bilateral iridodonesis and iris thinning. A-scan echography16 revealed increased anterior chamber depths !Figure 31. Lens thicknesses and axial length measurements were normal (Table 2). Cyckiplegic refraction was piano +0.50 * 90 OD and -^0.5U OS. The cup/disc ratios were 0.2. The fundua examinations were normal.
FIGURE 2A: Case 1 with mm ruler In approximate plane of the cornea.
FIGURE 2B: Similar photograph of Case 2.
This 32-year-old female (Figure 1. 11-21 sustained a penetrating injury to the nasal I imbal area OS at age four years. Visual acuity was 20/15 OD and 20/30 OS. A small posterior subcapsular lens opacity was present as was an anterior aynechia in association with the old peripheral corneal scar. The corneal diameters were 13.5 mm !Figure 2Bl with normal central thicknesses. The corneas were otherwise normal. Applanation tonometry was normal OU, as was gonioscopy of the right eye. Aside from the anterior synechia at 9 o'clock, the angle was normal OS. There was bilateral iris thinning and iridodonesis. ?-scan echography revealed increased anterior chamber depths. Lens thicknesses and axial length measurements were normal. Cycloplegic refraction was +2.00 OD and +2.50 + 3.50x105 OS. Keratometry readings were 42.75/42.75 OD and 41.25x15/44.75x105 OS. The cup/disc ratios were 0.1. The fuiidus examinations were normal.
FIGURE 3: Standardized ?-scan echogram of Case ?'? OS (Kretz 7200-MA) at measuring sensitivity with immersion technique. The arrows from left to right indicate: Il anterior and posterior corneal surfaces 2lanterier lens capsule 3! posterior lens capsule 4} retina.
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This 54-year-old female (Figure 1, 1-2) had visual acuities of 20/25 OU. The corneas were clear and of normal thickness. Horizontal corneal diameters were 13 mm. There was mild nuclear sclerosis OU. Applanation tonometry and gonioscopy were normal. Iridodonesis was present bilaterally. ?-scan echography showed normal iens thicknesses with increased anterior chamber depths and low-normal axial lengths. A cycloplegic refraction was +5.5 + 1.00x160 OD and +5.75+0.75x160 OS. Keratometry readings were 41.00x70/41.50x160 OD and 41.75x90/42.25x180 OS. The cup/disc ratios were 0.1 OU. The fundus examinations were normal.
Subjects ??-1, III-4. and 111-3 iFigure 1) had complete ophthalmic examinations which were normal.
The horizontal diameter of the normal adult cornea ranges from 11.0 mm to 12.5 mm with an average of 11.6 mm. Microcornea denotes a horizontal diameter of less than 11.0 mm while macrocornea (enlargement with no other signs) indicates a horizontal diameter over 12.5 mm.1-2 The major characteristics of megalocornea include: Da bilateral, non-progressive enlargement (diameters may exceed 18 mm) of clear corneas; 2) an hereditary disorder which is almost always x-linked recessive, and therefore almost exclusively limited to males; 3) absence of any evidence of glaucoma. Inconsistently associated ocular signs include deep anterior chambers, iridodonesis, iris thinning, meiosis, prominent Schwalbe's line, pigment dispersion, ectopia lentis and cataract.1,2,4
The etiology of megalocornea is unresolved. The "unitary" theory asserts that it is an arrested form of incomplete congenital glaucoma.17-19 Megalocornea and congenital glaucoma in the same pedigrees have been recorded. is Some of these patients with enlarged corneas, anterior chamber anomalies and initially normal intraocular pressures developed glaucoma in later years.
The "dualistic" view maintains that megalocornea and congenital glaucoma are separate disorders. 2,9.10,20,21 Various non-glaucomatous causes for megalocornea which have been proposed include hormonal stimulation of globe growth, hereditary hyperplasia, effects of abnormal rectus muscle insertion, calcium imbalance and a disparity between the optic cup ectoderm and ciliary ring growth rates.1,2
There is increasing evidence to support the dualistic view. The histopathologic findings of a normal anterior chamber angle and Jack of posterior segment enlargement are inconsistent with arrested congenital glaucoma.9 Similarly, the absence of enlarged posterior segmente in the M arche s ani syndrome patient studied echographically, speaks against congenital glaucoma.12 Also, normal endothelial cell densities observed in megalocornea suggest that the enlargement is due to corneal hyperplasia.10 If the megalocornea were caused by congenital glaucoma distention, endothelial cell counts should have been diminished.
The described patients are most unusual because of their sex and because they appear to have autosomal dominant isolated megalocornea. These patients present further support for the dualistic theory. There were no chamber angle abnormalities or late-onset glaucoma in these patients. Their echographic findings, the first to be described in isolated megalocornea, revealed increased anterior chamber depths with normal axial lengths. If their megalocorneas were secondary to arrested glaucoma, increased axial lengths and smaller lens thicknesses should have been observed.23
It seems reasonably certain that megalocornea exists as an anterior segment anomaly etio logically distinct from congenital glaucoma. Additional studies will determine if the differences in clinical characteristics are due to genetic heterogeneity, variation in expressivity, age dissimilarities, other factors or some combination thereof.
Karl Os so i n ig, M.D.,andAminNasr, M. D. provided the echographic evaluations.
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