Alex V. Levin
Posterior embryotoxon (PE) is a thickened and anteriorly displaced Schwalbe’s line. The PE may be discontinuous. Schwalbe’s line is the circumferential collagenous band at the junction of Descemet’s membrane and the trabecular meshwork.
• Its prevalence among normal population is 8–15%.
• It is almost an obligatory feature of the Axenfeld–Rieger spectrum and is seen in 80% of patients with Alagille syndrome.
• Patients with Alagille or Axenfeld–Rieger are at particular risk.
• PE is a congenital malformation without risk factor for acquisition.
• Most cases are not inherited and not heritable.
• Both patterns of autosomal dominant (more common) and recessive isolated PE have been reported.
• Otherwise, genetics parallels any associated syndrome (e.g., Axenfeld–Rieger and Alagille autosomal dominant).
Schwalbe’s line, trabecular meshwork, and Descemet’s membrane are derivatives of neural crest. PE develops when neural crest migration and differentiation in this region of the eye is disrupted, presumably due to defective genetic instruction. For example, JAG1, the gene, which when mutated causes Alagille type 1 (MIM 118450), encodes a protein, NOTCH1, which is involved with pathways that determine cell fates in early development.
COMMONLY ASSOCIATED CONDITIONS
• Few iris strands bridging the angle and attaching to the PE, cornea plana, corectopia, polycoria, aniridia, megalocornea, glaucoma, and unspecified anterior segment dysgenesis.
• Alagille syndrome (arteriohepatic dysplasia), is a rare genetic disorder with autosomal dominant transmission. Types 1 and 2. Ophthalmologist may be asked to assist in the diagnosis of this syndrome.
– Hepatic manifestations:
– Most often presents within the first 3 months of life
– Ranges from jaundice, mild cholestasis, and pruritus to progressive liver failure
– Cardiac manifestations:
– Ranges from benign heart murmurs to significant structural defects occur in 90–97%.
– Pulmonic stenosis is the most common cardiac finding (67%).
– Ophthalmologic manifestations
– PE (78–89%), iris abnormalities (45%), diffuse fundus hypopigmentation (57%), speckling of the retinal pigment epithelium (33%), and optic disc anomalies (76%) including optic disc drusen (94%) and papilledema
– Visual acuity is usually not significantly affected.
– Skeletal manifestations,
– The most common radiographic finding is butterfly vertebrae (33–87%)
– Most common in the thoracic vertebrae
– Facial features
Almost always present and include a prominent forehead, deep set eyes with moderate hypertelorism, pointed chin (“triangular facies”), and saddle or straight nose with a bulbous tip.
• Axenfeld–Rieger spectrum
• 22q11.2 deletion syndrome
• Noonan syndrome
• Aarskog (facial–digital–genital) syndrome
• May be history of at-risk associated disorders in family, especially if autosomal dominant
• May be history of glaucoma in family without known associated disorders or diagnoses
• Slit lamp examination:
– A glass-like hyaline membrane, sharply defined and concentric, on the inner surface of cornea, may be discontinuous, located 0.5–2 mm anterior to the posterior limbus. May not be visible without gonioscopy.
– Translucent posterior corneal surface between the PE and the limbus.
– The ring in many cases does not extend for a full 360°. More often temporally than nasally.
– May or may not have iridocornea strands attached to the PE
– Required if PE suspected and not visible at slit lamp
DIAGNOSTIC TESTS & INTERPRETATION
Initial lab tests
None required unless multisystem involvement suggests PE as part of a syndrome (e.g., liver function tests if suspect Alagille syndrome).
Follow-up & special considerations
• Risk for glaucoma in patients without iridocorneal adhesion is unknown. Ophthalmic follow-up suggested.
• PE accompanied with iris attachment should be followed for early glaucoma detection.
– Examine siblings, offspring, and parents where possible to identify others who may be at glaucoma risk.
High-resolution anterior segment optical coherence tomography (OCT) or ultrasound biomicroscopy may be helpful. Usually not required.
• Full ophthalmic examination with particular attention to possibility of glaucoma
• Review of systems to identify other possible syndromic findings.
• Hypertrophy or thickening and anterior displacement of Schwalbe’s line, in cross-section appears as a collagenous condensation.
• Thin Descemet’s membrane and endothelium on either side of Schwalbe’s line.
• Peripheral corneal endothelial or inner stromal opacification or deposition.
• Iridocorneal adhesion from other causes (e.g., peripheral anterior synechia)
– Surgical wounds (in particular stepped self-sealing wounds as in temporal cataract extraction) or external trauma
Treat glaucoma when identified.
Treat associated syndromic abnormalities as indicated.
Issues for Referral
• Genetics consultation if associated with systemic syndromes or other malformations
• Glaucoma referral as needed
• No surgery indicated or available to alleviate PE. Surgery if focuses on treatment of associated glaucoma if fails medical treatment
– No indication to lyse iridocorneal strands
• Follow for early identification and treatment of glaucoma.
• PE associated with any ocular or systemic problems should be followed for those conditions.
Pediatric Glaucoma and Cataract Family Association (www.pgcfa.org).
No known increased or decreased risk for adverse outcome secondary to glaucoma.
• Rennie CA, Chowdhury S, Khan J, et al. The prevalence and associated features of posterior embryotoxon in the general ophthalmic clinic. Eye (Lond) 2005;19(4):396–399.
• Krantz ID, Piccoli DA, Spinner NB. Alagille syndrome. J Med Genet 1997;34:152–157.
• Burian HM, Braley AE, Allen L. External and gonioscopic visibility of the ring of Schwalbe and the trabecular zone; an interpretation of the posterior corneal embryotoxon and the so-called congenital hyaline membranes on the posterior corneal surface. Trans Am Ophthalmol Soc 1954–1955;52:389–428.
• 377.21 Drusen of optic disc
• 743.43 Other congenital corneal opacities
• 743.44 Specified congenital anomalies of anterior chamber, chamber angle, and related structures
• Isolated PE is a common dominantly inherited ocular finding that may be associated with Axenfeld–Rieger spectrum or Alagille syndrome.
• Glaucoma risk should always be recognized except in Alagille syndrome where there does not appear to be a glaucoma risk.