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Congenital Optic Nerve Abnormalities Paul H 6 Congenital Optic Nerve Abnormalities Paul H. Phillips and Michael C. Brodsky he ophthalmologist is frequently called upon to evaluate Tinfants and children with decreased vision related to con- genital abnormalities of the optic nerve. Such evaluation necessitates a detailed understanding of the ophthalmoscopic features, associated neuro-ophthalmic findings, current theories of pathogenesis, and appropriate ancillary studies in each con- dition. This chapter examines congenital optic nerve abnor- malities, discusses current controversies surrounding their pathogeneses, and reviews associated neuroradiologic findings that may predicate the general medical management of affected patients. Certain general principles apply to the evaluation and man- agement of the child with congenital optic nerve abnormalities. 1. Children who have bilateral optic disc anomalies gener- ally present with poor vision and nystagmus in infancy; those with unilateral disc anomalies typically present with sensory strabismus. 2. Small optic discs are associated with a wide variety of central nervous system (CNS) malformations involving the cere- bral hemispheres, pituitary infundibulum, and midline intra- cranial structures (septum pellucidum, corpus callosum). A trans-sphenoidal basal encephalocele is common in patients with large discs of the morning glory variety. The presence of a basal encephalocele can often be predicted clinically by the finding of midfacial anomalies (hypertelorism, flat nasal bridge, cleft lip or palate).62 Large optic discs with a colobomatous con- figuration may be associated with systemic anomalies in a variety of coloboma syndromes. Magnetic resonance imaging 204 chapter 6: congenital optic nerve abnormalities 205 (MRI) is advisable in all children with small optic discs, and in children with large optic discs who either have facial character- istics suggestive of basal encephalocele or neurodevelopmental deficits. 3. Color vision is typically normal in an eye with a con- genitally anomalous optic disc (i.e., limited only by the visual acuity), in contradistinction to the severe dyschromatopsia that characterizes most acquired optic abnormalities. 4. A structural ocular abnormality that reduces visual acuity often leads to superimposed amblyopia. A trial of occlu- sion therapy is therefore warranted in most patients with uni- lateral optic disc anomalies and decreased vision. The findings of an afferent pupillary defect should not discourage this effort. 5. The finding of a discrete V- or tongue-shaped zone of infrapapillary retinochoroidal depigmentation in an eye with an anomalous optic disc should prompt a search for a trans- sphenoidal encephalocele.14 OPTIC NERVE HYPOPLASIA Incidence Optic nerve hypoplasia, once considered a rare anomaly, is now the most common congenital optic anomaly encountered in pediatric ophthalmic practice. Part of the increased prevalence may reflect a greater recognition of optic nerve hypoplasia on the part of clinicians. To the uninitiated observer, the small size of the hypoplastic disc is not readily apparent, and the disc may be interpreted as atrophic or even normal.9 Many cases of optic nerve hypoplasia were undoubtedly misdiagnosed in the past. On the other hand, some authors have suggested that drug and alcohol abuse, both of which have become more widespread in recent years, may account in part for the increased number of cases.46 Clinical Features Ophthalmoscopically, the hypoplastic disc appears as an abnor- mally small optic nerve head (Fig. 6-1). It may appear gray or pale in color and is often surrounded by a yellowish, mottled peripapillary halo, bordered by a dark pigment ring (double-ring sign).35 Retinal vascular tortuosity is commonly seen. Histolog- 206 handbook of pediatric neuro-ophthalmology FIGURE 6-1. Optic nerve hypoplasia with double-ring sign. Arrow denotes true margin of the hypoplastic disc. (From Lambert SR, Hoyt CS, Narahara MH. Optic nerve hypoplasia. Surv Ophthalmol 1987;32:1–9, with permission.46) ically, optic nerve hypoplasia is characterized by a subnormal number of optic nerve axons with normal mesodermal elements and glial supporting tissue.35 Visual acuity ranges from 20/20 to no light perception, and affected eyes invariably show localized visual field defects, often combined with a general constriction of the visual fields.27 Because visual acuity is determined pri- marily by the integrity of the papillomacular nerve fiber bundle, it does not necessarily correlate with the overall size of the disc. It is now appreciated that some forms of optic nerve hypoplasia are segmental (Fig. 6-2). A pathognomonic superior segmental hypoplasia with an inferior visual field defect occurs in some children of insulin-dependent diabetic mothers.15,43 Conversely, colobomatous malformations of the optic disc produce an inferior segmental hypoplasia.7 Intrauterine insults to the retina, optic nerve, optic tract, or occipital lobe are asso- ciated with hypoplasia of the affected portions of each optic nerve.57 chapter 6: congenital optic nerve abnormalities 207 Patients with periventricular leukomalacia (PVL) often have a unique form of optic nerve hypoplasia characterized by an abnormally large cup and a thin neuroretinal rim contained within a normal-sized optic disc.8,39 This morphological appearance has been attributed to bilateral injury to the optic radiations with retrograde transsynaptic degeneration of retinogeniculate axons after the scleral canals have established normal diameter. Optic disc cupping in patients with PVL may cause diagnostic confusion with glaucoma.8 Therefore, a history of prematurity and neuro- developmental delay should be sought in children who have abnormally large optic cups and normal intraocular pressures. Neuroimaging is necessary to confirm the diagnosis. Associated Features Optic nerve hypoplasia is often associated with other CNS mal- formations. Septo-optic dysplasia (de Morsier’s syndrome) refers FIGURE 6-2. Segmental optic nerve hypoplasia associated with macular coloboma. Note focal absence of temporal optic disc with corresponding temporal nerve fiber layer defect. (From Novakovic P, Taylor DSI, Hoyt WF. Localizing patterns of optic nerve hypoplasia: retina to occipital lobe. Br J Ophthalmol 1988;72:176–182, with permission.57) 208 handbook of pediatric neuro-ophthalmology to the constellation of small anterior visual pathways, absence of the septum pellucidum, and agenesis or thinning of the corpus callosum.22 The clinical association of septo-optic dysplasia and pituitary dwarfism was documented by Hoyt et al. in 1970.36 MRI is the optimal noninvasive neuroimaging modality for delineating congenital CNS malformations in patients with septo-optic dysplasia.13 Because MRI provides high-contrast res- olution and multiplanar imaging capability, the anterior visual pathways can be visualized as distinct, well-defined structures. Coronal and sagittal MRI consistently demonstrate a reduction in the intracranial optic nerve diameter in optic nerve hypopla- sia (Fig. 6-3). In bilateral optic nerve hypoplasia, coronal MRI shows diffuse thinning of the optic chiasm (chiasmal hypopla- sia). When MRI demonstrates a reduction in intracranial optic nerve size accompanied by other midline CNS features of septooptic dysplasia, the presumptive diagnosis of optic nerve hypoplasia can be made neuroradiologically.13 In addition to septo-optic dysplasia, MRI may demonstrate cerebral hemispheric migration anomalies (schizencephaly, cortical heterotopias) or intrauterine or perinatal hemispheric injury (periventricular leukomalacia, encephalomalacia). These abnormalities occur in approximately 45% of patients with optic nerve hypoplasia and are highly predictive of neurodevel- opmental deficits.12 Absence of the septum pellucidum alone does not portend neurodevelopmental deficits or pituitary hormone deficiency.72 Thinning or agenesis of the corpus callosum is predictive of neurodevelopmental problems only by virtue of its frequent association with cerebral hemispheric abnormalities. Magnetic resonance imaging demonstrates neurohypophy- seal abnormalities in approximately 15% of patients with optic nerve hypoplasia.12 In normal subjects, high-resolution cranial MRI delineates the pituitary infundibulum, anterior pituitary gland, and the posterior pituitary gland, which appears as a “bright spot” in the sella. The “posterior pituitary bright spot” has been attributed to the posterior pituitary hormones as well as the phospholipid content of the vesicles containing these hor- mones. In patients with optic nerve hypoplasia, absence of the infundibulum and posterior pituitary ectopia is associated with endocrine dysfunction.12,61,66 When the pituitary infundibulum and its surrounding portal venous system are absent, the hypo- thalamus is unable to stimulate the anterior pituitary gland, resulting in anterior pituitary hormone deficiency. Posterior chapter 6: congenital optic nerve abnormalities 209 AB CD FIGURE 6-3A–D. Magnetic resonance imaging in optic nerve hypoplasia. (A) Left optic nerve hypoplasia. T1-weighted coronal MR image shows normal right optic nerve (arrow) and no visible signal corresponding to the left optic nerve. (B) Septo-optic dysplasia. Clinically, this patient had mild right optic nerve hypoplasia and severe left optic nerve hypoplasia. T1-weighted coronal MR image shows thinning and signal attenuation of the left optic nerve compared to the right (lower arrow, right optic nerve). Also note absence of the septum pellucidum and squaring of
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