Congenital and Developmental Anomalies of the Optic Nerve
Abnormalities of Optic Nerve Size
Optic Nerve Hypoplasia
Optic nerve hypoplasia appears to be the result of excessive pruning of the optic nerve bundles during its development. The disc is pale and may be surrounded by a variably pigmented yellow–white ring. This appearance has been referred to as the “double ring” sign. While the nerve head is small, the retinal vessels are usually of normal caliber. It may occur in one or both eyes and be associated with mild to severe visual impairment, including limited visual acuity and visual field deficits.
Megalopapilla classically presents as an enlarged, but otherwise normal appearing optic disc. Patients often present with good visual acuity, but mild to moderate visual deficits may be seen.
Optic Nerve Aplasia
Optic nerve aplasia denotes total absence of the optic nerve, retinal blood vessels, and retinal ganglion cells. It is a rare condition, most commonly unilateral, and associated with microphthalmos, retinochoroidal colobomas, and cataracts. No light perception visual acuity and an afferent pupillary defect are common findings.
Congenital Prepapillary Vascular Loops
Congenital vascular anomalies, or congenital prepapillary vascular loops, usually appear as tortuous loops of an arteriole or venule that extend above the plane of the optic nerve and into the vitreous cavity. It may have a corkscrew or spiral shape and is often encased in a white fibroglial sheath as it enters the vitreous cavity. They are usually found in eyes with good visual acuity, though may be associated with retinal arterial obstruction in the distribution of the retina supplied by the loop. Amaurosis fugax and vitreous hemorrhage have also rarely been reported.
Persistent Fetal Vasculature (PFV)
Regression of the hyaloid artery usually begins in the third month of gestation and is complete by the eighth month of gestation. Failure of regression may result in variable findings, from only threadlike remnants protruding from the optic nerve to a grossly visible vessel attached to the posterior capsule of the lens, usually in an inferonasal location. If the hyaloid artery is encased by a glial sheath of neuroectodermal cells it is known as Bergmeister papilla.
Congenital Retinal Macrovessel
A congenital retinal macrovessel is a rare vascular anomaly that appears as an enlarged vessel exiting the optic nerve and traversing the macula with several first-order tributaries extending superior and inferior to its horizontal course. The optic nerve itself is generally normal. Venous macrovessels are more common than arterial and are most commonly located in an inferotemporal location. Visual acuity is usually preserved despite the vessel crossing over or through the foveal avascular zone.
Cilioretinal Artery Occlusion
A cilioretinal artery is seen in approximately 5 to 10% of patients and is generally temporal in location. It emerges from the optic nerve separately from the vessels derived from the central retinal artery. It often has a hook-like appearance and exits from the nerve substance and/or the edge of the nerve with approximate equal frequency. A cilioretinal artery becomes significant when an occlusion occurs. This may occur by itself or in conjunction with a central retinal vein occlusion. If the artery supplies blood to the papillomacular bundle and the macular region, significant visual loss may occur. The converse scenario may also be seen, in that in the setting of a central retinal artery occlusion, a cilioretinal artery may be protective in the preservation of central visual function.
Excavated and Colobomatous Defects
Optic Nerve, Retinochoroidal, and Iris Colobomas
Congenital coloboma of the optic nerve is characterized by absent tissue and may show enlargement of the papillary area, partial or total excavation with a white surface, and retinal vessels that enter and exit from the borders of the defect. The affected nerve is commonly larger than normal in diameter. The coloboma may be unilateral or bilateral, and it is thought to be secondary to a failure of fusion of the posterior part of the embryonic fissure. Coloboma of the optic nerve may be caused by a mutation in the PAX6 gene.
Morning Glory Disc Anomaly
The morning glory disc anomaly is a variant of an optic nerve coloboma. It is classically an enlarged, unilateral, funnel-shaped excavation of the optic nerve head with a central core of pale glial tissue. The margins are raised and chorioretinal pigmentary changes are commonly present at its border. Retinal vessels course near the margins, but are obscured centrally by glial tissue. It is more common in females and affects right eyes to a slightly greater degree than left eyes. There may be persistent hyaloidal remnants in the base of the excavation.
Optic Nerve Pit
(Also see Chapter 11 , Peripheral Retinal Degenerations and Rhegmatogenous Retinal Detachment)
An optic nerve head pit is an uncommon congenital anomaly that may result from imperfect closure of the superior edge of the embryonic fissure. It appears as a grey-white round or oval-shaped depression on the inferotemporal aspect of the optic nerve, often with adjacent peripapillary chorioretinal atrophy or retinal pigment epithelial pigment changes. Associated non-rhegmatogenous serous retinal detachments of the macula and schisis cavities are common. The origin of the subretinal fluid is controversial, but may arise from the vitreous cavity, leakage from retinal vessels within the pit or adjacent choroid, or may be cerebrospinal fluid leaking into the subretinal space via the subarachnoid space. In the presence of a long-standing serous macular detachment, pars plana vitrectomy surgery is often offered as treatment.
The retinal vessels exiting from the optic nerve generally progress directly temporally. However, on occasion they take a nasal bend, prior to heading temporally. The temporal half of the nerve generally appears full, and the optic cup is generally absent. Optic nerve hypoplasia may also be seen. This finding is also referred to as tilted disc syndrome (see below), nasal fundus ectasia, and Fuchs coloboma. Patients are generally asymptomatic.
Tilted Disc Syndrome
Features of tilted disc syndrome include inferonasal tilting, an inferior or inferonasal crescent, situs inversus of the retinal vessels, fundus ectasia, myopia, and astigmatism. About 75% of cases are bilateral. Choroidal neovascularization may occur due to weakness in Bruch membrane, usually near the crescent of a staphyloma in myopic eyes.
Peripapillary staphylomas are rare congenital anomalies characterized by a normal appearing optic nerve surrounded by a zone of staphylomatous excavation. Chorioretinal degeneration is a universal finding within the walls of the staphyloma. It is differentiated from myopic conus and staphyloma by a relatively normal refraction, normal appearing optic disc, absence of a progressive chorioretinal degeneration, and lack of a temporal predilection of the peripapillary pigmentary alterations.
Optic Nerve Head Drusen
Optic nerve head drusen are congenital intrapapillary refractile bodies unrelated to choroidal drusen. They occur in approximately 1% of the general population, are more common in Caucasian races, and are frequently bilateral. They may mimic papilledema of the optic nerve and are associated with spontaneous disc hemorrhages and arcuate visual field deficits. The drusen become increasingly visible with age and are rare before the teenage years. Buried drusen are readily detectable with B-scan ultrasonography and on spectral domain optical coherence tomography (SD-OCT) scans as well.
Optic Nerve Head Drusen with Juxtapapillary Choroidal Neovascularization (CNV)
Non-arteritic anterior ischemic optic neuropathy (NAION) is a common optic neuropathy affecting primarily adults over the age of 50 years. Classically, the condition affects hyperopic, small discs, with a crowded central cup (“disc-at-risk”). It may result in altitudinal or arcuate visual field defects, disc hemorrhage, or edema of the optic nerve. These eyes must be differentiated from a vasculitis or temporal arteritis, which has a more threatening prognosis and generally necessitates intervention with systemic corticosteroid and/or other immunosuppressive agents.
Optic Nerve Papillitis
Papillitis describes swelling of the optic disc caused by local inflammation of the optic nerve head. It is most commonly secondary to demyelinating disease in younger patients and ischemic optic neuropathy in patients older than 50 years of age. Infectious, inflammatory, autoimmune, and neoplastic entities must also be considered.