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Clinical Applications of Optical Coherence Tomography in Optic Nerve Disease
Emely Z. Karam, Thomas R. Hedges III, and Carlos E. Mendoza
Optical coherence tomography (OCT) is a relatively new technology that provides information about the retina, optic nerve, and nerve fiber layer (NFL) differently from other instruments.1 Newer versions of the OCT allow better resolution, which makes it useful for management of optic nerve disease. For neuro-ophthalmology, OCT methods employed for retinal as well as glaucoma analysis are used, because some optic nerve pathologies have macular complications, but also because many retinal conditions may mimic optic nerve disease.
The tomograms used are radial, linear, and circular. Radial scans are made up of linear sections through the optic nerve in a clockwise manner that may provide information regarding the diameter of the optic disc, physiologic/pathologic excavation and swelling, and the integrity of the neuroretinal rim. In addition to radial scans of the macula, linear scans can be made horizontally through both the disc and the macula, which can supply some information regarding the relationship of optic nerve head pathology to macular pathology. Circular tomograms centered at the optic disc can be made with different diameters. Each of these circular tomograms is displayed in an unwrapped manner and can be interpreted in three different ways: (1) following each clock hour, which produces 12 measurements of the nerve fiber layer; (2) by quadrants, producing four different measurements of the nerve fiber layer; and (3) as an average of the nerve fiber layer thickness of all four quadrants. These values can provide information regarding segmental nerve fiber layer thinning and swelling, as well as overall nerve fiber layer thickness.
Neuro-Ophthalmic Diseases
Optic Disc Pit
Optic disc pit is a congenital anomaly associated with retinal detachment in 25% to 75% of the cases, frequently involving the macular region. The debate about the origin of the subretinal fluid has not been completely settled, and includes speculation that it may arise from the cerebrospinal fluid, the
vitreous cavity, or the orbit. Chronic retinal detachment can result in lamellar macular holes, cystic retinal degeneration, or retinal pigment epithelial atrophy. There is also controversy regarding management.2,3
Radial tomograms are useful in demonstrating the optic disc pit, which appears as a focal excavation within the nerve head (Fig. 19.1). Circular as well as radial tomograms can show the neurosensory retinal detachment, and can document the relationship of the fluid to the macular region. 4 Optical coherence tomography can also demonstrate resolution of retinal detachment after surgical or laser treatment.2,3,5
Segmental Optic Nerve Hypoplasia
Segmental optic nerve hypoplasia, also called “topless disc,” is an anomaly characterized by incomplete development of the upper portion of the optic nerve head, although other regions of the optic nerve may be involved. In most cases the superior part of the optic nerve is absent, and the upper scleral halo is more prominent. The central retinal artery entrance is also more superior than nasal, and the retinal nerve fiber layer of this area is deficient, correlating with an inferior visual field defect. The pathogenesis is presumed to be due to interruption of normal migration of the retinal nerve fibers, or exaggerated dying-back of nerve fibers that do not normally reach the lateral geniculate body. This is more common among patients born of diabetic mothers.6
Unoki et al.7 demonstrated in vertically oriented optical coherence tomograms a disproportion between the normal lower part of the optic disc and the hypoplastic upper portion. There was thinning of the superior retinal nerve fiber layer and, in some cases, overlap or intrusion of the superior peripapillary pigment epithelium and choroid over the edge of the lamina cribrosa. Horizontal OCT sections through the optic disc and fovea showed that the papillomacular nerve fiber layer was intact, correlating with normal visual acuity and color vision seen in patients with segmental optic nerve hypoplasia, although in more severe cases there may be poor acuity and even nystagmus.
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FIG. 19.1. A 16-year-old girl was found to have an optic nerve pit. (A) This case demonstrated temporal coloboma and nasal pit of the optic disc; two cilioretinal arteries emerged from the margin of the pit. (B) Radial optical coherence tomography (OCT) demonstrated a full pit with some glial tissue over it. (C,D) The visual field showed superior arcuate defect (C) correlating with the inferior thinning of the nerve fiber layer demonstrated in the circular OCT (D).
19. Optical Coherence Tomography in Optic Nerve Disease
Optic Disc Tilt
Optic disc tilt is characterized by an oval optic nerve head that can be observed funduscopically in patients usually with myopic astigmatism. The superotemporal portion of the optic nerve appears elevated, whereas the inferonasal portion appears ectatic, often with associated thinning of the retinal pigment epithelium and choroid. It is also associated with situs inversus of the retinal arteries and veins. In some cases there may be degenerative findings including breaks in Bruch’s membrane, subretinal neovascularization, subretinal hemorrhage, or polypoidal subretinal pseudocysts. Optical coherence tomography demonstrates a normal appearance of the retinal layers, but it may be useful in documenting some of the rare complications of optic disc tilting.8
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prominent, visual field defects may occur. Most often these are arcuate visual field defects, but there may also be generalized constriction of the visual field as well as enlargement of the blind spot. Rare complications include peripapillary hemorrhages and anterior ischemic optic neuropathy. Optical coherence tomography may show optic disc drusen within the nerve head in the form of shadows similar to those caused by the central retinal artery and vein. It demonstrates thinning of the nerve fiber layer, and, as in glaucoma, OCT may be more sensitive than visual field testing in detecting the degrees and patterns of NFL loss (Fig. 19.3).10,11 Optical coherence tomography may be valuable in following the status of the nerve fiber layer in patients with optic nerve head drusen, 12 although no treatment is available for this condition.
Dominant Optic Atrophy
Dominant optic atrophy (DOA) or Kjer optic atrophy is an autosomal dominantly inherited optic neuropathy linked to the OPA-1 gene. The initial visual manifestations occur at 4 to 6 years of age, with symmetric reduction of visual acuity, blue-yellow axis dyschromatopsia or tritanopia, central/centrocecal scotomas, and temporal or diffuse pallor of the optic disc. Because patients with DOA tend to have large, triangular optic cups, they may be misdiagnosed as having low-ten- sion glaucoma.9 Association with sensorineural hearing loss and mental abnormalities can be observed. Optical coherence tomography shows excavation of the optic nerve head, and decreased optic nerve fiber layer thickness in the maculopapillary area, which correlates with what can be seen ophthalmoscopically (Fig. 19.2).
Leber’s Hereditary Optic Neuropathy
Leber’s hereditary optic neuropathy is initially associated with swelling of the retinal nerve fiber layer in the acute phase and with gradual loss of the maculopapillary nerve fiber layer in the chronic phase. Optical coherence tomography may demonstrate nerve fiber layer thickening before visual loss as well. Ultimately the appearance of the retinal nerve fiber layer both ophthalmoscopically as well as by OCT is similar to that seen in patients with nutritional and toxic optic neuropathy.
Optic Nerve Head Drusen
Optic nerve head drusen are hyaline bodies that result from stasis of axoplasmic transport associated with crowding of the axons in a smaller than normal scleral opening. The blood vessels also are crowded into the center of the optic disc and may show anomalous patterns. Optic nerve head drusen may be difficult to identify when they are buried within the optic nerve head. In this situation there may be some confusion with papilledema. As optic nerve head drusen become more
Papilledema
Papilledema refers to optic nerve head swelling due to increased intracranial pressure. In the initial stages, the visual acuity is relatively preserved, but with time there may be progressive loss of the nerve fiber layer, especially superiorly. Papilledema is also associated with macular complications in some cases. Visual field testing demonstrates an increase in the size of the blind spot, and progressive constriction of the visual field, especially inferonasally. Clinically, the distinction between mild papilledema and pseudopapilledema, or congenital crowding of the optic nerve, may be difficult. Unfortunately, this distinction cannot be made with OCT. In a review of patients with mild papilledema and those with crowded optic nerves, without optic disc drusen, OCT demonstrated that the nerve fiber layer is thicker in patients with mild papilledema, as expected, but also in patients with pseudopapilledema when compared to normal controlled subjects.13 However, OCT can be useful in following the retinal nerve fiber layer in patients with congenital crowding of the optic nerve in whom no changes occur over time, and in patients with papilledema in whom the optic nerve will change as the papilledema resolves or progresses. In monitoring patients with confirmed papilledema due to increased intracranial pressure, OCT changes need to be interpreted with caution. As the swollen retinal nerve fiber layer becomes thinner over time, one has to be aware that this could represent either improvement in the papilledema or damage to the nerve fiber layer from chronic papilledema. Therefore, correlation with visual field changes is critical. Optical coherence tomography can also demonstrate macular pathology in the presence of papilledema.
In some patients, subretinal fluid may develop with associated loss of visual acuity. In papilledema the subretinal fluid cannot be identified with fundus fluorescein angiography, but it is readily visible with OCT. The degree of thickening in the macular region is associated with the degree of visual acuity loss. This is a reversible phenomenon (Figs. 19.4 to 19.6).14
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FIG. 19.2. A 9-year-old boy was noted to have visual difficulties in school. His father also has visual difficulties. (A) The fundus of the eye showed triangular optics cups, with temporal pallor. (B) The visual field demonstrated a big blind spot with a superior arcuate defect. (C) Radial OCT in the right eye showed some optic disc excavation. (D) Circumferential OCT showed thinning of the retinal nerve fiber layers, especially temporally.
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FIG. 19.3. A 22-year-old man was followed over several years because of optic nerve drusen. (A,B) The right eye showed occult optic nerve drusen with a big blind spot and mild nasal defect. (C) In the radial OCT, the 90-degree tomogram showed elevation of the optic disc with shadows. (D,E) In the left eye, drusen were exposed and the visual field demonstrated an inferior arcuate defect. (F,G) The 90-degree section of the radial OCT showed elevation of the optic disc, shadows (F), and thinning of nerve fiber layer in the 180degree section as well (G).
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FIG. 19.4. A 41-year-old man developed transient visual obscurations and diplopia. He is obese, weighing 130 kg. A cerebral magnetic resonance imaging (MRI) was normal and the intracranial pressure by lumbar puncture was 480 mm H2O. The visual acuities were 20/20 OD and 20/40 OS. (A) There was visual field constriction of the visual fields, more marked in the left eye. (B) The optic discs showed marked swelling with adjacent hemorrhages, exudates, macular edema, and retinal folds. (C,D) Optical coherence tomography showed elevation of the both optic discs, especially in the inferotemporal area. (E) Macular scans of the left eye demonstrated subretinal fluid.
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FIG. 19.4. (continued).
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FIG. 19.5. After 1 month of treatment, the patient in Fig 19.4 showed improvement. The visual acuities were 20/20 in both eyes; the visual field showed reduction of the visual defect (A) and the optic discs were less swollen (B). The OCT demonstrated decreased edema (C,D) with absorption of the subretinal fluid in the macula area (E).
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FIG. 19.5. (continued).
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FIG. 19.6. (A) Papilledema in atrophic period. (B,C) Optical coherence tomography thinning of the nerve fiber layer.
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Optic Neuritis
When it is retrobulbar, optic neuritis is associated with NFL thinning, which may be observed with OCT. This can be correlated with electrophysiologic responses including pattern electroretinograms.15,16 When optic neuritis affects the optic nerve head as papillitis, OCT can document the degree of optic nerve head and nerve fiber layer swelling (Fig. 19.7).
Anterior Ischemic Optic Neuropathy
Anterior ischemic optic neuropathy is associated with acute loss of vision or visual field, which occasionally may be progressive. Optical coherence tomography can demonstrate the degree of nerve fiber layer thickening during the acute phase and demonstrates nerve fiber layer loss in affected quadrants, which ultimately occurs over time. Optical coherence tomography may be useful in detecting incipient ischemic optic neuropathy in the opposite eye, which may be difficult to detect ophthalmoscopically. As in patients with papilledema, subretinal fluid may also develop in rare cases (Figs. 19.8 and 19.9).
Big Blind Spot Syndrome
Big blind spot syndrome is a peripapillary chorioretinal disturbance of unknown etiology, which can occur in isolation or in association with different forms of chorioretinitis including multiple evanescent white dot syndrome (MEWDS), multifocal choroiditis with panuveitis (MCP), acute macular neuroretinitis (AMN), diffuse subretinal fibrosis (DSF), punctate inner choroidopathy (PIC), and pseudo– presumed ocular histoplasmosis syndrome (P-POHS).17 Abnormal findings have been demonstrated in visual fields, multifocal electroretinogram, and fluorescein and indocyanine green angiography. In the acute phase, when the peripapillary swelling is present, the OCT showed thickness of all the retinal layers and choroid around the optic nerve head that is correlated with the optic disc staining observed in the angiography fluorescein study (Fig. 19.10). After edema resolution, pigmentary findings can be found around the optic disc, and the OCT demonstrated fragmentation of the retinal pigment epithelium, which is related with the angiographic findings (Fig. 19.11).18
FIG. 19.7. A 48-year-old woman lost vision in the left eye. She had an afferent pupillary defect, optic disc swelling (A), and a central scotoma
(B). Several demyelinating plaques were seen on MRI. (C,D) Initial OCT showed retinal nerve fiber layer (RNFL) swelling. (E) She was treated with high-dose steroids and recovered much of her vision and visual field. (F) After 1 year, the fundus of the eye demonstrated loss of superior and papillomacular nerve fiber layer. (G,H) Follow-up OCT showed RFNL dropout in the same area.
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FIG. 19.7. (continued).
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FIG. 19.7. (continued).
FIG. 19.8. A 57-year-old man with hypertension woke up one morning with acute visual loss. (A) The visual field demonstrated an altitudinal defect. (B) There was optic disc swelling with peripapillary, flame-shape hemorrhages. (C) The OCT showed retinal nerve fiber layer thickening.
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FIG. 19.8. (continued).
FIG. 19.9. A 71-year-old man with hypertension, hypercholesterolemia, and mitral valve prolapse developed blurred vision in the right eye. The visual acuities were 20/60 OD and 20/20 OS. (A) The right visual field demonstrated an altitudinal visual field defect. (B) The optic disc was swollen with flame hemorrhages in OD and crowding (optic disc at risk) OS. (C) Optical coherence tomography showed thickness of the nerve fiber layer with subretinal fluid in OD, and a small disc without excavation OS. (D) After 1 month, the visual field improved. (E) The optic disc edema decreased and appeared pale in the superior area. (F) The OCT demonstrated thinning of the retinal nerve fiber layer without subretinal fluid.
FIG. 19.9. (continued).
FIG. 19.10. A 51-year-old man developed unilateral blindness in the temporal field of his left eye. Flickering lights appeared intermittently within the blind area. The visual acuities were 20/20 in both eyes. (A) The visual field revealed an enlarged blind spot in the left eye. (B) The left optic disc was slightly elevated with a halo surrounding it. (C) Fluorescein angiography demonstrated peripapillary hyperfluorescence in the venous phase and multiple hyperfluorescent spots scattered throughout the retina. (D) Optical coherence tomography showed, in the radial section, thickening of the nerve fiber and retinal layers in the peripapillary area, and some backscattering in the choroid with disruption in the retinal pigment epithelium. (E) The circular tomograms showed slight thickening of the nerve fiber layer.
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FIG. 19.10.(continued).
FIG. 19.11. A 34-year-old woman was followed because of several episodes blind enlargement. (A) During the last one, optic disc swelling and extensive pigmentary findings were observed in the superior area. (B) The visual field showed enlargement of the blind spot. (C) Optical coherence tomographic radial sections (90-degree) showed fragmentation of the retinal pigment epithelium in the peripapillary area. (D) The nerve fiber layer demonstrated slight diminution of the nerve fiber layer in the superior area.
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Optic Nerve Melanocytoma
Optic nerve melanocytoma is an optic nerve head pigmented tumor that can extend to the adjacent retina or lamina cribrosa. Local infiltration can cause obstruction of the axoplasmic fluid with consequent optic disc edema, central vein occlusion, or sporadic malignant melanoma transformation.19–22 In OCT, the melanocytoma shows a high reflectivity layer that continues to the high reflectance signal produced by nerve fiber layer, with optical shadowing behind it (Fig. 19.12). Optical coherence tomography is useful in following the tumor’s growth, local infiltration, and occasional neurosensorial retinal detachment.23
Optic Nerve Meningioma
Optic nerve meningioma and other compressive optic neuropathies lead to optic atrophy, which can be demonstrated by OCT (Fig. 19.13). However, in these cases, visual field loss precedes the nerve fiber layer loss and, ideally, treatment should be provided before permanent nerve fiber layer degeneration appears. In some cases, optic nerve sheath meningioma may lead to swelling of the optic nerve head, which in many ways resembles papilledema. This also could be documented with OCT.
FIG. 19.12. A 40-year-old woman was followed because of an optic nerve melanocytoma. (A) The visual field showed a big blind spot.
(B) The optic disc showed a pigmented lesion in the optic disc. (C,D) The OCT demonstrated superficial high reflectivity with shadowing behind the tumor.
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FIG. 19.13. (A) A chiasmal meningioma was associated with a visual deficit in both eyes, more marked in the right eye, in this patient. (B) The optic disc was pale. (C) The MRI demonstrated chiasmal tumor with the “tail” sign. (D,E) Optical coherence tomography showed depletion of the nerve fiber layer especially in the right eye.
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FIG. 19.13. (continued).
19. Optical Coherence Tomography in Optic Nerve Disease
Traumatic Optic Neuropathy
The retinal nerve fiber layer loss begins between 4 and 8 weeks after the onset of the injury. Over time, the nerve fiber layer becomes thinner, but some nerve fiber layer preservation is evident even when the optic nerve head becomes atrophic. Optical coherence tomography is useful in demonstrating the nerve fiber layer loss over time.24
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