Anterior ischemic optic neuropathy usually presents in the older patient with painless, often severe visual loss of sudden onset that can be irreversible. Ophthalmoscopy reveals pallid swelling of the optic disc that may be accompanied by superficial peripapillary hemorrhages. The findings probably relate to acute ischemia of the anterior portion of the optic nerve.^6,13,17 The initial report of severe visual loss in association with giant cell arteritis appears to be that of Jennings,²⁷ but the term ischemic optic neuritis was first used by Wagener.⁴⁶ It is now usually known as ischemic optic neuropathy (ION).³⁵ Clinical reviews have identified two groups of patients: those with giant cell arteritis (arteritic, AAION) and those without (nonarteritic, NAION).^3,17,34,39 Many nonarteritic cases are idiopathic, but systemic hypertension, ischemic heart disease, hypercholesterolaemia, and diabetes mellitus are risk factors.^17,39,41 There are reports of ION in association with hypotension,⁴⁴ migraine,^4,33,47 acute hypotension and anemia consequent on gunshot wound or lipsuction,^36,43 following internal carotid artery dissection,² and following cataract surgery.^30,38

Clinically, patients with NAION present with visual loss in one eye, possibly with previous involvement of the other eye. The optic disc is swollen, and the more extensive the disc swelling, the greater is the degree of visual impairment.³ Flame hemorrhages are usually present. The majority of patients have inferior altitudinal field defects, but approximately 20% have a central scotoma.⁷ The field defect may correlate poorly with the fundus appearance, but some patients have clear superior or inferior swelling with corresponding altitudinal field loss. In one large series,¹⁷ more than 35% of the NAION patients had a visual acuity of 6/36 or worse, but 30% had normal (6/9 or better) acuity.

Patients with AAION often have symptoms associated with temporal arteritis: malaise, muscle pain, scalp tenderness, etc., whereas the nonarteritic patients do not feel unwell. There is often generalized field constriction in the affected eye. Visual acuity may be severely reduced, with 60% having an acuity of counting fingers or worse,¹⁷ but also may be unimpaired. A percentage of both groups may have had previous transient visual dysfunction. The blood erythrocyte sedimentation rate (ESR) is usually raised in temporal arteritis, but a low ESR does not exclude the diagnosis,^17,34 and a positive temporal artery biopsy is necessary for confirmation. It is important to distinguish the cases due to temporal arteritis from idiopathic cases because high-dose steroids are the treatment of choice in arteritic ION.^9,18,34 The addition of methotrexate may be effective.²⁸ There have been reports of improvement following steroid administration in nonarteritic patients,¹⁷ but as yet there is no satisfactory treatment. Optic nerve sheath decompression initially seemed to improve outcome in some patients with progressive NAION,⁴² but the results from the Ischemic Optic Neuropathy Decompression Trial not only failed to confirm significant therapeutic benefit, but also suggested that nerve sheath decompression may actually be potentially harmful in NAION.^24–26

The histological changes in AAION were reviewed by Henkind et a1.¹⁹ The orbital vessels, including the posterior ciliary arteries, the ophthalmic artery, and the intraneural central retinal artery, may be involved in the arteritic process, but involvement of the intraocular retinal or choroidal vessels is unusual. Hayreh and Baines¹⁶ suggested that the posterior ciliary arteries feed fairly well delineated areas of the choroid and nerve head and that posterior ciliary artery occlusions may infarct the optic disc and adjacent retrolaminar optic nerve. The reader is referred elsewhere for a comprehensive discussion of the blood supply to the optic nerve head.^14,15 A case report, without clinical details, of the histopathological findings in nonarteritic ION showed focal infarction 3mm behind the lamina cribrosa that was caused by thromboembolism in three discrete pial and pial-derived arterioles.³² The temporal aspect of the macula showed ischemic necrosis. A recent large series further defined the histopathology of ischemic optic neuropathy in relation to localized ischemic edema, cavernous degeneration, or an area of atrophy located superior or inferior in the optic nerve.²⁹

The first detailed report of the electrophysiological findings in NAION was that of Wilson,⁴⁹ although “delays” in the pattern visual evoked potential (PVEP) had previously been mentioned.²⁰ Wilson⁴⁹ examined both PVEP and flash VEPs (FVEPs) in a mixed group of 15 arteritic and non-arteritic patients. Both PVEPs and FVEPs showed reduced amplitude, but only four patients showed minimal (<10ms) latency changes. The clinically uninvolved eye invariably had normal visual evoked potentials (VEPs). Those findings were contrasted with those in optic nerve demyelination, in which latency delays in excess of 10ms are common and there is often subclinical involvement of the fellow eye. Other authors^5,8,10,21,48 confirmed the high incidence of reduced amplitude, normal latency VEPs, but Glaser and Laflamme⁸ found a predominance of P100 component delays in acute cases. Harding's group¹⁰ noted that all affected eyes showed a reduced VEP and that those with a delayed or triphasic response to flash had temporal arteritis. The FVEP delay in association with temporal arteritis was confirmed by this author,²¹ who further reported that the PVEP was more sensitive than the FVEP in nonarteritic patients. Amplitude reductions were usually relative to the uninvolved eye. Typical findings are shown in figure 59.1. Cox et a1.⁵ compared the PVEPs from 24 eyes with NAION with 22 eyes with optic nerve demyelination. The mean latency difference between the involved and the uninvolved eyes was 3ms for NAION but 21ms in demyelination. Wildberger⁴⁸ found amplitude changes but also reported that patients with an inferior altitudinal defect touching the horizontal meridian showed apparent latency delays that were attributed to preservation of the normal longer latency response from the superior field.³¹

Figure 59.1.  

VEP findings in four patients with nonarteritic ischemic optic neuropathy. The affected eye in each patient is shown as the lower of the two pairs of traces for each patient. Patient A shows a broadening of the major positive P100 component with increased N135 component latency, but the dominant feature is amplitude reduction; patient B shows marked P100 amplitude reduction with mild latency increase; patient C shows amplitude reduction with no latency change; patient D shows a questionable P100, of normal latency if present. Calibration: 5µV, 80ms.

Definite latency delays have been reported,³⁷ but stimulus and recording parameters were not given. A later study⁴⁵ emphasized the difficulties in accurate component identification with a single midline recording channel. (See also chapter 15 for a discussion of normal PVEP components and their distribution.) Those authors, using a 15 degree radius, 50 minute check stimulus, found “delays” in some cases that could be explained by complete or partial substitution of the paramacular P135 subcomponent for the usually dominant, macular-derived P100 component. It should be remembered that this interpretation only applies with a large field. Most of their patients had single-channel recordings with central field stimulation. PVEPs were often extinguished, but delays were observed. Follow-up studies suggested that the abnormalities remained essentially unchanged.

Posterior ischemic optic neuropathy may also occur but is much less common^12,40 and has not been satisfactorily characterized electrophysiologically.

This author reported PERG abnormalities in seven cases of NAION, five with involvement of the P50 component and two with an abnormality confined to N95.²² As P50 component reduction is usually associated with dysfunction anterior to the retinal ganglion cells in the visual pathway,²³ the histopathological observations of macular necrosis in ION³² may be relevant.

To conclude, the finding of a normal latency, reduced amplitude PVEP suggests NAION in a patient with sudden, painless loss of vision and a swollen optic disc. If there has been a previous episode in the fellow eye with resultant disc pallor, the appearances may be mistaken for the Foster-Kennedy syndrome (see figure 59.2). An abnormal VEP is not a feature of papilloedema per se, and electrophysiology should help to resolve any diagnostic difficulties in such cases. The findings from clinically uninvolved eyes are normal. PVEP delays can occasionally be observed but are less marked than in optic nerve demyelination. There are usually associated systemic symptoms and elevation of the blood ESR with AAION.

Figure 59.2.  

Pattern and flash VEPs in a 62-year-old patient with pseudo Foster-Kennedy syndrome due to acute anterior ischemic optic neuropathy with disc swelling in the right eye and old anterior ischemic optic neuropathy with disk pallor in the left eye. Visual acuities were 6/12 right, 6/36 left. Pattern VEPs from both eyes fall within the normal latency range (vertical line = upper limit of normal for age) but show mild increase in P100 latency from the right eye relative to the left. Flash VEPs show mild interocular asymmetry in later components from the right eye but show no definite abnormality.