OPTIC NEUROPATHIES

Published on 10/04/2015 by admin

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CHAPTER 23 OPTIC NEUROPATHIES

Virtually every pathological process that can damage an organ in the body can damage the optic nerve. Thus, optic neuropathies can be produced by ischemia, inflammation, infection, compression, infiltration, toxic exposure, metabolic dysfunction, and trauma. Unfortunately, regardless of the cause of an acute optic neuropathy, the optic disc—the only portion of the optic nerve that can be observed with an ophthalmoscope—has only two possible appearances: swollen or normal. Even more confusing is that with chronic damage to the optic nerve, the optic disc simply becomes pale. Thus, the determination of the cause of an optic neuropathy usually cannot be made from the appearance of the optic disc alone. It can, however, be made from a complete assessment, including a complete history, a complete examination, and, in many cases, appropriate ancillary studies.

Damage to the optic nerve occurs in three main clinical settings: (1) optic disc swelling without visual loss; (2) acute visual loss with and without optic disc swelling; and (3) subacute or insidious visual loss with and without progression and with and without optic disc swelling. In this chapter, the common optic neuropathies within these settings are considered.

OPTIC DISC SWELLING WITHOUT VISUAL LOSS

The most common cause of optic disc swelling without visual loss is papilledema. Papilledema is defined as optic disc swelling caused by increased intracranial pressure.1 It may be produced by an intracranial mass, by blockage of the arachnoid villi by blood or protein (e.g., after a subarachnoid hemorrhage or from a spinal cord tumor), by obstruction of flow of cerebrospinal fluid through the ventricles, and by decreased flow of venous blood through dural sinuses.

The symptoms of patients with papilledema are mostly those of increased intracranial pressure (e.g., headache, nausea, vomiting). Visual symptoms are minor and include transient obscurations of vision and double vision.

The transient obscurations that occur in patients with papilledema are binocular and simultaneous, and they are extremely brief, lasting only a few seconds. They may occur once a day or dozens of times a day; they may be unassociated with activity or they may occur primarily during a change in posture, as in changing from lying down to sitting up or standing. This is in contrast to the transient visual obscurations that can occur in patients with emboli from the heart or internal carotid arteries. Those tend to be monocular, rarely have any relationship to activity, and tend to last at least 15 seconds and often many minutes. The presence of transient visual obscurations in patients with papilledema has no prognostic significance.

Double vision in patients with papilledema is almost always caused by a unilateral or bilateral sixth nerve paresis. The paresis is almost always incomplete. In most cases, it is caused by the effects of the increased intracranial pressure on the abducens nerve and not by direct compression of the nerve by an intracranial mass lesion.

Patients with acute papilledema usually do not complain of decreased or blurred vision unless there are hemorrhages or exudates in the macula or subretinal fluid beneath it or there is an intracranial mass that involves the optic nerve, optic chiasm, or both. The visual field in an eye with papilledema is usually initially normal; however, as disc swelling worsens, the physiological blind spot enlarges and may become noticed by the patient. When increased intracranial pressure causes death of axons, field defects similar to those that occur in patients with open-angle glaucoma (i.e., nasal steps and arcuate defects) occur. With continued damage, generalized field constriction occurs. Visual acuity is preserved until damage is very severe.

The appearance of papilledema varies with its severity. Early papilledema is characterized by mild swelling and hyperemia of the optic discs (Fig. 23-1). There are often no hemorrhages, and the retinal veins are not dilated. Visual function is usually normal at this time. As papilledema worsens, the disc becomes increasingly swollen and hyperemic, the vessels on the surface of the disc become obscured by the swollen tissue, and peripapillary flame-shaped hemorrhages may appear (Fig. 23-2). Patients with this fully developed papilledema continue to have normal visual acuity and color vision; however, their blind spots are enlarged, and they may have some mild, nonspecific field defects. If intracranial pressure is not lowered, chronic papilledema develops, characterized by a rounding up of the discs, which begin to become pale (Fig. 23-3). During this time, the hemorrhages resolve. The visual acuity may be slightly decreased, but the main visual finding is significant constriction of the visual field. The final stage of papilledema—atrophic papilledema—occurs when the swelling resolves as nerve fibers die, and the optic discs become pallid (Fig. 23-4). At this point, the visual acuity is reduced, and the visual field is markedly constricted, often to only 5 degrees or less.

Papilledema is an emergency. Patients in whom this condition is suspected require an immediate assessment, including neuroimaging and, if no mass is present, a lumbar puncture. Treatment is directed at the underlying process, the increased intracranial pressure, or both.

SUDDEN VISUAL LOSS WITH AND WITHOUT OPTIC DISC SWELLING

The most common causes of optic nerve–related acute visual loss are optic neuritis, ischemic optic neuropathy (ION), and Leber’s hereditary optic neuropathy (LHON).

Acute Optic Neuritis

Most patients with optic neuritis are women between 25 and 45 years of age, although this condition can also develop in children and older patients. Optic neuritis is characterized in more than 95% of cases by the sudden onset of pain, often quite severe, behind or around the eye, followed shortly thereafter by decreased central vision and, in many cases, by central field loss.2 The loss of central vision is variable. It may be extremely mild or quite severe; indeed, in some cases, all vision is lost. Affected patients have decreased color vision that may be worse than the acuity would suggest. A relative afferent pupillary defect is always present unless the patient has experienced a previous attack of optic neuritis or has some other optic neuropathy in the opposite eye or the acute process is bilateral. The affected optic disc appears normal in about two thirds of cases; in the other one third, it is swollen (Fig. 23-5).

Most cases of optic neuritis are idiopathic or demyelinating in origin; however, rare cases are caused by such inflammatory or infectious conditions as sarcoid, syphilis, Lyme disease, and cat-scratch disease.2

The natural history of demyelinating or idiopathic optic neuritis is to improve vision to 20/20 or better without treatment. Although a 3-day course of high-dose (1g/day) methylprednisolone followed by a 2-week course of low-dose (1 mg/kg/day) prednisone may speed recovery by several weeks to a month, this treatment does not affect the ultimate visual outcome. The use of low-dose steroids without first use of high-dose steroids in a patient with acute optic neuritis increases the risk of recurrent optic neuritis in the affected eye and the risk of an attack of acute optic neuritis in the other eye.

Patients who experience an attack of acute optic neuritis have an increased risk of developing multiple sclerosis, depending in large part on whether white-matter lesions are visible on magnetic resonance images at the time of the acute attack (Fig. 23-6). The presence of even one lesion doubles a patient’s risk of developing multiple sclerosis over the subsequent 10 years.3 Fortunately, there is evidence that the use of interferon β-1a reduces the risk of developing multiple sclerosis in these patients.4

Patients who experience an attack of optic neuritis in one eye have a 10% to 20% risk of developing a similar event in the opposite eye.2 Risk factors for second-eye involvement include white-matter lesions on magnetic resonance images, a family history of multiple sclerosis, and neurological symptoms.

A variant of optic neuritis that has a very different prognosis from the demyelinating or idiopathic form is neuroretinitis.5 This condition begins as an apparently straightforward anterior optic neuritis in which vitreous cells may or may not be present; however, within 1 to 3 weeks, a macular star develops that often persists after the optic disc swelling resolves (Fig. 23-7). Neuroretinitis may be caused by cat-scratch disease, sarcoid, syphilis, tuberculosis, or Lyme disease; however, it is never caused by multiple sclerosis.

Ischemic Optic Neuropathy

The second major cause of acute visual loss with and without optic disc swelling is ION.6 This condition occurs in three main settings: (1) as a complication of systemic noninflammatory vascular diseases, such as diabetes mellitus, hypertension, and hypercholesterolemia; (2) in the perioperative period, most often after cardiac surgery or back surgery in the prone position; and (3) as a complication of vasculitis, most often temporal (giant cell) arteritis.

Nonarteritic ION usually occurs in patients older than 55; equal numbers of men and women are affected. At least one underlying systemic vasculopathy is usually present, but it may or may not have been identified at the time vision is lost.

Nonarteritic ION is usually painless. When eye pain is present, it is usually mild, and pain on eye movement is very rare. As in the case of optic neuritis, the degree of visual loss in patients with nonarteritic ION is variable, ranging from 20/20 to hand motion vision or worse. Color vision usually mirrors acuity; the worse the central vision is, the worse the color vision is. The visual field usually shows an altitudinal or arcuate field defect. A relative afferent pupillary defect is always present if the condition is unilateral and there is no optic neuropathy in the opposite eye.

Nonarteritic ION may be of the anterior or the retrobulbar variety.7 In anterior ION, which constitutes about 90% of all cases, the optic disc is usually hyperemic, and peripapillary flame-shaped hemorrhages are often present (Fig. 23-8); however, soft exudates (cotton-wool spots) are usually absent. The opposite optic disc is almost always small with little or no cup (see Fig. 23-8

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