Optic Nerve

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Decreases in visual acuity and color vision, unilateral visual field loss, and an afferent pupillary defect suggest an optic neuropathy. Commonly associated field defects include arcuate, central, altitudinal scotomas, and constriction. The funduscopic examination may reveal an optic disc that is normal, swollen, or pale, depending on the etiology and temporal profile of the optic neuropathy. A glaucomatous disc has a normal rim, normal overall color, and an enlarged cup. Optic disc swelling, which is characterized by hyperemia, nerve fiber layer edema, venous congestion, obscuration of retinal vessels, and peripapillary hemorrhages, is indicative of active processes such as raised intracranial pressure or optic nerve inflammation, ischemia, or infiltration. Retrobulbar optic neuropathies by definition have no disc swelling because the pathology occurs more proximally, away from the optic nerve head. Optic disc pallor suggests an atrophic process and is seen at least to some degree in all chronic optic neuropathies when the damage has been present for at least 4 weeks.

Although the most common optic neuropathy results from glaucoma (increased intraocular pressure), others associated with neurological disease include inflammatory, vascular, compressive, infectious, infiltrative, neoplastic, nutritional, toxic, congenital, and hereditary etiologies.

Patients with optic neuritis, which is an inflammatory optic neuropathy, present with acute monocular (usually) visual loss with pain exacerbated by eye movements. Two thirds of the cases are retrobulbar, y and visual loss results from inflammatory demyelination of the optic nerve. Most cases resolve spontaneously, y but some patients continue to notice a transient loss of vision in the previously afflicted eye (Uhthoff's symptom) during periods of elevated body temperature during exercise or showering.

Some vascular etiologies of optic nerve dysfunction include ischemic optic neuropathy (nonarteritic or arteritic, secondary to giant cell arteritis), blood loss, and hypotension. Nonarteritic anterior ischemic optic neuropathy is an idiopathic, ischemic insult of the optic nerve head. y Acute, painless visual loss (decreased acuity and an altitudinal defect, usually) and optic disc swelling characterize this disorder. The prognosis for visual recovery is poor (in contrast to optic neuritis), and patients are typically older. y Presumed risk factors include a crowded, cupless optic disc; diabetes; and hypertension. Arteritic anterior ischemic optic neuropathy is associated with more severe visual loss,y and the disc swelling tends to be pallid.

The most common compressive lesions causing optic neuropathies are carotid-ophthalmic artery aneurysms and sellar masses (craniopharyngioma, meningioma, or pituitary adenoma/apoplexy). Primary neoplastic processes affecting the optic nerve include optic nerve gliomas (often associated with neurofibromatosis type 1), and optic nerve sheath meningiomas. Infiltrative processes (sarcoidosis, carcinomatous meningitis, leukemia, and lymphoma); nutritional

deficiencies (cobalamin and thiamine deficiency); infections (syphilis); trauma; toxin exposures (ethambutol, methanol, and isoniazid); and congenital disc abnormalities (disc hypoplasia, congenital disc elevation, and optic nerve head drusen) may all cause optic neuropathies. The cause of tobacco/alcohol amblyopia, which appears classically as disc pallor and cecocentral scotomas, is unclear. The most common form of heredodegenerative optic neuropathy is the maternally inherited Leber's hereditary optic neuropathy. y Appearing in young males with severe, often sequential visual loss, this disorder is typically associated funduscopically with nonedematous disc elevation and peripapillary telangiectasias.

Pseudotumor cerebri (idiopathic intracranial hypertension) should be mentioned here because its major morbidity is visual loss related to optic nerve dysfunction. We dislike the term benign intracranial hypertension because the visual deficits can be severe and blinding. Patients should satisfy the following (modified Dandy's) y criteria: (1) signs and symptoms due to elevated intracranial pressure; (2) a normal result on neurological examination except for an abducens palsy; (3) modern neuroimaging excluding a mass lesion or other cause of elevated intracranial pressure; and (4) normal CSF parameters, except an elevated opening pressure (greater than 250 mm H2 O). Patients are usually young obese females who may complain of headache, transient visual obscurations (seconds), pulsatile intracranial noises, or double vision. Almost uniformly, patients have papilledema; other causes of disc elevation, such as pseudopapilledema (optic nerve drusen or congenital nerve head elevation, for instance), should be excluded. Typically, visual acuity and color are preserved, but optic nerve-related visual field defects, which are best detected with computerized threshold perimetry, are present in over 90 percent of patients and include enlarged blind spots, generalized constriction, and inferior nasal field loss. ^

The combination of optic disc pallor and papilledema in the fellow eye is termed Foster Kennedy's syndrome. Classically, the culprit lesion is a subfrontal mass, typically a meningioma, which compresses the ipsilateral optic nerve, causing disc atrophy. If the lesion is large enough to cause elevated intracranial pressure, papilledema results in the contralateral eye, but the ipsilateral optic nerve cannot swell because it is atrophic. Bilateral optic nerve compression is another possible mechanism.[35] Nontumor causes, resulting in the pseudo-Foster Kennedy syndrome, are actually more common. Examples include consecutive anterior ischemic optic neuropathy, characterized by new ischemic disc swelling in one eye accompanied by longstanding disc atrophy resulting from a previous event.

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