Fig. 5.1 a, b (a) Lesions of the visual pathways. (Adapted from Rohkamm R. Color Atlas of Neurology. New York, NY: Thieme; 2004:81.) (b) Types of visual field defects secondary to lesions along the visual pathways. Note that the visual pathways (a) are shown with the right eye on the left and the left eye on the right (similar to a computed tomographic or magnetic resonance imaging scan); the visual field defects (b) are by convention shown with the right eye on the right and the left eye on the left.
This chapter gives a brief review of the approach to the patient with visual loss, then provides an overview of the many causes of monocular and binocular visual loss.
5.1 Approach to the Patient with Visual Loss
The most important part of the history, when evaluating a patient with visual loss, is to establish whether the visual loss is monocular or binocular. Monocular visual loss results from lesions anterior to the chiasm (i.e., the eye itself or the optic nerve), whereas binocular visual loss results from either bilateral anterior lesions or chiasmal or retrochiasmal lesions.
Clinical examination allows for anatomical localization of the lesion and determination of the mechanism of visual loss. Further workup looks for a specific cause.
Evaluation of the patient with visual loss also involves determining whether the loss is transient or permanent, acute or progressive. Identification of any associated symptoms, such as ocular pain, red eye, proptosis, chemosis, diplopia, and headache, as well as neurologic symptoms and elevated blood pressure, is essential.
Pearls
If visual acuity can be improved by the patient looking through a pinhole, the problem is refractive or ocular, not neurologic in origin.
Reduction in the saturation or brightness of colors may be an early sign of optic nerve disease.
A relative afferent pupillary defect (RAPD) ipsilateral to visual loss indicates an optic neuropathy or severe retinal disease (in which case the retina looks abnormal on funduscopic examination). Ocular disease, such as corneal abnormalities, cataracts, and most retinal disorders, do not cause RAPDs.
If nonorganic visual loss is suspected, stereovision should be tested first: normal stereovision implies 20/20 visual acuity in both eyes.
A positive response to the optokinetic nystagmus stimulus indicates a visual acuity of at least 20/400 in the eye tested.
5.2 Monocular Visual Loss
Monocular visual loss always results from lesions anterior to the chiasm (the eye or the optic nerve). Transient or permanent monocular visual loss may result from various mechanisms. Those resulting from disorders of the eye itself are usually easily diagnosed by the ophthalmologist. Optic neuropathies and some retinal and ocular vascular disorders that cause monocular visual loss may present to the neurologist.
Any lesion involving the eye (▶ Fig. 5.2) or optic nerve may produce monocular visual loss, including the following:
Fig. 5.2 Anatomy of the eye. (From Schuenke M, Schulte E, Schumacher U, Ross LM, Lamperti ED, Voll M. THIEME Atlas of Anatomy, Head and Neuroanatomy. Stuttgart, Germany: Thieme; 2007. Illustration by Karl Wesker.)
Refractive errors
Corneal diseases
Anterior chamber inflammation (anterior uveitis)
Anterior chamber hemorrhage (hyphema)
Lens opacities (cataract), dislocation
Vitreous hemorrhage
Vitreous inflammation (posterior uveitis)
Retinal diseases
Choroidal diseases
Optic neuropathies
5.2.1 Causes of Transient Monocular Visual Loss
Numerous ocular disorders as well as transient ocular ischemia may produce episodes of transient monocular visual loss (TMVL) (see Chapter ▶ 6).
There are three main causes of TMVL: vascular disorders, ocular diseases, and optic nerve disorders:
The most common vascular disorder of the eye is sometimes referred to as amaurosis fugax, the temporary loss of vision of one eye caused by decreased blood flow (ischemia) to the retina.
Orbital (ophthalmic artery), retinal (central retinal artery and its branches), optic nerve (short posterior ciliary arteries), and choroidal (posterior ciliary arteries) ischemia can all produce TMVL.
Ocular diseases involving the anterior segment of the eye that can cause TMVL include dry eyes, keratoconus, hyphema, and angle closure glaucoma. Retinal detachment can also rarely produce transient visual loss.
Optic nerve disorders that can cause TMVL include papilledema, optic disc drusen, and congenitally anomalous optic discs, all of which can produce transient visual obscurations. Gaze-evoked transient monocular visual loss can be the result of optic nerve compression. The Uhthoff phenomenon occurs in patients with optic neuropathies, (especially of demyelinating etiology), whose vision may worsen with heat or exercise.
5.2.2 Ocular Causes of Permanent Monocular Visual Loss
In cases of permanent visual loss, the ocular examination usually allows direct visualization of the cause of visual loss. It is when the eye itself appears normal, or when the optic nerve is not normal, that a neuro-ophthalmic disorder is suspected.
Permanent visual loss may result from disorders involving the anterior segment of the eye (seen with a penlight or direct ophthalmoscope at bedside, or with a slit lamp) or from disorders involving the posterior segment of the eye (seen with a direct ophthalmoscope, with a slit lamp, or with an indirect ophthalmoscope).
▶ Fig. 5.3, ▶ Fig. 5.4, ▶ Fig. 5.5, ▶ Fig. 5.6, ▶ Fig. 5.7, and ▶ Fig. 5.8 show examples of anterior segment disorders presenting with acute or chronic visual loss.
Fig. 5.3 Infectious keratitis. Visual loss with pain and ocular redness from infectious keratitis and conjunctivitis. Illumination of the eye shows conjunctival hyperemia with discharge and hazy cornea.
Fig. 5.4 Acanthamoeba keratitis. Visual loss with severe pain and ocular redness from infectious keratitis related to acanthamoeba infection. The infection is a classic complication of contact lens wear. Illumination of the eye shows mild conjunctival hyperemia and hazy cornea.
Fig. 5.5 Corneal ulcer. Severe visual loss with pain and ocular redness secondary to infectious corneal ulcer. The cornea is opaque.
Fig. 5.6 Keratoconus. Chronic, painless visual loss. Examination of the red reflex (focus on the cornea) shows an abnormal reflection with a conical deformation of the cornea.
Fig. 5.7 Traumatic hyphema. Severe visual loss with pain after an ocular injury. There is blood in the anterior chamber (hyphema).
Fig. 5.8 Cataract (nuclear sclerosis). Progressive, painless visual loss secondary to a cataract. The lens is yellow and opaque. The eye is quiet (no redness).
Funduscopic examination is essential in patients with visual loss (see Chapter ▶ 2). Optic neuropathies and retinal disorders involving the macula are common causes of central visual loss in patients with clear ocular media. Neurologists need to be able to reliably examine the optic nerve and the macula, which can be done with a direct ophthalmoscope at the bedside (▶ Fig. 5.9). Ophthalmologists use the slit lamp and lenses of various powers to visualize the optic nerve and macula (posterior pole) and an indirect ophthalmoscope to examine the entire retina and its vasculature. The slit lamp and the indirect ophthalmoscope require more practice than the direct ophthalmoscope and are only rarely used by neurologists. These techniques allow stereo views of the fundus and therefore better appreciation of optic nerve elevation, cupping, and contour, as well as macular thickness and edema.
Fig. 5.9 Funduscopic examination with a direct ophthalmoscope in neuro-ophthalmology focuses on the optic nerve (short arrow) and the macula (long arrow).
The red reflex is the first step of the funduscopic examination (▶ Fig. 5.10; see Chapter ▶ 2) and is used to screen for opacities or irregularities in the cornea, lens, or vitreous. An abnormal red reflex suggests a problem with the ocular media or severe retinal disease involving the posterior pole (▶ Fig. 5.11). Patients with optic neuropathies have a normal red reflex. When the red reflex is abnormal and the fundus cannot be visualized (hazy or no view), then a media opacity is obstructing the view (▶ Fig. 5.12a), and B-scan ultrasonography of the eye is helpful in this setting as it may provide an explanation for the media opacity and allow examination of the retina to see if it is attached or not and if there is a retinal tear (▶ Fig. 5.12b) (see Chapter ▶ 4).
Fig. 5.10 a–c Examples of red reflexes. (a) Normal and symmetric red reflex; (b) absence of right red reflex from a right retinal detachment; (c) and right leukocoria from a retinoblastoma.
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