Pattern of Visual Loss

Central Visual Loss

A defect in the visual field surrounded by normal vision is called a scotoma, from the Greek word meaning “darkness.” Loss of central vision, resulting in a central or cecocentral scotoma, is usually quickly noticed and reported. Peripheral visual field defects, such as homonymous hemianopia, can be asymptomatic but when noticed are frequently referred to the eye with the greater extent of field loss (i.e., the eye with temporal field loss) (Fig. 14.1). Central and cecocentral scotomas are usually due to lesions of the central retina or optic nerve. When the lesion is at the junction of the optic nerve and chiasm, there will be an ipsilateral central scotoma due to optic nerve involvement, and a contralateral temporal defect due to chiasmal involvement; this highly-localizing visual field defect is known as a junctional scotoma (Fig. 14.2). Patients with junctional scotomas are often unaware of the contralateral temporal defect, emphasizing the importance of assessing each eye separately during history taking and visual field evaluation.

Fig. 14.1 Right homonymous hemianopia. The visual loss is often referred to the right eye, because the right temporal visual field is larger than the left nasal visual field. Numbers refer to the normal extent of the visual field in degrees.

Fig. 14.2 Junctional scotoma from a lesion at the junction of the optic nerve and chiasm. The lesion affects both the right optic nerve, producing a cecocentral scotoma in the right eye, and crossing fibers in the optic chiasm, producing an upper temporal visual field defect in the left eye. The temporal field defect of a junctional scotoma often goes unnoticed by the patient and may only be detected with visual field testing (see Chapter 36).

In general, scotomas caused by retinal disease are so-called positive scotomas, since they are perceived as a black or gray spot in the visual field. Patients with macular pathology can also have metamorphopsia, where there is distortion of images such that straight edges or geometrical figures appear warped. Metamorphopsia is almost always caused by retinal disease. In contrast, optic nerve lesions characteristically produce negative scotomas, areas of absent vision that are otherwise not perceivable, in conjunction with decreased color vision, contrast vision, and light brightness perception. On occasion, paradoxical photophobia, especially with fluorescent lighting, can occur with optic nerve lesions. Photopsias (light flashes) can occur with vitreoretinal traction (e.g., posterior vitreous detachment), retinal disease (e.g., cancer-associated retinopathy), toxicity from certain drugs (e.g., digitalis), or optic nerve disease (e.g., in the healing phase of optic neuritis, in which case they may be evoked by sound). Photopsias can also occur as part of migrainous visual aura. Aside from ocular diseases, bilateral central visual loss can result from lesions involving both optic nerves, the optic chiasm, or the part of the occipital cortex concerned with central vision. The possibility of nonorganic visual loss must also be considered, but it remains a diagnosis of exclusion (see Chapter 36).

Peripheral Visual Loss

For simplicity, visual field defects can be classified into one of three groups: prechiasmal, chiasmal, or retrochiasmal. Unilateral prechiasmal lesions affect the visual field of one eye only, chiasmal lesions affect the fields of both eyes in a non-homonymous bitemporal fashion, and retrochiasmal lesions cause homonymous field defects with variable degrees of congruity (i.e., similarity) depending on their location (Fig. 14.3). See Chapter 36 for further discussion of patterns of visual field loss.

Fig. 14.3 Topographical diagnosis of visual field defects.

(Reprinted with permission from Vaughn, C., Asbury, T., Tabbara, K.F., 1989. General Ophthalmology, twelfth ed. Appleton & Lange, Norwalk, CT, p. 244.)

Temporal Profile of Visual Loss

Sudden-Onset Visual Loss

Visual loss of sudden onset can be divided into three temporal patterns: transient (Box 14.1) (Thurtell and Rucker, 2009), nonprogressive, and progressive.

Box 14.1 Causes of Transient Monocular Visual Loss

Embolic cerebrovascular disease

Migraine/vasospasm

Hypoperfusion (hypotension, hyperviscosity, hypercoagulability)

Ocular (optic disc edema, intermittent angle-closure glaucoma, hyphema, impending central retinal vein occlusion)

Vasculitis (e.g., giant cell arteritis)

Other (Uhthoff phenomenon, idiopathic, nonorganic)

Transient Monocular Visual Loss

Some patients with reduced blood supply to the eye due to a high-grade stenosis or occlusion of the internal carotid artery report TMVL in bright light, which is likely due to impaired regeneration of photopigments secondary to ocular ischemia (Kaiboriboon et al., 2001). The TMVL can also occur following meals or with postural changes. A variety of ophthalmic abnormalities, including midperipheral retinal hemorrhages, can be present and collectively comprise the ocular ischemic syndrome (Chen and Miller, 2007). Other retinal diseases, such as cone dystrophies and age-related macular degeneration, can cause evanescent visual loss in bright light, also known as hemeralopia or day blindness. The visual loss in these diseases is usually bilateral, whereas it is unilateral in patients with unilateral carotid disease.