Visual Disturbances

Optical Disturbances

People with myopia have poorer visual acuity at increasingly greater distances. Myopia often becomes troublesome during adolescence when it causes difficulty with seeing blackboards, watching movies, and driving. Because reading and other close-up activities that require “near vision” remain unimpaired, common parlance labels people with myopia as “nearsighted.”

The usual causes of myopia are optical rather than neurologic, such as a lens that is too “thick” or a globe that is too “long” ( Fig. 12.2 ). Occasionally, medicines cause myopia. For example, topiramate (Topamax), a widely prescribed antimigraine and antiepileptic drug (AED), may produce acutely occurring but transient myopia. (Topiramate can also lead to angle-closure glaucoma [see later].)

In contrast to myopia, people with hyperopia or hypermetropia have poorer visual acuity at increasingly shorter distances. The lay public commonly labels them “farsighted” because they can see distant objects, such as street signs, more clearly than closely held ones, such as newspapers. In hyperopia, the lens is usually too “thin,” rendering its refractive strength insufficient. Occasionally, the globe is too “short.”

In presbyopia , older individuals cannot focus on closely held objects because their relatively inelastic and dehydrated lenses are unable to change shape. With their impaired near vision, people with presbyopia, as well as those with hyperopia, tend to hold newspapers or sewing at arms' length. Reading glasses usually can compensate for the refractory error by bringing the focal point into the proper working distance.

Disruption of the accommodation reflex is another common cause of visual disturbance. Normally, when a person looks at a nearby object, this reflex causes the ciliary body muscles to contract, thereby thickening the lens so that the image falls on the retina. This response also constricts the pupils and converges the eyes. In other words, the accommodation reflex focuses on the image of closely held objects on the retina.

Because the parasympathetic nervous system mediates the accommodation reflex, many medications with anticholinergic side effects impair visual acuity for closely held objects by disrupting this reflex ( Fig. 12.3 ). These medicines include selective serotonin reuptake inhibitors (SSRIs), selective norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants, and clozapine. For example, duloxetine (Cymbalta) causes blurred vision in approximately 3% of patients; sertraline (Zoloft) and paroxetine (Paxil) in 4%; and venlafaxine (Effexor) at a dose of 75 mg in 9%. This side effect may be unsuspected because these medicines can impair accommodation without producing other anticholinergic effects, such as dry mouth, constipation, and urinary hesitancy.

Abnormalities of the Lens, Retina, and Optic Nerve

Cataracts (loss of lens transparency) may result from advanced age (senile cataract), trauma, diabetes, myotonic dystrophy (see Chapter 6 ), and chronic use of certain medicines, such as steroids. In prolonged high doses, phenothiazines and some second-generation neuroleptics may produce minute lens opacities, but these rarely are dense enough to impair vision.

Pigmentary changes in the retina can be a manifestation of injury, degenerative diseases, diabetes, infection, or the use of high doses of the phenothiazine drug thioridazine ( Fig. 12.4 ). Among infants and children, nonaccidental head injury (child abuse), particularly violent head shaking or direct trauma, creates retinal hemorrhages. Other stigmata of repeated trauma—spiral fractures of the long bones, multiple skull fractures, and burns (see Chapter 22 )—frequently accompany these retinal hemorrhages.

In 25% or more of Americans older than 65 years, the cells of the retina's pigment epithelium, mostly in the macula, degenerate through a variety of mechanisms, including a proliferation of the underlying blood vessels. When degeneration involves cells in the macula, a condition known as macular degeneration disrupts central vision, which is critical for tasks such as reading. Patients use their remaining peripheral vision to negotiate around their living areas, but progressive deterioration ultimately may deprive them of all their eyesight. As with individuals who develop blindness from any cause, those beset by macular degeneration are at risk of losing their independence, appearing to have cognitive dysfunction, and experiencing visual hallucinations (especially if they also have hearing or cognitive impairments [see Box 12.1 ]).

Patients with acquired immune deficiency syndrome (AIDS) patients are at risk for retinal infection by opportunistic organisms such as cytomegalovirus. Current antiretroviral regimens have greatly reduced visual complications. In a more benign situation, several medicines lead to visual discoloration. For example, digoxin at toxic concentrations casts a yellow hue ( xanthopsia [Greek, xanthos , yellow; opsis , sight]), and sildenafil (Viagra), a blue or yellow hue. On the other hand, a few drugs, such as the AED vigabatrin (Sabril), sometimes cause permanent retinopathy.

Optic Nerve

Damage to the optic nerves, which are projections of the central nervous system (CNS), results in vision loss that may be limited to a scotoma (an area of blindness [see Fig. 15.2 ]) but may also encompass the entire visual field. In addition, because the optic nerves serve as the afferent limb of the pupillary light reflex, unilateral (or asymmetric) optic nerve injury also causes an afferent pupillary defect : when the examiner shines a light into an eye with optic nerve dysfunction, both pupils fail to constrict; however, when the same light shines into the unaffected eye, both pupils constrict normally (see Fig. 4.2 ). With time, optic nerve injuries usually lead to atrophy of the optic disk (the most anterior segment of the optic nerve), visible as disk pallor on fundoscopic examination.

Optic nerve dysfunction may occur in isolation or in conjunction with disorders of the cerebrum or other parts of the CNS. One of the most common is optic neuritis , inflammation of one or both optic nerves, which causes sudden visual loss (often affecting central vision), pain with eye movements, and an afferent pupillary defect ( Fig. 12.5 ). In addition, color vision becomes “desaturated.” For example, patients cannot appreciate the difference between fire engine red and brick red. In severe cases, they cannot distinguish red from green.

When optic neuritis affects the optic disk, neurologists can see inflammation of the disk (papillitis) on fundoscopic examination. If the inflammation affects only the segment of the optic nerve posterior to the disk (retrobulbar neuritis) , the optic disk will appear normal on fundoscopic examination. With recurrent attacks of optic neuritis, the optic nerve becomes atrophic, the disk white, the pupil unreactive, and the eye blind.

Of the many conditions that cause optic neuritis, demyelinating illnesses, particularly multiple sclerosis (MS) and its close relative neuromyelitis optica (NMO) , are the most common (see Chapter 15 ). Importantly, optic neuritis frequently precedes other manifestations of MS or NMO. If an otherwise asymptomatic patient develops optic neuritis and the magnetic resonance imaging (MRI) shows two or more hyperintense lesions in the brain, that patient has a greater than 70% risk of developing MS. On the other hand, an otherwise asymptomatic optic neuritis patient who has no MRI lesions in the brain has only a 25% risk of developing MS.

Treatment of optic neuritis often includes a course of high-dose intravenous steroids; this may shorten an attack but probably does not alter the long-term outcome. Neurologists cautiously use steroids because of their potential side effects, which include euphoria, agitation, and, in the extreme, psychosis.

Toxins, including some medications, can also damage the optic nerves. For example, alcoholics may inadvertently drink methanol (CH ₃ OH), a solvent component of antifreeze and cooking fuels, such as Sterno, and an illicit adulterant of ethanol (C ₂ H ₅ OH). Drinking methanol causes a combination of gastroenteritis, delirium, and visual problems, particularly blurry vision and a scotoma. With severe methanol intoxication or merely its chronic intermittent consumption, optic nerves atrophy, and victims become blind. Accidental or deliberate drinking of ethylene glycol ([Ch ₂ OH) ₂ ], another component of antifreeze, will result in brain damage, acidosis, and a distinctive calcium oxalate renal sediment, as well as blindness in survivors.

Temporal arteritis or giant cell arteritis , a form of vasculitis (an inflammatory condition of the blood vessels), typically begins in the temporal arteries but often involves the arteries that supply the optic nerve, brain, or both (see Chapter 9 ). When it involves the arteries that supply the optic nerve, this disorder may cause blindness. Characteristically affecting only people older than 60 years, temporal arteritis usually begins with headache, malaise, and joint pains lasting weeks to months. These initial nonspecific symptoms understandably may lend the appearance of depression or a somatoform disorder. However, neurologists should avoid missing this diagnosis because, if untreated, it can result in blindness and stroke. Finding giant cells and other signs of inflammation in a temporal artery biopsy will confirm the diagnosis. High-dose steroids usually reverse the illness and prevent blindness and strokes.

Leber hereditary optic atrophy , an illness attributable to a mitochondrial DNA mutation, also involves the optic nerves but no other part of the CNS or the musculature (see Chapter 6 ). Most commonly affecting young males, it causes painless progressive visual loss culminating in blindness in one and then, within months, the other eye.

Several conditions affect both the cerebrum and the optic nerves. These conditions produce cognitive decline and personality changes, as well as blindness. MS would be one example. Another is Tay-Sachs disease , a lysosomal storage disease due to a deficiency in hexosaminidase A, which is almost always fatal by age 5 years.

Another classic example of simultaneous disruption of both the optic nerve and cerebrum is an olfactory groove or sphenoid wing meningioma . This tumor may compress the optic nerve (see Chapter 19, Chapter 20 ) and the overlying frontal or temporal lobe. The cerebral damage can trigger complex partial seizures and cause cognitive decline and personality changes. At the same time, optic nerve damage causes optic atrophy and blindness in one eye.

Similarly, tumors of the pituitary region, such as adenomas or craniopharyngiomas , may also produce visual loss accompanied by psychologic changes. Unless detected and removed early, these tumors grow slowly upward to compress the optic chiasm and hypothalamus and downward to infiltrate the pituitary gland (see Fig. 19.5 ). Compression of the optic chiasm causes bitemporal hemianopia. Compression of the hypothalamus and pituitary gland causes headache and panhypopituitarism that result in decreased libido, diabetes insipidus, and loss of secondary sexual characteristics.

Open-Angle Glaucoma

Open-angle or wide-angle glaucoma occurs seven times more frequently than closed-angle glaucoma. People at greatest risk are those older than 65 years, those with diabetes or myopia, and relatives of glaucoma patients. Because symptoms are usually absent at the onset, this variety of glaucoma might be diagnosed only when an ophthalmologist detects elevated intraocular pressure, certain visual field losses, or changes in the optic nerve. Later, when central vision or acuity is impaired, the optic cup is abnormally deep and permanently damaged. Lack of symptoms in the initial phase of open-angle glaucoma is one of the most compelling reasons for regular ophthalmologic examinations. Because glaucoma poses such a threat to sight, individuals older than 40 years should have intraocular pressure measured every 2 years and those older than 65 years, every year.

Open-angle glaucoma usually responds to topical medications (eye drops) or laser treatment. Psychotropic medications do not precipitate open-angle glaucoma. In general, patients with open-angle glaucoma may take antidepressants and other psychotropic medications, provided that they continue their glaucoma treatment.

Angle-Closure Glaucoma

In angle-closure glaucoma , which is also called closed-angle or narrow-angle glaucoma , intraocular pressure is usually elevated due to impaired outflow of aqueous humor at the trabecular meshwork, trapping the fluid behind the iris (see Fig. 12.6A ). Patients with narrow-angle glaucoma are usually older than 40 years and often have a family history of the disorder, but they also frequently have a history of hyperopia and longstanding narrow angles. Few have the classic symptoms, such as seeing halos around lights, preceding an attack of angle-closure glaucoma. In contrast to the relatively normal appearance of the eye in open-angle glaucoma, in acute angle-closure glaucoma, the eye is red, the pupil dilated and unreactive, and the cornea hazy. Moreover, the eye and forehead are painful, and vision is impaired.

Angle-closure glaucoma is sometimes iatrogenic. For example, when pupils are dilated for ocular examinations, the “bunched-up” iris can block the angle (see Fig. 12.6B ). Likewise, medicines with anticholinergic properties can precipitate angle-closure glaucoma, probably because they dilate the pupil.

The actual risk of glaucoma with tricyclic antidepressant use is low, and with SSRIs, it is almost nonexistent. However, as neurologists and other neurologists prescribe tricyclics for numerous conditions—including chronic pain, headache, and diabetic neuropathy—many patients become vulnerable. In addition, other medications for neurologic diseases, particularly topiramate, can cause angle-closure glaucoma.

Whatever the cause of angle-closure glaucoma, prompt treatment can preserve vision. Topical and systemic medications open the angle (by constricting the pupil) and reduce aqueous humor production. Laser iridotomy immediately and painlessly creates a passage directly through the iris that drains aqueous humor.

Most patients who are under treatment for either form of glaucoma may safely receive psychotropic medications. Glaucoma medications such as pilocarpine (a cholinergic medicine that constricts the pupils) and ophthalmic beta-blockers such as timolol (Timoptic) may seep into the systemic circulation and create psychologic and cardiovascular side effects, including orthostatic lightheadedness, bradycardia, and even heart block. Not surprisingly, elderly patients who use beta-blocker eye drops sometimes experience brief periods of confusion. Children are also susceptible to systemic absorption of eye drops and may become agitated after receiving scopolamine or other atropine-like eye drops for ocular examination.

Anton Syndrome

The dramatic neuropsychologic phenomenon of Anton syndrome —in which patients with cortical blindness explicitly or implicitly deny that they have lost all vision—characteristically complicates the sudden onset of blindness. Some patients demonstrate anosognosia (see Chapter 8 ); some simply refuse to admit that they have lost vision, while others blame external factors, like dim light, for their problem. Some may, if pressed, acknowledge visual loss but confabulate by “describing” their room, clothing, and various other objects. Blind patients with Anton syndrome, behaving as though they still have normal vision, stumble about their room.

For example, a 76-year-old man sustained a right-sided posterior cerebral artery stroke that was superimposed on a prior left-sided posterior cerebral artery stroke. He first blamed his inability to see the examiner's blouse on poor lighting and having misplaced his glasses. He then claimed to be uninterested in the exercise. When urged, still implicitly denying his blindness, he calmly described the blouse as “lovely” and “becoming,” at one time elaborating that it was “obviously finely sewn and made from expensive material.”