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and progressively affect the photoreceptors and pigment epithelium. The disorder can be inherited in autosomal dominant, autosomal recessive or X- linked pattern. This patient had no known family history of visual loss but he was an only child whose mother died at an early age in a car accident, and so the genetic information was incomplete. Onset of symptoms in retinitis pigmentosa is highly variable, ranging from early childhood to mid-life. Early symptoms usually consist of prolonged dark adaptation and night blindness. Visual field loss typically begins in the mid-periphery, corresponding to the retinal area of maximal rod density, and progresses relentlessly with continued loss of photoreceptors. The peripheral isopters slowly contract and eventually merge into the scotoma, leaving only a tiny central island of vision at fixation. For most of the course of the disease, Goldmann perimetry is the preferred technique for monitoring visual loss. In end-stage disease, strategies designed to test the macular region using automated perimetry become more useful for assessing the remaining vision. Total blindness is uncommon, but patients with severe loss of peripheral field are functionally disabled.

A full-field electroretinogram, in conjunction with Goldmann kinetic perimetry, is both diagnostic and prognostic. There is marked and often complete loss of both rod and cone signals, usually worse for rods. The fundus typically shows arteriolar narrowing, a waxy pale color of the optic disc and characteristic bone-spicule pigment deposits in the mid-to-far peripheral retina. Early in the course of disease, these pigment deposits may be subtle or even absent. Cystoid macular edema is sometimes present. While characteristic, none of the fundus, ERG or visual field findings is pathognomonic for retinitis pigmentosa. Acquired causes of panretinal dysfunction and degeneration should be ruled out, particularly paraneoplastic retinopathy (CAR syndrome), syphilis, other forms of uveitis and drug toxicity. Systemic diseases that may be associated with retinopathy should be considered in individual cases depending on the clinical findings.

Standardized automated perimetry (including the Humphrey 30–2 or 24–2, Humphrey Fastpac,

Humphrey SITA and Octopus G1) is a differential light threshold test designed to examine the central 24–30 degrees of the visual field. Limiting the test to the central field in this manner saves time and is generally effective because most neurologic visual loss involves this area of the field. While this test strategy is usually accurate in detecting disease involving the afferent visual pathways, there are cases in which important information is omitted, as in the case under discussion. This patient’s automated visual fields were mistakenly interpreted as showing severe generalized constriction, whereas, in reality, the far periphery had not been measured and no information about his visual field outside the central 24 degrees can be inferred. In this case, the presence of intact peripheral field, not picked up by the automated test, created the impression of inconsistency between the patient’s visual behavior and the results of visual function tests, thus leading to a mistaken diagnosis of non-organic visual loss.

Diagnosis: Retinitis pigmentosa

Tip: Commonly used automated visual field strategies do not test the peripheral field. Awareness of this limitation is important for the proper interpretation of the results in selected cases.

Constricted fields after herniation

Case: A 50-year-old clerk experienced increasing headaches for one month, culminating in a particularly severe episode during which she became unresponsive. She was taken to a local emergency room where she was noted to have a dilated right pupil and extensor posturing. Hydrocephalus was found and a ventriculoperitoneal shunt was placed. After regaining consciousness post-operatively, her general neurologic examination returned to normal but she complained of difficulty seeing. Ophthalmic examination initially suggested severe visual loss because she was unable to read even the big “E” on the Snellen chart. On closer investigation it was discovered that, despite her inability to see large letters, she could read the letters on the 20/25 line

as well as the numbers on her mobile telephone. Pupillary responses and fundus examination were normal. Confrontation testing showed severe visual field constriction and Goldmann perimetry similarly showed only three degrees of remaining central field in each eye (Figure 8.4). Based on her visual field defect and the apparent inconsistency in results of vision testing, non-organic visual loss was suspected.

What bedside test can help distinguish non-organic field loss from true constriction of the visual field?

Testing the visual field at different distances from the patient is usually diagnostic. Most patients with non-organic field loss exhibit a similar degree of constriction at all distances, referred to as a “tunnel” or “gun-barrel” field. In contrast, the field in

patients with organic constriction shows expansion with increasing distances. This distinctive feature can be demonstrated using simple confrontation techniques at one meter and six meters. A tangent screen, if available, provides comparable information.

The most common causes of severe organic visual field constriction are end-stage glaucoma, severe retinitis pigmentosa and marked post-papilledema optic atrophy, and in these disorders corroborative clinical and fundus findings usually reveal the correct diagnosis. In contrast, cortical visual loss, a less common cause of severe field constriction, may not be accompanied by other ophthalmic or neurologic abnormalities and is therefore more easily mistaken for non-organic visual loss. This patient’s confrontational visual field testing at two distances showed physiologic expansion of the field with increasing distance, consistent with organic

122 Chapter 8: Misinterpretation of visual fields

Figure 8.5 Axial non-contrast CT of the above patient with severe visual field constriction. There is a ventriculoperitoneal shunt in the anterior horn of the right lateral ventricle. The area of low density in the distribution of the posterior cerebral arteries is consistent with bilateral occipital stroke.

disease. A subsequent CT scan showed bilateral occipital infarction with sparing of the occipital tip (Figure 8.5).

Discussion: This patient suffered bilateral occipital lobe infarction due to compression of the posterior cerebral arteries as a consequence of uncal herniation. Due to the dual blood supply to the occipital tip, there was macular sparing, which is represented in the very small area (less than five degree) of preserved central vision. This visual field pattern strongly resembles what is often seen in nonorganic field constriction, including the tendency for all the Goldmann isopters to line up together, an unusual characteristic for most neurologic visual loss.

In most cases, a comparison of the visual field at different distances from the patient will successfully distinguish organic from non-organic constriction. Patients with genuine loss show expansion of the field at increasing distance whereas those with non-organic constriction do not. One note of caution should be observed when using this technique, however. In some cases of genuine and profound visual field loss, the constriction is so severe that it is difficult to appreciate this expansion. An additional technique that may be helpful for determining the nature of severe field constriction is to test the peripheral field without the patient’s awareness of the purpose of the test. For example, one can perform the standard finger–nose test that is used to assess cerebellar function, presenting the examiner’s finger in various parts of the peripheral field, asking the patient to touch the target each time. Most patients with non-organic constriction will readily find and accurately point to the target finger in areas that just a few minutes earlier were apparently blind, whereas those with organic visual loss will either fail to respond at all or will use some form of searching strategy to locate the target.

Diagnosis: Bilateral occipital stroke with macular sparing

Tip: A comparison of the visual field at varying distances from the patient usually distinguishes organic from non-organic constriction.

Sudden difficulty reading the paper

Case: This 55-year-old businessman awoke with a mild headache and at breakfast that morning noted difficulty reading the newspaper. Specifically, he reported a temporal blur in his right eye that blocked parts of words and caused him to lose his place on the line. He was taking no medications and had no known health problems. On examination, visual acuity was 20/20 at distance and at near. Results of pupillary, ocular motility, slit-lamp and funduscopic examinations were unremarkable. An Octopus G1 visual field examination was performed

and interpreted as showing a single focus of depression in the nasal hemifield of his left eye (Figure 8.6). It was hard to reconcile this patient’s visual symptoms with his relatively normal examination findings and he was discharged without a specific diagnosis. He returned one week later reporting persistent difficulty reading.

What simple “bedside” test could be performed to further investigate this patient’s symptom?

The Amsler grid is a useful test for investigating abnormalities of the central visual field. In this patient, an Amsler grid test revealed a scotoma just to the right of fixation in each eye. A central 10 degree visual field confirmed a right homonymous hemianopic central scotoma (Figure 8.7A and B).

A non-contrasted MR scan was obtained, which showed a small area of hyperintensity at the left occipital pole consistent with acute hemorrhagic infarction. Further investigation for the origin of his presumed embolic stroke revealed a patent foramen ovale which was subsequently repaired. The patient reported progressive improvement of vision over the

next two months and his visual field one year later was nearly normal (Figure 8.8).

Discussion: A unilateral lesion at the tip of the occipital lobe causes a congruous, central, homonymous hemianopic scotoma. Because the lesion is confined to one hemisphere, visual acuity is always preserved. Because the area of damage manifests clinically as a small hemianopic scotoma near fixation, it is often undetectable with confrontational visual field testing and may also be overlooked on standard perimetry. Because the lesion is postgeniculate, the pupils and fundus appearance are normal. In the case of a stroke, visual loss is sudden, painless and typically unassociated with other focal neurologic deficits. Thus, despite the patient’s report of an acute visual disturbance, the examination may be surprisingly normal. Further compounding these difficulties in diagnosis, some of these occipital lesions are so small that they go undetected on neuro-imaging studies.

The history in cases of a unilateral occipital tip lesion can be very helpful in pointing to the correct localization. Because the hemianopic scotoma

is contained within the central 10 degrees, the visual disturbance is most noticeable when fine visual discrimination is required, such as reading. Some patients recognize that their visual disturbance is related to seeing only half of words. Other patients simply describe their vision as “blurred” when read-

ing and yet others, like this patient above, interpret their homonymous visual loss as being monocular (in the eye with the temporal scotoma). The visual defect is less bothersome or even inapparent during tasks such as driving or watching television because, when viewing large objects at distance, the