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Figure 8.8 Follow-up macular visual field testing (central 10 degrees) one year later shows near-complete resolution of this patient’s previous homonymous defect.
missing information in the scotoma is compensated by information obtained from the rest of the visual field.
Due to the very small size and central location of the homonymous scotoma, it can be overlooked on standard automated threshold tests, dismissed as a non-specific parafoveal depression or misinterpreted as a test artifact from unstable fixation. Amsler grid testing is particularly useful in detecting such subtle visual field defects, provided that visual acuity is around 20/40 or better. The most commonly used Amsler grid consists of a series of parallel black lines on a white background, forming a grid of 400 squares covering a 10 cm × 10 cm area. When the patient views the fixation dot in the center of the grid with one eye at a distance of 30 cm, each square represents 1 degree of visual angle and the test assesses the central 10 degrees of the visual field. The patient is asked to fixate on the center of the grid and mark with a pencil any regions where the lines are blurred, missing, or distorted. In this way, the patient can map out his/her own paracentral scotoma. As with other psychophysical tests, the reliability of the Amsler grid test depends on patient comprehension and
cooperation. Other effective methods for assessing the central field are Goldmann perimetry (using the smallest visible stimulus), central automated fields like the Humphrey 20–10 or Octopus M2 and the tangent screen.
Diagnosis: Small homonymous scotoma due to occipital stroke
Tip: A small homonymous hemianopic scotoma can often be suspected from the history. The Amsler grid is a quick and effective method for assessing the central visual field in such cases.
Post-cardiac bypass visual loss
Case: A 63-year-old machinist experienced bilateral visual loss upon awakening after coronary artery bypass surgery. Visual acuity was 20/200 OU with markedly decreased color vision. By confrontation he appeared to have bilateral central scotomas, stating that he could see the examiner’s hair, chin and ears but not eyes, nose and mouth. Ischemic optic neuropathy was suspected but the pupillary responses and optic disc appearance were normal.
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Figure 8.9 Goldmann perimetry in the above patient with bilateral visual loss following cardiac surgery. (A) His initial visual field was interpreted as showing bilateral central scotomas. (B) On closer inspection, these defects are actually bilateral homonymous hemianopic scotomas with a small mismatch along the vertical meridian.
Is there another possible explanation for this patient’s visual loss, and how would you investigate this alternative mechanism?
Infarction of the retrobulbar segment of the optic nerves, called posterior ischemic optic neuropathy, would be consistent with a normal fundus appearance acutely. However, the presence of brisk pupil reflexes is not. The combination of marked bilateral central visual loss and normal pupillary responses suggests cortical visual loss, specifically due to a lesion involving the occipital tips. Initial examination of the field with Goldmann perimetry suggested bilateral central scotomas with an odd vertically ovoid shape (Figure 8.9A). Based on this appearance and the other clinical features, the central portion of the field was tested again, this time including careful exploration of the vertical meridian to either side of fixation. With this technique it was found that what appeared to be bilateral central scotomas were indeed matched, bilateral, homonymous hemianopic scotomas in the central field with a small vertical step between the two sides (Figure 8.9B). As expected based on the clinical findings, a CT scan revealed bilateral infarcts at the occipital tips (Figure 8.10). Based on the clinical context and radiographic appearance, the mechanism was presumed to be embolic.
Discussion: The topographic representation of information carried in the afferent visual pathways is displayed within the occipital cortex in a very precise arrangement. The central (macular) visual field is transmitted to the posterior aspect of the occipital lobes, also termed the “occipital tip”. Information from the peripheral field is displayed more anteriorly, adjacent to the genu of the corpus callosum. The occipital lobes are separated by the interhemispheric fissure, and each receives its own blood supply. While damage to the occipital tips can produce bilateral homonymous defects, it is unlikely that the right and left defects would be perfectly symmetric. The resultant scotomas, therefore, show a mismatch along the vertical meridian, as in the above case. Because the posterior cerebral
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Figure 8.10 Axial post-contrast CT of the head shows areas of low density at the occipital tip bilaterally with adjacent gyriform enhancement on the right side.
arteries that supply the occipital lobes arise from a common trunk, the basilar artery, it is not uncommon for emboli traveling in this arterial system to arrive in both occipital lobes.
Although a unilateral post-geniculate lesion never affects visual acuity (still normal in the intact hemifield), a bilateral lesion involving macular fibers does produce loss of acuity of varying degree that is always symmetric in the two eyes. The resultant bilateral central visual loss often suggests bilateral optic neuropathy or maculopathy as the cause, and the finding of bilateral central scotomas furthers this impression. In such cases, careful inspection of the contours of the central scotomas, with particular attention to each side of the vertical meridian, should provide the correct localization. Goldmann perimetry is particularly well suited for such exploration but an automated field that concentrates on the central field, such as the Humphrey 10–2 program, should also provide comparable information.