Clinical Reasoning: An 80-Year-Old Woman With a Homonymous Hemianopsia

Section 1

An 80-year-old woman with hypertension, hyperlipidemia, and type 2 diabetes mellitus reported 2 years of progressive difficulty reading and loss of depth perception. More recently, she stopped driving and was running into objects while walking. Initial evaluation by a local ophthalmologist identified bilateral cataracts resulting in surgical extraction, which did not improve the visual symptoms. Automated visual field (AVF) testing demonstrated a left homonymous hemianopsia (HH). Routine laboratory evaluation showed normal blood counts and chemistries. The patient was referred to a neurologist, and a normal neurologic examination was noted. A brain MRI with contrast showed normal age-related changes, but specifically, there was no evidence of a prior stroke, restricted diffusion on the diffusion-weighted images, tumor, abnormal posterior cortical asymmetry, or enhancement. The patient was referred to neuro-ophthalmology for further evaluation.

On our evaluation, visual acuity was 20/30 in each eye. She identified only 1 of 13 plates in each eye from the Ishihara pseudochromatic book. Confrontation testing was inconsistent to finger counting, but there was a nasal defect in the right eye and a temporal defect in the left eye. Pupils were equal with normal light reflexes and no relative afferent pupillary defect. Slitlamp examination showed centered posterior chamber intraocular lenses with normal intraocular pressure. The optic disc, macula, and retina appeared normal in each eye. Repeat AVF testing confirmed the left HH (Figure 1A).

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Figure 1 Interpretation of Humphrey Visual Field Test Printout

(A) 24-2 Humphrey analyzer (Carl Zeiss Meditec AG, Dublin, CA). There is a left homonymous hemianopsia. Annotations highlight the various metrics needed to interpret the results. (B) Approach to automated perimetry interpretation.

Section 2

The pattern of a visual field defect can often determine the anatomic location of the lesion (Figure 2). In general, the following characteristics of a visual field defect should be noted: monocular or binocular, homonymous or heteronymous, complete or incomplete, and incongruous or congruous. Historically, a congruent HH was considered to originate from a lesion involving the occipital lobe, but this paradigm has been shown not to be a reliable localizing sign.¹ Therefore, in our patient, the presence of a left HH localizes the site of the lesion to anywhere along the right retrochiasmal visual pathway (i.e., right optic tract, optic radiations, or occipital lobe).

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Figure 2 Visual Pathway Anatomy With Localization of Visual Field Defects

Illustration of the anterior and posterior visual pathway with various locations of a left-sided lesion resulting in corresponding visual field defects. With permission from the Mayo Foundation for Medical Education and Research.

AVF is designed to detect visual field defects by projecting lights of different brightness, size, and location while the patient fixates at the center of a hemispherical bowl. Typically, the central 24–30° of vision is tested with a target size of III (4 mm²) stimulus. These stimuli are algorithmically planned to decrease patient anticipation and increase reliability. For the test to be valid, the patient must be able to reliably participate and complete the test. Patient factors such as familiarity with the test (i.e., learning curve), age, cognition, and physical abilities affect reliability.^2,3 Fixation losses, false-positive responses (a positive response but there is no target displayed), and false-negative responses (failure to signal at a target that was previously seen in the same location) are metrics measured and displayed on the printout that determine the reliability of the test. Fixation losses should be less than 20% and false-negative errors and false-positive errors less than 20%–30% for the test to be reliable (Figure 1B).

Any visual field defect should be interpreted within the context of other clinical signs and symptoms. The Ishihara pseudochromatic color plates are often used clinically to screen for dyschromatopsia by displaying a symbol (number or geometric figure) composed of colored dots against a background of differently colored dots; however, they can also be used as a screening tool for simultanagnosia. Patients with simultanagnosia are unable to perceive more than a single image at a time and often have trouble depicting the numbers from the background without actual impairment in color vision because of the inherent nature of the test.⁴ Further testing was notable for alexia and inability to name single objects in overlapping figures. Our patient's difficulty with the Ishihara color plates was likely because of simultanagnosia because she did not have a history of congenital color blindness and there was no evidence of optic neuropathy or retinopathy on examination. She scored 27 of 38 on the Kokmen Short Test of Mental Status, by obtaining 8 of 8 points in Orientation, 6 of 7 in Attention, 4 of 4 in Registration, 0 of 4 in Calculation, 2 of 3 in Similarities, 1 of 4 in Construction, 4 of 4 in Knowledge, and 2 of 4 in Recall.

An HH without a structural lesion on brain MRI has a short but important differential diagnosis that includes the Heidenhain variant of Creutzfeldt-Jacob disease (CJD), posterior cortical atrophy (PCA) syndrome, nonketotic hyperglycemia, seizures, subtle occipital ischemia, or a functional visual disorder.^5,6 In our patient, the duration of symptoms and lack of diffusion restriction on the brain MRI made CJD unlikely. Normal laboratory evaluation and the progressive nature of the symptoms made hyperglycemia or a vascular etiology unlikely. The history and examination findings were not consistent with seizures or a functional visual disorder.

Section 3

[¹⁸F]-fluorodeoxyglucose (FDG) PET-CT showed asymmetric moderate hypometabolism involving the right posterior cerebral hemisphere in a pattern consistent with PCA (eFigure 1, links.lww.com/WNL/C290). Studies have shown that FDG PET-CT, which measures functionality, is more sensitive and specific when evaluating patients with mild cognitive impairment compared with MRI, which is a structural measurement.^7,8 She was referred to neurology for management but declined further neuropsychiatric testing or initiation of treatment.