128Chapter 8: Misinterpretation of visual fields

Tip: Lesions at the occipital tips bilaterally can mimic bilateral optic neuropathy. Careful perimetry looking for a vertical mismatch should be diagnostic.

Pseudo-bitemporal defects

Optic nerve fibers originating in the nasal retina and serving the temporal visual field decussate at the optic chiasm. This anatomic fact explains the most common pattern of visual loss associated with a chiasmal lesion: a bitemporal hemianopia. There is no other place in the visual pathway where a single lesion can cause bilateral, non-homonymous visual field loss, and thus the presence of a bitemporal visual field defect that respects the vertical meridian is absolutely localizing to the optic chiasm (Figure 8.11). In principle, this concept is clear; in practice it is sometimes less straightforward.

Certain ocular conditions, when bilateral, can cause simultaneous temporal field losses. These

include papilledema (bilaterally enlarged physiologic blindspots), disorders of the outer retina termed “big blindspot syndromes”, retinal degenerations, macular disease, congenital disc anomalies and glaucoma. The bitemporal visual field defect found in these ocular disorders does not strictly respect the vertical meridian and is sometimes referred to as a pseudo-bitemporal hemianopia. Clinicians should be able to distinguish between a pseudo-bitemporal and a true bitemporal hemianopia in order to correctly localize the visual loss to the eye or to the chiasm. The following three cases highlight clues that are helpful in making this distinction.

Incidental field defect

Case: This 45-year-old librarian described a oneyear history of difficulty reading, and gradual blurring of vision in both eyes. Her past medical history was significant only for pre-eclampsia during her

last pregnancy five years ago. On examination, she was found to be presbyopic and her reading problems were alleviated with over-the-counter readers. A routine visual field test was also performed

(Octopus 101, Interzeag AG, Bern, Switzerland), and rather unexpectedly the results were not normal (Figure 8.12). The remainder of her examination was unremarkable.

130 Chapter 8: Misinterpretation of visual fields

The visual field appeared to show a superior bitemporal hemianopia, suggesting the possibility of a chiasmal lesion. The patient underwent an MRI of brain and orbits which showed no abnormalities in the sellar region.

What is the next step in this patient’s evaluation?

The issue at hand is whether or not this patient has chiasmal visual loss. Alignment of a temporal scotoma along the vertical meridian best distinguishes a chiasmal lesion from other conditions that produce similar field loss. In reviewing her automated test, we find that the individual threshold values show a clear temporal–nasal asymmetry across the vertical meridian superiorly, suggesting a true hemianopic defect; but there is also mildly decreased sensitivity that extends into the nasal field superiorly (green shading on the color scale representation). How accurately does the Octopus G1 program assess vertical alignment of a scotoma? The G1 measures the visual threshold at 59 loci within the central 26 degrees of visual field, and the loci are not evenly distributed throughout the field. At the foveal area, there are more test locations and the test points straddling the vertical meridian are very close to actual midline. With increasing eccentricity, the location of test points along the vertical meridian diverges, so that defining a true hemianopic border is less certain.

A more precise exploration of the vertical meridian is needed and there are several methods for accomplishing this. One is to select the automated program with a testing strategy specifically designed for this purpose, sometimes referred to as the “neuro program”. Another option is testing with the Goldmann perimeter, paying particular attention to points on each side of the vertical meridian. A tangent screen is also effective for this purpose, with the patient seated one meter from the screen and tested with a small red target. In this case, the patient returned for Goldmann perimetry which demonstrated that the temporal scotomas actually extended into the adjacent nasal

field, and thus represented a pseudo-bitemporal defect (Figure 8.13).

Based on the new interpretation of this patient’s visual field defect, what feature of the examination should be reconsidered?

A more critical fundus examination revealed tilted optic discs, a congenital anomaly often associated with a pseudo-bitemporal hemianopic defect (Figure 8.14). The patient was reassured as to the benign nature of her visual field abnormality.

Discussion: The tilted disc is a congenital anomaly in which the superotemporal optic disc is elevated and the inferonasal disc is posteriorly displaced. The vertical axis of the disc is oriented obliquely, with angulation of the inferonasal aspect, which appears flattened and even hypoplastic. The tilted disc syndrome may include one or more associated findings: peri-papillary atrophy (scleral crescent), inferonasal chorioretinal hypoplasia and ectasia, and situs inversus of the retinal vessels. The degree of temporal field loss is variable and may appear as a scotoma or an area of depression. The field defect typically emerges from the physiologic blindspot, involves the superior temporal quadrant and, most importantly, crosses over into the superior nasal quadrant. The defect is at least in part refractive, due to ectasia of the inferonasal peri-papillary retina, which produces more myopia in this area compared to the surrounding retina. Consequently, the field defect can be reduced or even eliminated with the use of additional minus correction when testing that portion of the field. This disc anomaly often occurs in eyes with myopic astigmatism, is non-progressive and, unlike some other congenital ocular anomalies, is not associated with any abnormalities involving other organ systems.

Diagnosis: Tilted disc syndrome

Tip: Techniques that allow accurate assessment along the vertical meridian can distinguish chiasmal visual loss from other conditions that affect the temporal field.

Abnormal fields and temporal disc pallor

Case: A 35-year-old homemaker was referred for neuro-ophthalmic consultation because of abnormal visual fields found during a routine screening test. She had no visual symptoms, though she did recall having had some difficulty reading the eye chart at her last driver’s examination but had passed nevertheless. She was generally healthy and took no medications. She was a non-smoker, consumed occasional alcohol and had a normal diet. Her family history included long-standing visual loss in a

sister and in her father. Visual acuity was correctable to 20/25 in each eye, pupillary responses were normal and she identified 13 of 15 pseudoisochromatic plates OU. The optic discs were pale temporally and fundus examination was otherwise normal (Figure 8.15B). Humphrey perimetry (Zeiss-Humphrey, San Leandro, CA, USA) showed a superior temporal defect in each eye that appeared to respect the vertical meridian (Figure 8.15A). This visual field pattern suggested a lesion involving the optic chiasm, however a high quality MRI of this area was completely normal.