On Seeing With Defective Vision: 2
Heteronymous are all those visual field defects that are neither monocular, nor homonymous. Heteronymous defects are attributable to bilateral disorders of the eyes and/or the optic nerves, or chiasmal lesions. The variety of heteronymous defects is immense and hinders a unified approach to simulation. Here, I will concentrate on bitemporal and binasal defects.
The former offer an easier prediction of functional effects for the simple reason that they pay better respect to the vertical visual field meridian.
When asked about their visual problems in daily life, patients with bitemporal field defects frequently answer in terms of tunnel vision or bitemporal contractions. On closer questioning, it usually becomes clear that tunnel vision or bitemporal contractions actually rarely were detected by the patients themselves but were demonstrated in formal visual field examinations.
What the patients themselves may note usually takes place in central rather than in peripheral vision and is described in terms like blurring or doubling. Blurring is often equated with impaired visual acuity but the reading problems associated with heteronymous defects have much more complex and intriguing grounds, and so has "doubling". The following sections aim to illuminate some key aspects.
Reading on the edge
At the time when suprasellar lesions become visually symptomatic, the chiasm suffers both elevation and deformation. Although the crossing nerve fiber bundles take the brunt of damage, the non-crossing fibers do not escape involvement. The scene is then set for a bilateral loss of visual acuity. The acuity losses have opposing spatial gradients in the two eyes: the right eye has greater difficulties reading from left-to-right than in the opposite direction whereas the reverse conditions apply to the left eye. This is easy enough to
understand but there is also a peculiar, dys-cognitive aspect to reading with bitemporal field defects, namely, a
tendency to stick to one margin of the test chart, or "reading on the edge".
Everyone knows what an acuity chart looks like, with several rows of letters. In the presence of bitemporal field defects this knowledge suddenly seems to evaporate when one eye is covered, so that the right eye sees something like the center panel, and the left eye something like the right panel. After 40+ years in the clinic, I am still waiting to meet a patient who comments spontaneously on the apparent change in chart lay-out when switching from one eye to the other...
Patients with homonymous field defects also tend to read on the edge but they stick to one and the same edge when switching eyes. Because homonymous hemianopia commonly is associated with cognitive defects and defects of selective attention, including neglect and extinction, homonymous reading on the edge is perhaps not very surprising. What is surprising is that patients with isolated suprasellar lesions behave in a similar fashion. The phenomenon is well known but largely unexplained. Very little has been written on this intriguing topic
The lost strip and vertical steps
Although striking to the examiner, the reading on the edge phenomenon usually escapes the attention of patients. Observant patients are more prone to note other types of reading difficulties. These are sometimes described as a drop-out or doubling of letters. The drop-out is not only quite annoying but may actually have serious implications. One of my patients, an accountant with a complete bitemporal hemianopia, described great difficulties reading numbers correctly at work. For example, the number series 1234 would sometimes look like 14, or 34, or 124, and sometimes like 122334, and there was no way for the patient to decide what the true answer was (he did not try looking with one eye only).
Descriptions of lost or doubled letters can be understood in terms of a minimally defective eye alignment. A correct alignment of left and right eye images will be difficult if function in the temporal hemifields is defective or absent. If the nasal hemifields happen to overlap slightly, the patient will see two images of one and the same target. Conversely, if the nasal hemifields happen to slide apart slightly, targets situated within the gap will not be seen. Even if the nasal hemifield edges abutt correctly sideways, alignment may fail vertically, so that the left eye image appears displaced vertically relative to the right image. This is what must have happened to one of my patients who once was standing on the beach, looking out toward the horizon, when the horizon suddenly presented a vertical step straight ahead.
The display below aims to simulate the effects of a sliding apart of nasal hemifields and an overlap, respectively. Click the left mouse button to see how a narrow vertical strip of text disappears and move the mouse to see how the strip follows the gaze.
Click the right mouse button to see doubling of a narrow vertical strip of text. Tap keyboard key F5 to reset.
Localized defects of depth perception
Different parts of the visual fields cover different parts of visual space. For example, the prism-like region beyond the fixation point is covered by the temporal hemifields. Without temporal hemifields, nothing can be seen in this region (right image; green areas represent nasal hemifield coverage). The opposite situation applies for binasal hemifield loss, where nothing can be seen in the prism-like region in between the eyes and inside the fixation point. Such localized defects of perception in depth may cause considerable problems in daily life. For example, a Rolls Royce driver with bitemporal field defects who proudly contemplates the Spirit of Ecstacy sculpture on top of the radiator, may be unable to see a stop signal further ahead. On a more mundane basis, patients with binasal or bitemporal field defects may share considerable problems with near work involving a precise discrimination of depth, e g, cutting their nails or threading needles. The display below aims to illustrate similar phenomena. Play around with the controls to see what happens.