Case report

A 29 year old man with no history of neurological disease was attacked and shot in the back of his head. When regaining consciousness in the hospital, he complained of a complete loss of vision. Initially he also had anosmia, reduced vigilance, and a mild left sided hemiparesis. Reduction of vigilance and hemiparesis recovered completely within days. After 1 week, vision had recovered in the right upper quadrant, and central vision was sufficient to perform a neuropsychological examination which disclosed a slowing of reaction times as well as an impairment of short term memory and visuospatial functions. Verbal intelligence was normal.

Six months after the incident, we examined the patient for the first time in our department. General and cardiovascular examination yielded normal results. Higher cortical functions and cranial nerves were normal except for a bilateral lower quadrantanopia and hyposmia. Best corrected visual acuity was 100%. Muscle tone, muscle strength, and reflexes were normal and symmetric. There was hypaesthesia in a region at the back of the head where the bullet had entered, otherwise sensation was normal. In a detailed neuropsychological examination, his performance was normal, except in visuospatial tasks and in tests involving a considerable amount of visual scanning (see below). Subjectively, the patient had difficulties in estimating direction and speed of moving objects. This caused problems riding a bicycle, pouring liquids into a glass, and walking through crowds of people.

Axial CT images (2 mm slice thickness) and coronal reconstructions showed that the bullet had entered the left occipital lobe (about 5 cm cranial and 3 cm lateral to the inion) and followed a trajectory through the interhemispheric fissure into the right temporal lobe, where the bullet is still located. A hypodense area involving the upper part of the cuneus and the caudal part of the precuneus bilaterally could be identified. The lesion extended bilaterally to the ventral part of the upper calcarine bank, but spared striate cortex in the lower calcarine bank and the occipital poles. In the right hemisphere the hypodense area extended into the posterior part of the medial temporal gyrus.

Results

After initial blindness, the patient reported a diffuse perception of light 5 days after the incident. Visual field size increased rapidly, starting in the upper right quadrant (fig 1). In our first examination, 6 months after the lesion, we found an incomplete bilateral lower quadrantanopia (fig 1). He showed a mild impairment of form perception, visual orientation, recognition of object size, and spatial relations. The patient showed no deficits in memory, speech, or intelligence, nor was he slowed in a simple reaction task. Moreover, we found no sign of visuospatial neglect, but selective attention in a Stroop paradigm was impaired. During the first 4 weeks, he had complex pseudohallucinations, including people and scenarios, located in the lower visual quadrants.10

Twenty eight weeks after lesion, baseline testing with computer based perimetry7 was begun. Unexpectedly, visual field size still increased over the next months so that restitution training was not started. During that period, we found a gain of more than 20% in perimetric tests of the lower visual field until measurements stabilised 16 months after the gunshot. This improvement was found almost exclusively in partially defective areas (transition zones) at the visual field border (fig 1). In parallel, colour perception and form recognition recovered in the transition zone, and he subjectively perceived an improvement of visual orientation and spatial perception.

After visual field size had remained constant for more than 7 months (week 51 to 94), visual restitution training was started.

The patient showed a remarkable but variable further increase of visual field size over the next 80 weeks of training (fig 1). Again, this improvement was found almost exclusively in the transition zones of the scotoma with peripheral parts of the visual field defect recovering more and faster than central parts. Interestingly, we found a striking discrepancy between the position of the visual field border in our standard perimetric measurements versus the training related perimetric results. Obviously, our patient was able to detect light stimuli during the training program when they were presented for 2000 ms, but he could not perceive a light stimulus at the same position when it appeared only for 150 ms in the perimetric test.

We quantified recovery over time using two measures (fig 2): (1) increase of per cent intact lower visual field and (2) number of detected stimuli. Mean size of the lower intact visual field in the period of spontaneous recovery (week 1 to 51) was 27.8% (SD12.9%) —that is, 137 of 499 stimuli (SD 65.2) detected in high resolution perimetry. During that time, visual field size increased substantially from 0% to 51% intact. Between weeks 51 and 94 visual field testing yielded stable results (baseline) with an average intact lower field of 49.6% (SD1.41%)—that is, 250/499 detected stimuli (SD7.4). In the period of visual field training (weeks 94 to 176), mean visual field size increased again with occasional peak levels reaching 66% intact visual field size (330/499 stimuli). Albeit, mean size of intact visual field during training was 54.8% (SD 8.6%)—that is, 273/499 stimuli (SD42.6), which was not significantly different from baseline (weeks 51 to 94). However, during the last two measurements of the training period his visual field size had clearly improved in comparison with the baseline level. He detected luminous stimuli, in particular regions of the transition zone where he had never perceived them before.

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Figure 2

Course of spontaneous and training induced recovery. Graphs show the enlargement of visual field over the complete course of spontaneous and training induced recovery. The dotted line with the open circles depicts the increase of per cent intact lower visual field as measured by high resolution perimetry (fixation point at upper edge of the screen). The dashed line with the filled circles shows changes of per cent intact area of the central visual field measured with standard perimetry. The solid line demonstrates the parallel increase of the number of detected stimuli in high resolution perimetry of the lower visual field.

Subjectively, the patient reported an improvement in everyday function—for example, less difficulties riding a bicycle, shaking somebody's hand, or avoiding obstacles on the ground. However, visual exploration and the perception of movements were still difficult for him, although neither a test of visual scanning nor tests of higher order visual functions (object agnosia, prosopagnosia, depth perception) showed any impairment.

Discussion

The bilateral lower quadrantanopia was a result of traumatic injury of both occipital lobes and is in full agreement with earlier descriptions of visual field defects. Our research interest was focused on the time course and phenomenology of spontaneous recovery from visual field defects and additional effects of systematic visual field training.

Three findings are unusual in the case reported here. In most studies on spontaneous recovery, the maximum duration of visual field increase ranged between 3 weeks and 6 months.11 By contrast, the patient's spontaneous recovery ended only 16 months after lesion. Secondly, the amount of spontaneous improvement was striking in that he regained almost two thirds of his visual field although average recovery in patients with complete cortical blindness usually amounts to about 28%.11 Thirdly, even after the termination of spontaneous recovery and a long period of stable visual field size, further—albeit smaller and inconstant—improvement could be achieved by systematic stimulation during visual field training starting almost 2 years after the lesion.

In concordance with earlier studies,4 12 improvement was mainly found at the borders of visual field defects—that is, in partially defective areas.13 Analysis of reaction times to light stimuli in the perimetric test showed a considerable slowing of responses to stimuli presented in the transition zone compared with those appearing in completely intact areas. Subjectively, stimuli seem to be reduced in brightness and do not have clear contours. Additionally, light stimuli cannot be perceived when presentation time is too short, explaining the difference between the visual field border found in our standard perimetric tests and training related perimetric measurements. These findings suggest that perceptual thresholds are increased in areas of residual vision. During the process of spontaneous as well as training induced recovery, these thresholds are reduced so that perception is gradually normalised. This mechanism has been postulated by Zihl and von Cramon1 as the basis of their training results, but it has not been related before to detailed observations of spontaneous and training induced increase of visual field size like in the case presented here. Therefore, we conclude that partially defective neuronal areas representing transition zones in the visual field may be crucial not only for spontaneous improvement but brain plasticity and functional recovery in these areas can also be induced by systematic stimulation as provided during visual restitution training.