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Charles Bonnet syndrome (visual hallucinations) following enucleation

Sir,

We read with interest the excellent article by Drs Ross and Rahman, describing visual hallucinations in a patient following enucleation.1 These hallucinations were characteristic of Charles Bonnet syndrome (CBS) and, interestingly, disappeared with eye closure. We would like to propose a possible pathophysiologic mechanism to explain this observation.

A visual acuity of 6/6 in the patient's other eye is not incompatible with the diagnosis of CBS, which has been described both in people with good visual acuity in the fellow eye2, 3 and in patients who have visual field defects and good central visual acuity in the affected eye.2, 4, 5 Shiraishi et al6 proposed that it is the dynamic reduction in visual acuity, rather than the actual visual acuity, that has a greater impact on CBS.2, 7

This case is intriguing because the hallucinations ceased when the patient's eyes were closed, only to return when he opened his eyes. Although it is well known that eye closure may terminate hallucinations in patients with CBS, this is the first case in which it appears that transient reduction of light perception on closure of the fellow eye is associated with its cessation. The enucleated eye constantly has no light perception and lid closure will not have any additional effect. It is possible, as the authors suggest, that closing the eyes results in secondary normalization of sensory input, thus abolishing the abnormal independent impulses and resultant complex imagery.1 Another possibility might be that deafferentation induced changes in the cortical neurons, resulting in reorganization of the receptive field and increased sensitivity to sensory input.8 Stimulation of these hypersensitive areas by normal sensory impulses (in this case from the left eye) may trigger visual hallucinations.7, 9, 10 However, a minimum amount of sensory input is required in order to trigger the hallucinations. Therefore, when the patient closes both his eyes, normal input is abolished and the hallucinations cease, only to return when he opens his eyes. This theory would also explain why some hallucinations cease when patients eventually lose all light perception. This possibility is illustrated in another patient who experienced CBS following cortical resection for cortical dysplasia.5 In this patient, the hallucinations diminished with eye closure, and varied in intensity with blinking, light intensity, and the sight of moving objects—factors that vary the intensity of the visual stimulation.

Regardless of the mechanism, we agree with the authors that it is important to recognize CBS and its possible occurrence following sudden loss of vision.

References

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  8. Eysel UT, Schweigart G, Mittmann T, Eyding D, Qu Y, Vandesande F et al. Reorganization in the visual cortex after retinal and cortical damage. Restor Neurol Neurosci 1999; 15 (2–3): 153–164.

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Correspondence to C S H Tan.

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The authors have not received any financial support in the preparation of this article and do not have any financial or proprietary interests in the article

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Tan, C., Sabel, B. & Eong, KG. Charles Bonnet syndrome (visual hallucinations) following enucleation. Eye 20, 1394–1395 (2006). https://doi.org/10.1038/sj.eye.6702236

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