Dear Editor

We read with interest the paper by Tan et al. [1] on Charles Bonnet Syndrome (CBS) in Asian patients. Their finding of a lower CBS prevalence than European or North American surveys demands further investigation, although this may reflect the stringent criteria of hallucination complexity they used in making the diagnosis (thus excluding the commonest CBS hallucinations of coloured blobs and grid-like ‘tesselloptic’ patterns [2,3]) and, as pointed out in the accompanying editorial comment, the relatively low prevalence of macular disease in their cohort. However, it is not this aspect of the report we found most intriguing - it was the observation that CBS occurred with good acuity. In fact, 3 of the 4 CBS patients described had a degree of impairment which placed them at risk for CBS (best eye acuity 0.3 or worse [4]). It is the remaining patient (patient three, a 72 year old male) who is of particular importance as his relative preservation of acuity bilaterally (20/30 RE, 20/40 LE) challenges the view that significant acuity loss is a prerequisite for ‘ophthalmological’ visual hallucinations. This case mirrors 4 patients we have recently studied with Charles Bonnet Syndrome secondary to glaucoma and bilaterally good acuity. We describe the cases below and offer a pathophysiological mechanism for the association.

In one sense, the finding that CBS occurs with preserved acuity is hardly novel. As cited by Tan et al.[1], several previous reports have found such an association. However, all is not as it seems, the term Charles Bonnet Syndrome (CBS) being used in different ways by different authors. Some use the term to describe visual hallucinations with insight, irrespective of the presence of eye disease, age or clinical context [5,6]. Others use the term to describe the association of visual hallucinations with age and intact cognition, without reference to eye disease or hallucination phenomenology [7,8]. Under these definitions it is hardly surprising that a patient with ‘CBS’ has preserved acuity, the patients in these studies having a diverse range of conditions from delirium to Parkinson’s disease and beyond. In contrast, ophthalmologists and neurologists have used CBS to emphasise eye or visual pathway disease, with the phenomenology of the hallucinations and age being of secondary importance [9,10]. Although each definition of CBS has its merits, the ophthalmological definition reminds us best of Bonnet’s original description and helps characterise a distinctive subgroup of visually hallucinating patients with predicable prognosis and specific pathophysiology [2,11]. However, even CBS as defined ophthalmologically carries with it an inherent ambiguity: is it eye disease itself or the loss of acuity that is the important factor? The consistent finding of acuity loss as a risk factor [4,12,13] suggests the latter, or at least that the central retina plays a key role in the underlying pathophysiological mechanism.

As part of a larger study into the visual phenomenology of CBS, we have recruited 4 patients with advanced glaucoma (3 POAG and 1 chronic narrow angle) but preserved visual acuity. The age range of the patients was 81 to 91, 3 men and 1 woman. Their visual acuities ranged from 6/6 to 6/12 monocularly with all patients having 6/9 or better in their better eye. All had extensive field defects bilaterally and cup to disc ratios of 0.8 or greater in both eyes. 2 patients had bilateral trabeculectomies now off treatment, one was on g.bimatoprost and g.trusopt to both eyes and one on g.timolol 0.25% to both eyes. 2 patients were bilaterally pseudophakic. The patient with chronic narrow angle glaucoma had previous surgical iridectomies. The duration of their hallucinations ranged from 6 months to 6 years. 3 patients hallucinated in colour and one in black and white. The most common hallucination was of tessellopsia [2] experienced by all the patients, with 2 patients seeing, in addition, formed buildings and 2 patients, letter-like shapes. There were also single reports of hallucinations of groups of people, animals, branching shapes (dendropsia [2]) and one patient described visual allesthesia [14]. In three of the patients the hallucinations encompassed the entire visual field, in the fourth they were restricted to the visual field defect. None had hallucinations in other sensory modalities and all had insight into the nature of the experiences. The phenomenology of the hallucinations and the relative frequency of the different hallucination categories are consistent with previous descriptions of ophthalmologically-defined CBS [2,3]. Non-ophthalmological causes of visual hallucinations [2,3] were excluded. As far as we are aware this is the largest case series of patients with visual hallucinations secondary to eye disease and bilaterally preserved visual acuity yet to be reported.

Current aetiological theories of CBS emphasise the importance of deafferentation [15] (both ‘physiological’ through ganglion cell loss and ‘functional’, e.g. related to blindfolding or cataract), the loss of visual input resulting in a change in cortical excitability [2]. Although it has been assumed that deafferentation of sufficient severity to precipitate CBS implies a consequent loss of acuity, our cases and that of Tan et al.[1] suggest otherwise. Patients with advanced glaucoma can have a significant degree of ganglion cell loss and consequent physiological deafferentation without a loss of acuity, placing them at risk for CBS. This contrasts with age related macular disease where the loss of central retinal ganglion cells leads, indirectly, to an association of CBS with acuity loss. We conclude that reduced acuity is not a necessary prerequisite for ophthalmologically-defined CBS and that ophthalmologists should be aware that patients with preserved acuity but significant deafferenting ocular disease are at risk of the syndrome.

References

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