Apart from damage or dysfunction of the CNS visual hallucinations may also arise from a pure peripheral pathology caused by lesions of the optical nerves or an ocular pathology as in macular degeneration, retinopathy, or cataract. This association of impairment of peripheral vision and complex visual hallucinations in aged psychologically normal people is called Charles Bonnet's syndrome. Typically there are no concomitant psychotic symptoms and the patient is usually aware of the unreality of his experiences. However, despite a widespread agreement about hallmarks of the phenomenology, a universally accepted definition has not been found yet.1

Little is known about the underlying pathophysiology. A widely accepted hypothesis postulates a reduced afferent input causing a “release” with disinhibition of engrams in the visual association cortex that are experienced as hallucinations. Indeed a recent fMRI study has shown an increased activation of the ventral extrastriate cortex in Charles Bonnet's syndrome.2 Some authors suggest that hallucinations in the syndrome may share a common mechanism that also evokes hallucinations in some central disorders (for example, after infarction of the visual cortex) involving cholinergic and serotonergic pathways.3

Although often neglected or misdiagnosed in clinical practice4 a peripheral visual pathology seems to be an important differential diagnosis of complex visual hallucinations. In a large study of 500 visually handicapped patients Teunisseet al 5 found a prevalence of Charles Bonnet's syndrome of 11%. The occurrence of the syndrome was significantly associated with older age (>64) and a severe impairment of visual acuity (<0.3 in the best eye).

Therapeutic options for Charles Bonnet's syndrome still remain poor and of uncertain benefit for the individual patient. Even without any intervention in some patients the hallucinations can fade away within a few weeks or months. However, there are also many reports of a continuous course with …