The formed visual hallucinations associated with vision loss

Patients with acquired visual impairment, but without mental illness, may experience repetitive episodes of vividly detailed, clearly focused, formed visual hallucinations. Yet these may not be shared voluntarily with clinicians. We wanted to learn: (1) the prevalence of formed visual hallucinations and (2) the nature of any changes in the hallucinations after more than two years.

Part I--Eighty-eight visually impaired adult patients at the Massachusetts Eye and Ear Infirmary were surveyed during 1995. Age, gender, ocular diagnoses, visual acuities, and hallucination descriptions were recorded. Part II--Those subjects reporting hallucinations in 1995 were selected to participate in a second survey--more than two years later-to ascertain changes.

Part I--Nine of 88 subjects (10.2% prevalence) reported formed visual hallucinations, frequently human faces or figures. Five additional subjects (5.6%) reported a history of formed hallucinations-beginning after their vision loss--that had either disappeared or changed to unformed hallucinations (lights and colors) before this survey. Part II--Seven of the eligible nine subjects were surveyed more than two years later. Hallucinations persisted for six. Five reported reduced frequency of hallucinations, despite failing acuity among four of these five.

Formed visual hallucinations are not uncommon among patents with significant acquired losses in visual acuity and/or visual field (homonymous hemianopsia). Although formed hallucinations may persist for years, there appears to be a tendency toward reduced frequency of episodes over time. Silent isolated visual hallucinations, in the absence of psychopathology, imply an organic etiology. Eliciting these phenomena through an appropriate clinical history provides a valuable opportunity for referral, or patient education and reassurance.