Charles Bonnet syndrome may explain visual hallucinations in patients with low vision

February 15, 2009
Nancy Groves

Complex visual hallucinations experienced by visually impaired patients are often indicative of Charles Bonnet syndrome. Individuals who experience these hallucinations usually understand that they are not real but will benefit from a doctor's explanation that they are associated with their vision loss and are unlikely to be a sign of mental illness.

Key Points

Atlanta-If patients with low vision due to glaucoma or retinal disease report seeing a giraffe on the patio, flowering trees blooming in midwinter, or uninvited "guests" in their homes, they most likely need reassurance and sympathy rather than a psychiatric referral.

It is not uncommon for these patients with visual impairment to experience complex visual hallucinations, a condition known as Charles Bonnet syndrome (CBS), and an empathetic response is the best medicine for patients and their families, said Lylas G. Mogk, MD, who described the syndrome in a presentation at the annual meeting of the American Academy of Ophthalmology.

"Because of its prevalence, it's really important for ophthalmologists, in particular, to be aware of CBS," said Dr. Mogk, director, Henry Ford Visual Rehabilitation and Research Centers, Detroit. "Explaining this to the patient can have a tremendous therapeutic effect in itself."

Complex hallucinations

CBS is a condition characterized by recurrent, complex visual hallucinations of a pleasant or neutral nature. Images appear when individuals are wide awake and fully conscious, often in daylight, and they may appear and disappear spontaneously, said Dr. Mogk.

Patients may see a variety of images, either in color or black and white, ranging from simple patterns such as straight lines to detailed pictures of people or buildings. Other common subjects include animals, flowers, trees, and houses, and the images often have vertical and horizontal orientation suggestive of a grid.

Figures in the images may be miniature or distorted, but often they are normal. Whether they remain still or move, they do not approach or engage the viewer and may vanish if the viewer simply looks away and diverts his or her attention. The frequency and duration of the images varies considerably, and they may stop occurring to an individual for no apparent reason.

"The people and animals and places in these scenes are generic," Dr. Mogk said. "They are not your grandfather or your pet from childhood."

As part of their research on CBS, Dr. Mogk and colleagues asked patients at a vision rehabilitation center to draw pictures of their hallucinations. Among the pictures were scenes described as a red brick wall with green windows; a quilt in shades of blue and teal; a chain link fence; a repetitive wallpaper pattern made up of babies' faces; flowers and branching trees; or a lake surrounded by long grass.

Individuals and groups of people commonly were reported, such as the Royal Canadian Mounted Police that one patient described; the quartet of people in Elizabethan-era attire who periodically sat in another patient's living room; and the little girls in white dresses with pink sashes and bows who played in a patient's backyard.

Typically, the same figure or scene repeats itself in a patient's hallucinations, which are exclusively visual. Although these images can be extremely vivid, those who experience them are aware that they are not real.

Two theories

Two main theories, according to Dr. Mogk, exist about the cause of CBS:

"The disinhibition of higher cortical centers is higher in the elderly, and the preponderance of the visually impaired are also elderly, so some have suggested that this is a confluence of age and vision loss," Dr. Mogk said. "However, hallucinations have been reported in children who experience rapid vision loss and in young adults in sensory deprivation experiences."

CBS usually is associated with poor visual acuity, and Dr. Mogk has observed that few of her patients with this condition have acuity better than 20/100. It recently was reported, though, that CBS was more highly correlated with loss of contrast sensitivity than with visual acuity levels.

"This finding speaks to the enormous perceptual and functional deficit invoked by loss of contrast sensitivity," Dr. Mogk said.

Prevalence rates for CBS vary. In different studies, rates among adults seen at low vision clinics were 17.5%, 20%, and 35%. The condition is associated with social isolation, which is endemic among the visually impaired elderly. This connection is supported by evidence that the hallucinations may cease temporarily among people who are hospitalized.

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