Charles Bonnet syndrome
Though Charles Bonnet syndrome (CBS) was first described in 1760, it is still a phenomenon that is not widely known. Some medical providers know very little about CBS, and some patients are even referred for inappropriate inpatient psychiatric care upon presenting with symptoms of CBS. Additionally, most patients are unaware that hallucinations may be a function of vision loss. This may lead to unnecessary suffering and worry for the patient and a diagnostic dilemma for the practitioner.
No universally agreed-upon definition for CBS exists. It is agreed that those with CBS must have intact cognition, but mental status evaluation may be performed many ways. Though many variations exist, CBS is best described as visual hallucinations in patients with acquired vision loss, without cognitive impairment. Patients are also very often reluctant to report hallucinations for fear of being labeled as demented or mentally incompetent. Most patients with CBS are elderly, visually impaired, and have already experienced some loss of independence. The additional stigma of psychiatric illness often prevents patients from discussing their symptoms with their physicians, friends, and family. Most patients are not aware that visual hallucinations may be a normal function of loss of vision. CBS should be considered a diagnosis of exclusion, and other etiologies causing visual hallucinations must be investigated.
Loss of vision in those who experience CBS is generally bilateral. Patients with CBS retain insight into the unreal nature of the hallucinations and do not have hallucinations in any other sensory modalities. The episodes can last from seconds to hours, and the duration of the syndrome may be days to years. These episodes may occur in clusters or regular frequency, with asymptomatic intervals in between, and tend to be self-limiting. The frequency of occurrences generally diminishes over time.
The true pathophysiology underlying Charles Bonnet syndrome is not completely understood. The most commonly accepted hypothesis is the deafferentation theory. Deafferentation is defined as a loss of sensory input from a portion of the body, usually caused by interruption of the peripheral sensory fibers. This decreased sensory input may stimulate intracerebral perception. In other words, spontaneous neuronal discharge (manifesting in the form of hallucinations) is caused by lack of visual-sensory input into the cortex. The brain activity in the absence of visual input has been compared with what occurs in "phantom limb" or "phantom pain" syndromes. Relating CBS to a visual analogue of phantom limb syndrome has been helpful and comforting in explaining to patients why these hallucinations occur. Another theory is the "release phenomenon" where missing input to primary visual areas causes a disinhibition of visual association areas, contributing to a release of visual hallucinations. The adaptive nature of neural networks may explain why visual hallucinations may occur frequently after the onset of vision loss, but often seem to lose sustainability over time.
Although any loss of vision in CBS may be causative, the most commonly reported ocular disease associated with CBS is AMD. Given that AMD is one of the most common causes of sustained central vision loss in developed nations, this is not surprising. Although most reports are in elderly or adult patients who have a greater prevalence of acquired visual impairment, reports of CBS in children have been published. Any ocular pathology or surgical procedure causing a loss of vision can precipitate CBS.
Several situations have been found to increase recurrence of CBS. Social or physical isolation, dimly lit conditions, evening hours, and states of drowsiness or relaxation have all been associated with higher rates of hallucinations in those with CBS. With regard to social isolation, the quality of social contact and not the quantity was most significant in CBS. Subjective feelings of loneliness, bereavement, and shyness may contribute to an increased risk. Rate of vision loss, and not the nature of pathology or even severity of visual impairment, has been found to increase the risk of developing CBS. Patients who suffer from a sudden onset of vision loss (exudative AMD, after laser photocoagulation) and sudden onset of visual field loss with cerebral disease are more likely to experience hallucinations than those experiencing more gradual loss of vision.
There are many options for prevention and treatment of CBS. For patients not bothered by the visual hallucinations, treatment may not necessarily be indicated. Many patients need only education and reassurance of the benign nature of the visions. Some patients find rapid blinking, closure of the eyes, walking away from the hallucination, improved lighting, and diversionary activities may halt or prevent hallucinations. Increasing socialization and forming stronger social networks should also be encouraged. Optical devices, such as telescopes and prism, used to maximize residual vision and enhance visual field, have been found to be beneficial in reducing hallucinations.
Charles Bonnet syndrome remains an under-recognized yet common disorder of persons with visual impairment. Speculation still exists concerning the true pathophysiology of visual hallucinations in CBS, and medical treatments are not well established. Other more serious etiologies of visual hallucinations must be ruled out before arriving at a diagnosis of CBS, including metabolic, toxic, neurologic, and psychiatric disorders. Once these are adequately ruled out, many patients require only assurance of the benign nature of the hallucinations and education that the symptoms are commonly a function of vision loss and are often self-limiting. Increasing knowledge of characteristics of CBS by both practitioners and patients can lead to the relief of anxiety associated with these hallucinations.
Optometry. 2009 Jul;80(7):360-6