Complex Visual Hallucinationsand
by Erik A. Steen
Located in the center of the retina, the sensitive macula providesus with sight in the center of our field of vision. When we lookdirectly at something, the macula allows us to see the fine details.This sharp, straight-ahead vision is necessary for driving, reading,recognizing faces, and doing close work, such as sewing. Maculardegeneration is the impairment of this central macular area.Age-related macular degeneration (AMD) is the most common cause ofvision loss in the Western world in the over 50 age group. It mostcommonly affects those of northern European descent and is uncommonin African-Americans and Hispanics. The prevalence increases withage. It affects about 15% of the population by age 55, and over 30%are affected by age 75 (Miller, 1992).
In macular degeneration a layer beneath the retina, called theretinal pigment epithelium (RPE), gradually wears out from itslifelong duties of disposing of retinal waste products. Eventuallythe capacity of the RPE layer to handle these metabolic products isoverwhelmed and the RPE begins to degenerate (Miller, 1992). Otherdeep layers involved in development of macular degeneration areBruch's membrane and the choroid layer. Additional factors that mayhasten loss of the RPE layer, and the resulting degeneration of thecentral retina (macula) include hereditary factors, ultraviolet raysfrom sunlight, and blue iris color (more UV rays reach the retina inblue-eyed patients, probably because of the lower pigment density inthe eye).
There are basically two forms of macular degeneration. So-calleddry (or atrophic) macular degeneration, which accounts for 90% ofcases, is caused by the aging and thinning of the tissues of themacula (Miller, 1992). This type is characterized by tiny yellowishdeposits under the macular part of the retina. These deposits areknown as drusen and may increase in size and number over time. Areasof loss of retinal and RPE layers in the macula may gradually appear.Small clumps of brown pigment from the degenerating RPE layer alsoare commonly seen.
The other major form of this disease is wet (or exudative maculardegeneration) which is a much greater threat to vision loss eventhough it accounts for only 10% of cases. This type can be associatedwith a more sudden loss of vision due to leakage or bleeding underthe macula from abnormal vessels, called the choriocapillaris,arising from one of the deeper layers. Eventually these areas ofbleeding or fluid accumulation can develop into a dense mass of scartissue beneath the retina, resulting in permanent loss of centralvision (Foerster, 1976).
For a small percentage of people diagnosed with maculardegeneration the loss of central vision is the least of theirproblems.The occurrence of complex visual hallucinations in peoplewith age-related macular degeneration is not uncommon. These visualhallucinations in the elderly are sometimes known as the CharlesBonnet syndrome (CBS), named after the Swiss philosopher whodescribed the phenomenon in his grandfather, Charles Lullin, in 1769(Berrios & Brook, 1982). Lullin underwent bilateral cataractextraction followed, at age 89, by an episode of vividly formedvisual hallucinations in the absence of any cognitive impairment.Lullin retained insight into the unreality of the experiences.Morsier first applied Bonnet's name to the syndrome but discountedthe role of ocular disease, instead postulating pathology involvingthe processing of visual stimuli in the central nervous system. Morerecently, the term has been applied to any elderly patient withvisual hallucinations. Various explanations have been proposed forvisual hallucinations in partial blindness. Psychologicalexplanations focus on regression and emergence of the primary processin conditions of reduced sensory input (Rosenbaum & Freedman,1987). Physiological explanations have also been proposed. Lack ofthe usual afferent impulses due to ocular pathology, post surgicaleye patching, or deprivation of visual sensory stimulation may unmaskspontaneous neuronal activity at any of the various levels ofprocessing of visual stimuli (Rosenbaum & Freedman, 1987). Theseare sometimes referred to as release hallucinations.
Charles Bonnet hallucinations occur in elderly people with severevisual sensory deprivation. This disease is frequently overlooked ormisdiagnosed by both ophthalmologists and psychiatrist. CBS patientsexperience these complex visual hallucinations during clearconsciousness. That is, psychosis, substance abuse, sleep disorders,focal neurological lesions and acute eye disease are absent. CBSsubjects have reduced vision due to peripheral or central eyedisease, i.e. macular degeneration. Individual hallucinations canlast from a few seconds to most of the day. Episodes can occur forperiods ranging from days to years, with hallucinations changing inboth frequency and complexity during this time. The hallucinationsmay be triggered or stopped by many factors that operate via ageneral arousal mechanism (Casey & Wandzilak, 1988).
CBS patients most often report people, animals, buildings andscenery. These images may appear static, dynamic or animated.Subjects may react positively or negatively to their visualhallucinations. Additional common characteristics include no personalmeaning for the content of the hallucinations and hallucinationsvanishing when patients close their eyes. Also, most CBS subjectssuffer from social isolation. Several theories may explain CBS. Themost popular theory builds on sensory deprivation and emphasizesvisual system activity following sensory loss producing nerveimpulses and visual experiences (Lalla & Primeau, 1993). Reducedvisual input cannot be the only cause of CBS because not all visuallydisabled people have visual hallucinations, and CBS has beendescribed in patients with normal vision.
Previous studies link cognitive deficits with visualhallucinations. Cognitive deficits in Parkinson's disorder patientshave also been associated with risk of hallucinations. A study ofpatients with Charles Bonnet Syndrome revealed that two of sixsubsequently developed dementia, which raises the question of whethervisual hallucinations in patients with age-related maculardegeneration may be a risk factor for or, more likely, an earlysymptom of dementia (Lalla & Primeau, 1993). A follow-up study ofthese patients may answer this question. The nearly significantassociation between older age and hallucinations could also indicatethat greater age related cortical atrophy is a risk factor, but nothere is no data to support or refute this possibility (James, 1993).The work of Foerster (1976), who produced complex visualhallucinations by stimulating the visual association cortex, makesvisual association cortex area 19 a possible candidate for the brainregion "releasing" visual hallucinations. Interestingly, anabnormally high number of neurofibrillary tangles are found in thevisual association cortex of patients with Alzheimer's disease, adisease in which 10% of patients have visual hallucinations, withrelative sparing of the primary visual cortex (Holroyd, et al. 1992).An examination of phenomena in other visual disorders and over abroader age range is now underway. These new studies may help clarifythe importance of age as a risk factor. It will also help clarifywhether this phenomenon is present in visual disorders with differentpathologies and will further define other factors found inassociation with macular degeneration with hallucinations. Futurestudies of visual disorders in young patients, and of brain structureand function with magnetic resonance imaging, single-photon emissioncomputed tomography, and autopsy to examine the role of abnormalneuroanatomy and neurophysiology would be of value to study thisphenomenon also.
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