Charles Bonnet Syndrome
Charles Bonnet Syndrome (CBS), named after the Swiss scientist who first described visual hallucinations in his grandfather in the 1970s, has three features: hallucinations, ocular pathology, and intact cognition.  
CBS most commonly occurs in elderly individuals who have a severe visual impairment with age related macular degeneration being the most common cause. Other ocular impairments causing Charles Bonnet syndrome include retinitis pigmentosa, severe myopia, glaucoma, cataract, diabetic retinopathy, and optic neuritis  . CBS has also been found in patients with vascular associated vision loss including central retinal artery occlusion, retinal vein occlusion, temporal arteritis, and occipital infarction . In addition, it has been reported in patients following ocular procedures including CE/PCIOL, bilateral laser iridotomies, anti-VEGF injections, and enucleation.   
It occurs with sudden and unexpected vision loss and has not been found to occur in lifetime visual blindness. Although it is mostly seen in the elderly, there have been cases reported in children experiencing sudden vision loss suggesting that it appears more commonly in elderly due to an increased prevalence of visual impairment in this population. There is no consensus on whether males or females are more commonly affected .
CBS is believed to be underrepresented due to under-reporting of visual hallucinations out of fear of being considered mentally unstable, as these patients are aware that they are seeing things that they should not be . Due to this, the estimates of CBS prevalence in visually impaired individuals varies from 0.4-34% .
Content of the hallucinations is extremely varied. Images of people, animals, plants, inanimate objects and full scenes have all been described and commonly an individuals will experience different hallucinations in each episode. The images may be familiar to the person or completely unfamiliar, they may be in color or black and white, some hallucinations may fit in well with what was going on in reality whereas others found them to stick out from their surroundings. Some reported personal and emotional meaning to their hallucinations while others did not, and the patient is generally unable to consciously influence their hallucinations. All individuals are able to recognize that their hallucinations are not reality 
Patients have reported images lasting for varying amounts of time from seconds to hours. Many triggers have been suggested such as fatigue, stress, and differing levels of light. Resolution of the hallucinations often occurs spontaneously but, again, with many different causes reported .
The course of Charles Bonnet Syndrome is also varied with some episodes of hallucinations being reported to occur daily, weekly, monthly, as well as only a few episodes per year . Categorization of the differing patterns have been described as episodic, periodic, or continuous. Episodic consists of a single episode of hallucinations occurring over a period of days to months and then resolving permanently. Periodic is where there are occurrences of hallucinations followed by hallucination free periods. People experiencing continuous have no remission of the hallucinations .
The underlying cause of the hallucinations is unknown but many theories have been proposed including phantom limb in the context of vision, the idea of dreams and hallucinations being on the same continuum with reduced sensory input resulting in a state more similar to sleep causing the brain to create images, and the theory of perceptual release where normally filtered visual information is allowed to enter the consciousness as a result of decreased visual input . Another theory centers on the idea of deafferentation where the loss of vision results in an excessive response by the neurons creates visual hallucinations .
There is limited data regarding the effect of CBS on mortality. One study found patients with CBS to have a higher mortality overall when compared to the general population indicating that this may be a marker of mortality however, further investigation is required. This same study found many patients later develop lewy body dementia suggesting that some cases of CBS may actually be early stage lewy body dementia .
For those with CBS due to correctable eye diseases, the hallucinations may go away with treatment of the underlying cause. For example, patients with CBS caused by macular degeneration reported resolution of hallucinations with PDT and patients with cataracts had resolution of CBS with CE/PCIOL  .
Most often there is no known treatment for this condition. Management requires awareness by physicians treating patients with visual impairments and primarily consists of providing empathy and reassurance. Some medications have been tried such as antipsychotics, antiepileptics, cholinesterase inhibitors, and SSRIs showing benefit in individual patients    . Pharmacologic therapy may be considered in cases of disturbing hallucinations causing significant distress to the patient. However, non-pharmacologic methods are preferred with eye closure, appropriate lighting, and reducing patient isolation as possible approaches . Ultimately, it is very important for health care providers to recognize this syndrome and to avoid misdiagnosing a cognitively intact individual as this can lead to unnecessary and potential harmful treatment.
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