Charles Bonnet syndrome is a common although rather obscure and underresearched condition that is becoming ever more prevalent as the population ages and people live longer. It causes intricate, lifelike and recurring hallucinations in patients who have significant vision loss, especially in those diagnosed with macular degeneration, diabetic retinopathy or glaucoma. 

Charles Bonnet first described the syndrome that bears his name in the 18th century. He had observed such hallucinations in his 87-year-old grandfather, who had almost complete vision loss in both eyes due to glaucoma, and yet perceived women, children, buildings, geometric patterns, scenes and physically impossible circumstances. He saw that his grandfather was cognitively well, and concluded that the hallucinations were a result of his vision loss.1

View the Video
The Macular Society has posted a video about Charles Bonnet Syndrome.

Etiology of Bonnet Syndrome

Charles Bonnet syndrome can affect any person of any age. However, the affected population is predominantly older because vision loss is more common in this age group. 

As causes of blindness became more clearly differentiated, cases of Charles Bonnet syndrome were separated into the group of macular disease. The prevalence of Charles Bonnet syndrome among patients with vision loss has been estimated at 10 to 38 percent.1 Differing definitions of the syndrome, as well as patients’ unwillingness to report symptoms for fear of being labeled as mentally incompetent, may explain the wide range in the reported incidence.

Charles Bonnet is by no means a mental illness, nor is it a symptom of neurological disease. The images Charles Bonnet sufferers see are nonpsychotic hallucinations. While the experience is involuntary, the person experiencing the phenomenon recognizes that it is not the product of external stimuli and is not real.2 This differentiates it from a hallucination experienced as a result of mental illness, in which the person does not perceive the images as imaginary.  

Hallucinations of Charles Bonnet patients can range from simple images, such as colored patterns, to complex scenes, such as children playing. Patients are often able to vividly describe and recall the images, which is highly characteristic of Charles Bonnet syndrome (Figure).1

“Everywhere I walk I see disembodied gargoyle heads,” is how one patient with Charles Bonnet Syndrome described her hallucinations. (Image courtesy of Macular Society).

Brain ‘Sees’ What Eyes Don’t

Medical experts have speculated about the causes of Charles Bonnet syndrome hallucinations, but the consensus is that the brain is reacting to a lack of visual input. As one experiences vision loss, the brain will continue to interpret visual data, even without corresponding visual input. Lacking that input, the brain will invent images, and visual brain cells will begin to fire spontaneously in order to compensate for lack of visual data. As the brain adjusts to the vision loss, the frequency of the hallucinations will wane and eventually cease entirely.3 A study in which 13 normally sighted and mentally healthy subjects were blindfolded for five consecutive days supports this theory. After one day, 10 of the patients reported visual hallucinations, ranging from simple to complex.4

Similar effects to Charles Bonnet syndrome can be seen in a number of conditions affecting the elderly. An estimated 36 percent of those with Parkinson’s disease have experienced hallucinations. Parkinson’s patients experiencing hallucinations will also have insight into the nature of the images, suggesting that such hallucinations have aspects in common with Charles Bonnet syndrome.5

Hallucinations are also prominent in a number of psychiatric illnesses, such as bipolar disorder and psychosis. They can be triggered by drug or alcohol use. Patients in withdrawal often experience hallucinations. Those who have hallucinations that arise from a mental illness or drug use do not have insight into their nature, another characteristic that differentiates them from hallucinations in Charles Bonnet syndrome.

‘Phantom Vision Syndrome’

Charles Bonnet syndrome is also known as “Phantom Vision Syndrome,” and can be grouped in with a number of other “phantom” conditions that occur when the body loses one of its parts or functions. 

The most famous of these is phantom limb syndrome, whereby the loss of a limb causes a patient to experience sensations in the lost appendage. The sensations can include light touch or even pain, and patients with this syndrome often have a sense of weight or movement in their phantom limb.6 The prevalence of this syndrome is astonishing, with an estimated 49 to 88 percent of amputees having experienced it.7

Perhaps the most parallel condition to Charles Bonnet Syndrome is paracusis, or auditory hallucination, in deaf or hearing-impaired people. These conditions have helped in determining the cause of Charles Bonnet syndrome. The same mechanism causes all of these conditions: the brain’s reaction to a lack of information from a sense or limb.

The literature on Charles Bonnet syndrome is meager. Most of the published research in this area has been primarily descriptive. Investigative studies of issues that relate to visual perception, generally a subject for experimental psychologists, have not been published. 

Our Research In Visual Perception

Our research group, composed of several full-time experimental psychologists and clinicians, has published the results of a number of visual perception studies in the ophthalmic literature on topics such as fixation stability following successful macular disease treatment and binocular and monocular fixation behavior in AMD.8-11

Most recently, our group has followed a number of patients with Charles Bonnet syndrome to try to more rigorously define the nature of their visual hallucinations from the perspective of visual perception abnormalities. We’re attempting to determine if the observed visual images change in relative size, and what might be the determinants of this phenomenon, or if they display “size constancy.” We hope to be able to shed new light on this condition, which has been known for more than 200 years and yet is still poorly understood. 

The authors are with University Health Network, Toronto.

1. Jackson ML, Ferencz J. Charles Bonnet syndrome: Visual loss and hallucinations. Can Med Assoc J. 2009;181:175- 176.
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3. Jan T, del Castillo J. Visual hallucinations: Charles Bonnet syndrome. W J Emerg Med. 2012;13:544-547.
4. Merabet LB, Maguire D, Warde A, et al. Visual hallucinations during prolonged blindfolding in sighted subjects. J Neuroophthalmol. 2004;24:109–113.
5. Biousse V, Skibell BC, Watts R, et al. Ophthalmologic features of Parkinson’s disease. Neurology. 2004;62:177– 180. 6. Sacks OW. The Man Who Mistook His Wife for a Hat: And Other Clinical Tales. London: Picador; 2015.
7. Ahmed A, Bhatnagar S, Mishra S, Khurana D, Joshi S, Ahmad SM. Prevalence of phantom limb pain, stump pain and phantom limb sensation among the amputated cancer patients in India: A prospective, observational study. Indian J Palliat Care. 2017;23:24-35.
8. Tarita-Nistor L, González EG, Mandelcorn M, Steinbach MJ. Fixation stability, fi xation location, and visual acuity after successful macular hole surgery. Invest Ophthalmol Vis Sci. 2009;50: 84-90.
9. Mandelcorn MS, Podbielski D, Mandelcorn ED. Fixation stability as a goal in the treatment of macular disease. Can J Ophthalmol. 2013;48: 364-367.
10. Kisilevsky E, Tarita-Nistor L, González EG, et al. Characteristics of the preferred retinal loci in the better and worse seeing eyes of patients with a central scotoma. Can J Ophthalmol. 2016;51:362-367.
11. González EG, Tarita-Nistor L, Mandelcorn E, Mandelcorn M, Steinbach MJ. Mechanisms of image stabilization in central vision loss: Smooth pursuit. Optom Vis Sci. 2018;95:60-69.