When It Is the Eyes, Not the Dementia

She is seeing people who are not there, and everyone assumes the dementia has moved on. If her sight is failing, the cause may be her eyes, not the dementia.

What is Charles Bonnet syndrome?

Visual hallucinations caused by disease of the visual system rather than of the mind. The brain, deprived of input, fills the gap with images.

The definition used clinically is recurrent or persistent complex visual hallucinations in someone with a disease of the visual system, with intellect intact and no psychiatric illness driving it.

The images are not vague. They are well-defined, organized, detailed, and the person has almost no control over them. They arrive and leave on their own. The most commonly reported images are of people — figures, faces, sometimes children, sometimes crowds. Patterns and repeating geometric shapes are also common.

They are typically silent. That detail matters diagnostically: hallucinations involving voices point somewhere else.

How common is it?

Common enough that anyone with significant sight loss and new visual hallucinations should be assessed for it before anything else is assumed.

Reported prevalence runs between 10% and 40%, with an Australian study finding 17.5%. Estimates vary by setting — as low as 0.4% in general ophthalmology clinics, and approaching 30% in people with advanced age-related macular degeneration.

The main risk factor in late macular degeneration is poor visual acuity in the better eye. Advancing age and sight loss in both eyes raise the risk further. Mean age of onset is roughly 70 to 75, which is squarely the age band where dementia is also being diagnosed.

People conceal it. The true prevalence is considered an underestimate precisely because so many hide the images, fearing they will be thought to be losing their minds. Clinicians are advised to ask directly, because the question is rarely volunteered.

Why does this get missed in dementia?

Because the textbook definition requires intact cognition, so people assume it cannot apply. In practice the two conditions sit in the same person all the time.

Here is the trap. Charles Bonnet was described in people with normal cognition and preserved insight — they know perfectly well the figures are not real. That definition is what most clinicians carry.

So when someone already has dementia, new visual hallucinations get attributed to the dementia by default, or to a psychiatric cause, and the eyes never come into the conversation. Meanwhile the person has macular degeneration, glaucoma or cataracts that nobody connected to the symptom.

What complicates it further is that dementia can erode the insight. Somebody with intact cognition says "I know they are not really there." Somebody with dementia may not be able to make that distinction, or hold onto it. The mechanism can still be the eyes even when the reassurance no longer lands.

• Ask what her vision is actually like. When was the last eye examination? Is there known macular degeneration, glaucoma, cataract?
• Note whether the images are silent. Voices point away from Charles Bonnet.
• Note whether they are detailed and stable. Well-formed, organized figures fit; vague shifting shadows fit less well.
• Note when they happen. Poor light and low stimulation make them more likely.

What else could it be?

Several things, and two of them are urgent. This is a symptom that deserves a proper look rather than a label.

• Delirium. New confusion and hallucination developing over hours or days, with drowsiness or agitation, is delirium until proven otherwise. Infection, dehydration, pain and medication are the usual causes, and it is a medical emergency in an older person.
• Dementia with Lewy bodies. Recurrent, detailed visual hallucinations are a core feature — often of people or animals — usually alongside fluctuating attention and sometimes parkinsonian movement changes. It matters enormously to identify, because people with Lewy body dementia can react severely to antipsychotics.
• Medication. A long list can cause hallucinations, including some Parkinson's drugs, opioids, steroids and anticholinergics. A medication review is quick and often productive.
• Sight loss plus one of the above. These are not mutually exclusive, which is the point of getting it assessed rather than guessing.

What actually helps?

Explanation first. For most people the treatment is understanding what it is, and knowing they are not losing their mind on top of everything else.

Most people in the research were not distressed by the images themselves — but reassurance about their benign nature was welcomed. The distress often comes from the fear about what the images mean, not from the images.

Practical things that help:

• Say what it is, plainly and more than once. "Your eyes are making pictures because they are not getting enough to look at. It is not your mind."
• Turn the lights up. Low light and low visual stimulation are when the images tend to arrive.
• Give the eyes something to do. Television, movement, going to another room, blinking hard or looking rapidly from side to side. Several people find the images break when they change what they are looking at.
• Get the eyes treated where treatment exists. Cataract surgery, updated glasses, better lighting. Improving the input can reduce the phenomenon.
• Do not argue about whether the figures are real. If insight is gone, arguing achieves nothing. Address the feeling and change the environment.
• Keep a note of what she sees and when. Take it to the appointment. It is the single most useful thing you can hand a doctor.