Charles Bonnet syndrome: what it is, why it happens and what helps

Charles Bonnet syndrome (CBS) causes people with significant sight loss to see images that are not really there. The Macular Society explains that it is named after the eighteenth-century Swiss scientist who first described it. The hallucinations are the same experience as really seeing something, but the thing seen is not present.

It is important to be clear about what CBS is not. The NHS says it is not caused by a mental health problem or dementia, and RNIB repeats that the hallucinations are caused by sight loss and are not a sign of a mental health problem. Many people keep quiet about what they see because they fear what it might mean, which is one reason the condition is under-recognised.

RNIB notes that CBS hallucinations are silent and do not involve any other senses, and that people are usually aware that what they are seeing is not real, even when it is very vivid.

According to RNIB, the images fall into two broad groups: simple repeated patterns or shapes, such as grids, brickwork or colours, and complex images of things such as people, animals, buildings and landscapes. A person may have one type, both, or one after the other.

The Macular Society adds that they can range from simple flashes of light, colours or shapes to geometrical grids and lattices, and that some people report gardens, animals, people or processions of miniature costumed figures. They are often seen in more vivid detail than real life. The NHS says they can be moving or still, in black and white or colour, can start suddenly, and can last from a few minutes to several hours.

Reactions vary. Some people find the images amusing or interesting; for others they are frightening or frustrating. RNIB says that over time people often become more comfortable with them and may start to recognise the same figures or scenes.

The NHS explains that when you lose your sight your brain gets less information from your eyes than it is used to, and the brain sometimes makes up for this by creating hallucinations. The Macular Society says researchers do not fully understand why, but believe the brain starts trying to fill in the gaps when the eye does not send enough visual information.

Not everyone who loses sight gets CBS, and the reason is not known. RNIB says it usually starts in the weeks and months following a big deterioration in sight but can develop at any time. The NHS lists a sudden worsening of sight and loss of sight in both eyes as things that make it more likely. The Macular Society adds that it can happen if only one eye is affected and that it affects anyone with sight loss, not only people with macular degeneration.

RNIB says sight loss from any eye condition can cause it, and that anyone of any age may develop it, although because sight loss is more common in older people, many of those affected are older.

RNIB says it is difficult to know how many people in the UK have the condition because people are often unaware that sight loss can cause hallucinations. They may think they are developing a mental health condition and so may not want to tell doctors what they are seeing.

RNIB says research suggests about one in five adults who experience sight loss develop Charles Bonnet syndrome, which it estimates means at least 1 million adults in the UK. It adds that children with sight loss are just as likely to develop it as adults but have only just begun to be included in research. The Macular Society says up to half of all people with macular degeneration are thought to experience it at some time.

The Macular Society says individual hallucinations come and go unannounced, lasting from a few seconds to a day or more, and that the syndrome itself can last from days to many years. For most people the hallucinations improve with time and happen only occasionally.

RNIB says that when it first develops, the hallucinations may happen quite often, even every day for long periods. For some people they become less frequent over a few months and may eventually stop, while for others the condition continues for years or lifelong. A study cited by RNIB found it is not unusual to still have hallucinations five years after they first started.

RNIB also notes that if the hallucinations stop they may come back if you lose more sight, and that they can return or get worse during an infection such as a urinary tract or chest infection.

There is no single test. RNIB says a doctor will want to rule out other causes of hallucinations, usually by talking with you and sometimes by carrying out tests. If nothing else is found and you have recently lost some sight, Charles Bonnet syndrome is the most likely explanation. The NHS advises asking for an urgent GP appointment or NHS 111 help whenever hallucinations occur.

RNIB lists Parkinson's disease, Alzheimer's disease, strokes, serious mental illnesses and other brain conditions as things that can affect the parts of the brain concerned with seeing and may cause hallucinations. It says having Charles Bonnet syndrome does not mean you are more likely to develop any of these. The Macular Society lists features that suggest something other than Charles Bonnet syndrome and says to contact a GP or ophthalmologist if you notice them.

There is currently no cure, but there are practical techniques. The NHS suggests the following when a hallucination appears: turn on the lights or move somewhere lighter if it is dark, or turn them off or move somewhere darker if it is light; move your eyes from left to right without moving your head, 15 to 30 times, then pause and repeat; stand up and move around; watch television or listen to the radio; reach out to touch the hallucination; and use a cane or walking stick to check what is real around you.

The Macular Society describes the eye movement exercise as looking between two points about a metre apart once a second or faster for 15 to 30 seconds, and says that if it has not worked after four or five attempts it is unlikely to work for that hallucination, so you may try again another time.

Longer-term measures include being well rested, because the NHS says hallucinations may be worse when you are tired or stressed; using bright light bulbs and magnifiers to make the most of the sight you have; and having regular eye tests. RNIB adds that hallucinations may be worse when you are anxious, isolated or stressed, and that they can return or worsen with an infection such as a urinary or chest infection.

The NHS says that if the hallucinations suddenly return or get worse this could be caused by an infection or a medicine you are taking, and a GP can check; treating the infection or changing the medicine might help. RNIB notes that anticholinergic medicines, which are used for conditions such as motion sickness and bowel and bladder problems, can sometimes make hallucinations worse, especially in older people, and that some eye drops can too. It advises not to stop prescribed medicines without talking to the GP.

If you find the hallucinations upsetting, the NHS says a GP can refer you for talking therapy that can help with hallucinations. For very distressing cases, the Macular Society says medication is rarely offered, usually prescribed for other conditions and decided by a specialist after checking it is safe. RNIB says there is no proven drug and that these medicines can have serious side effects. Research into causes and treatments continues, including a trial of non-invasive brain stimulation reported by the Macular Society that needs further work.

A GP, optician or ophthalmologist can refer you to a low-vision service for advice, which the NHS lists as one of the ways of making the most of the sight you have.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you or someone else have hallucinations, because it is important to rule out more serious conditions that cause hallucinations. You can call 111 or use 111 online.

The Macular Society says that if you notice hearing voices, confusion, memory problems, or not realising the images are not real, you should contact your GP or ophthalmologist.

The NHS also says that if hallucinations suddenly return or get worse this could be caused by an infection or a medicine, so speak to a GP who can check.

For most people the hallucinations become less frequent. The Macular Society says CBS can last from days to many years but that, for most people, the hallucinations improve with time and happen only occasionally. RNIB cautions that it is not unusual to still have them five years after they started, and that they may return if sight worsens.

Support is available from the Royal National Institute of Blind People, the Macular Society and Esme's Umbrella, all of which are listed by the NHS as sources of information and support. Simply knowing what the condition is can make the experience less frightening, and RNIB suggests that telling friends and family about it may also help.

On the NHS, a GP, optometrist or ophthalmologist can assess the hallucinations, check for infection and medicines, and refer to a specialist low-vision service. The NHS says a GP can also refer you for talking therapy if the hallucinations are upsetting. Treatment of the underlying eye condition, for example macular degeneration or cataract, continues alongside.

Private eye care can offer assessment and treatment of the eye condition, often with shorter waiting times, but the advice on coping and the charities that offer support are the same whichever route you use.