Most people are familiar with psychiatric disorders such as schizophrenia and bipolar disorder, but some conditions are so rare that many psychiatrists won't come across a single case in their professional lives.

Here I present five of the rarest – and strangest – syndromes known to psychiatry.

1. Fregoli syndrome

Fregoli syndrome is where someone believes that different people are in fact the same person who just changes their appearance. People with this syndrome often feel persecuted by those they believe to be in disguise.

The disorder is named after Leopoldo Fregoli, an Italian theatre actor who was known for his remarkable ability to quickly change his appearance while on stage.

Fregoli syndrome typically occurs with other mental disorders, such as bipolar disorder, schizophrenia, and obsessive-compulsive disorder. It can also be caused by brain injury and use of the drug levodopa in the treatment of Parkinson's disease.

A review in 2018 found that fewer than 50 cases worldwide had been reported since the condition was first described. However, a more recent study (2020) reported an incidence of 1.1 percent among patients following a stroke, so certainly more than 50 cases but still very rare.

There is no known cure for Fragoli syndrome, but treatment with antipsychotic drugs may lessen the symptoms.

2. Cotard's syndrome

Cotard's syndrome, also known as "walking corpse syndrome", is where people hold the delusional belief that they are dead and do not exist. Others believe that body parts are missing.

The syndrome is named after the 19th-century French neurologist Jules Cotard, who first described the condition in 1882.

Schizophrenia, depression and bipolar disorder are risk factors for Cotard's syndrome. However, it has also been reported as a rare side-effect of the anti-viral drug acyclovir.

The syndrome is thought to originate from a disconnection between the areas of the brain that recognize faces and the areas that associate emotional content with such facial recognition.

This rare condition is usually treated with antidepressants, antipsychotics, and mood stabilizers, as well as electroconvulsive therapy.

3. Alien hand syndrome

Alien hand syndrome is one of the strangest neurological disorders. It is where a person's hand appears to have a mind of its own and acts autonomously, and where the person feels as though their hand doesn't belong to them.

This syndrome was first identified in 1908 but was not clearly defined until the early 1970s. The term "alien hand syndrome" was coined by Joseph Bogen, an American neurophysiologist, to describe a curious wayward behavior occasionally seen during recovery from certain types of brain surgery.

People with alien hand syndrome typically have sensory processing disorders and dissociate themselves from the actions of their hand. Research indicates that people with the syndrome often personify the alien hand, and may believe it is possessed by some other spirit or alien life form.

Causes of the syndrome include dementia, strokes, prion disease (a fatal brain disease), tumors and seizures. Cases of alien hand syndrome have also been reported among patients who have had surgery to separate the left and right brain hemispheres to treat severe epilepsy.

The syndrome is very rare. A review in 2013 found just 150 cases in the medical journals.

Although there is no cure for alien hand syndrome, symptoms can be minimized and managed to some extent by keeping the affected hand occupied and involved in a task – for example, by giving it an object to hold in its grasp. Other treatments have included injections of botulinum toxin and mirror box therapy (see video). Stroke patients appear to have the best treatment success.

4. Ekbom's syndrome

Ekbom's syndrome is a tactile hallucination in which sufferers believe they are infested with parasites – often experienced as insects crawling under their skin.

The syndrome is named after Karl Ekbom, a Swedish neurologist who first described the condition in the late 1930s.

The exact number of people who suffer from this syndrome is unknown, but one study reported around 20 new cases a year in a large US referral clinic.

According to a meta-analysis of 1,223 Ekbom cases, the syndrome is more common among women (two-thirds women, one-third men), and more common in those over the age of 40. Symptoms typically lasted three to four years.

Ekbom's syndrome is associated with several conditions, including paranoid schizophrenia, organic brain disease, neurosis and paranoid personality disorder. It has also been reported in some people undergoing alcohol withdrawal, cocaine misuse, strokes, dementia, and lesions in a part of the brain called the thalamus.

Psychological treatments are often not wanted by Ekbom's syndrome sufferers, as they are convinced that the problem needs medical treatment.

5. Alice in Wonderland syndrome

Alice in Wonderland syndrome, also known as Todd syndrome, refers to when a person's sense of body image, vision, hearing, touch and space/time are distorted. People with the condition typically experience objects to be smaller than they actually are, while people seem bigger than they are. Or the opposite: objects are perceived to be larger than they are and people appear smaller. These experiences may be accompanied by feelings of paranoia.

Little is known about how common this disorder is. Sufferers are most commonly children and migraine sufferers.

People with the condition can become frightened and panic-stricken, so successful treatment often includes rest and relaxation. In most cases, it is a relatively short-lived condition. The most recent review on Alice in Wonderland syndrome reported that close to half of all sufferers are treated successfully.The Conversation

Mark Griffiths, Director of the International Gaming Research Unit and Professor of Behavioral Addiction, Nottingham Trent University

This article is republished from The Conversation under a Creative Commons license. Read the original article.