Lewy body dementia is very similar to Alzheimer's Disease disease with progressive loss of memory, language, calculation and reasoning as well as other higher mental functions. However the progress of the illness may be more rapid than seen in Alzheimer's Disease disease. It is characterised by 'Cortical Lewy bodies', abnormal brain cells which are distributed in varying degrees throughout all areas of the brain. These are distinct from the Pick bodies of Pick's Disease and the neurofibrillary tangles of Alzheimer's Disease, but like them, they are only identifiable in post-mortem examination.

People with Lewy body dementia suffer periodic episodes of confusion which may vary from hour to hour. Because the 'confusion' is not there all the time, carers may feel that the person is 'pretending' to be confused. Hallucinations may also occur, commonly involving seeing objects or people in the same place (e.g. always seeing a little girl sitting on a particular chair). These are often worse during the times of acute confusion. Other problems include short term memory loss, word finding difficulties, difficulty sustaining a line of thought and locating objects in space. Sufferers may also experience symptoms of anxiety and depression.

Some patients develop the features of Parkinson's disease (rigidity, tremor, stooped posture, slow shuffling movements), followed later by the fluctuating cognitive performance, visual hallucinations, memory loss and a progressive dementia. Others experience the cognitive symptoms first and go on to develop Parkinsonian features later in the disease. In some patients, sensitivity to certain medications (particularly some "sedatives" can cause or worsen these Parkinsonian features).

Anybody can develop Lewy body dementia. As yet risk factors for developing the disease have not been identified. Rare 'familial' cases of Lewy body disease have been described. In these families it is inherited as an 'autosomal dominant' disease which means that if a person carries the gene, he/she will eventually develop the disease. Their children have a 50% chance of inheriting the illness. It appears to affect men and women alike.

Sometimes it occurs alone as the presenting illness and sometimes it occurs simultaneously with Alzheimer's or Parkinson's disease.