In some patients who present with Parkinson disease at a young age, with or without a family history, there will be an underlying genetic cause.

Mitochondrial neuro-gastrointestinal encephalopathy is a rare but important differential diagnosis in patients presenting with acute gastrointestinal and neurological symptoms.

Some cases of young-onset motor neurone disease (also known as amyotrophic lateral sclerosis (ALS)) will have a genetic cause.

Genetic causes of dementia are rare, and are typically associated with early disease onset and/or complex neurological symptoms. There is often a significant family history.

Spinal and bulbar muscular atrophy, also known as Kennedy disease, is a genetic cause of progressive limb and bulbar weakness.

Rhabdomyolysis is the rapid breakdown of damaged skeletal muscle. In some cases, there may be a genetic cause.

Ptosis and ophthalmoplegia can have numerous causes. Once acquired causes are excluded, genetic conditions should be considered.

In some individuals with weakness of the proximal muscles, there may be a genetic cause.

Pathogenic variants in the POLG gene are one of the most common causes of primary mitochondrial disease. Patients may present with differing features, depending on the age of onset.

Optic neuropathy occurs when there is damage to the optic nerve. Some cases will have a genetic cause.