Inborn errors of metabolism (IEMs) are a wide and variegate, mostly recessively inherited group of multi-systemic diseases that often involve the central nervous system (CNS). Onset of symptoms in childhood or adolescence is generally the rule, but presentation in adulthood is not exceptional and should be taken into consideration in the differential diagnosis even in adult patients presenting with complex movement disorders.
Neurologists may face two scenarios in metabolic disorders of adulthood: late-onset metabolic diseases sometimes manifesting with neurological symptoms, including movement disorders or movement disorders developing later in the context of an earlier-onset and previously diagnosed IEM.
The latter circumstance is becoming increasingly frequent as early diagnosis, improvement of standards of care for patients diagnosed in early childhood, and a multidisciplinary approach in the treatment of these disorders, as well as the availability of new therapies, has considerably prolonged the life expectancy of these patients, so that several long-term neurological manifestations due to CNS involvement, including movement disorders, are now being observed with increasing frequency. Movement disorders in IEMs are clinically heterogeneous and include a wide spectrum of manifestations, from dystonia to ataxia, with different patterns and course; for this reason, considering metabolic diseases in the differential diagnosis is of primary importance in adults, especially when presenting with signs and symptoms of involvement of other organs. Most importantly, such diagnoses should not be missed as some of these disorders (for example, Wilson’s disease) are treatable and some of the movement disorders themselves can also be treated symptomatically.