Neurometabolic disorders
Authors: McShane Tony , Clayton Peter , Donaghy Michael , Surtees Robert 
Published: January 2012

Various disorders result from genetically determined abnormalities of enzymes, the metabolic consequences of which affect the development or functioning of the nervous system. The range of metabolic disturbances is wide, as is the resultant range of clinical syndromes. Although most occur in children, some can present in adult life, and increasing numbers of affected children survive into adult life. In some, specific treatments are possible or are being developed. The last 20 years has seen a considerable expansion in our understanding of the genetic and metabolic basis for many neurological conditions. Particular clinical presentations of neurometabolic disorders include ataxias, movement disorders, childhood epilepsies, or peripheral neuropathy. Detailed coverage of the entire range of inherited metabolic diseases of the nervous system is available in other texts (Brett 1997; Scriver et al. 2001; Menkes et al. 2005).

Treatment is possible for some metabolic diseases. For instance, the devastating neurological effects of phenylketonuria have been recognized for many years. Neonatal screening for this disorder and dietary modification in the developed world has removed phenylketonuria from the list of important causes of serious neurological disability in children. This success has led to new challenges in the management of the adult with phenylketonuria and unexpected and devastating effect of the disorder on the unborn child of an untreated Phenylketonuria mother. More recently Biotinidase deficiency has been recognized as an important and easily treatable cause of serious neurological disease usually presenting with early onset drug resistant seizures. This and some other neurometabolic diseases can be identified on neonatal blood screening although a full range of screening is not yet routine in the United Kingdom. More disorders are likely to be picked up at an earlier asymptomatic stage as the sophistication of screening tests increases (Wilcken et al. 2003; Bodamer et al. 2007).

Although individual metabolic disorders are rare, collectively such disorders are relatively common. In reality most clinicians will see an individual condition only rarely in a career. Furthermore, patients with certain rare conditions are often concentrated in specialist referral centres, further reducing the exposure of general and paediatric neurologists to these disorders. A recent study into progressive intellectual and neurological deterioration, PIND, gives some information about the relative frequency and distribution of some childhood neurodegenerative diseases in the United Kingdom (Verity et al. 2000; Devereux et al. 2004). Although primarily designed to identify any childhood cases of variant Creutzfeldt- Jakob disease, the study also provided much information about the distribution of neurometabolic disease in children in the United Kingdom. The commonest five causes of progressive intellectual and neurological deterioration over 5 years were Sanfilippo syndrome, 41 cases, adrenoleukodystrophy, 32 cases, late infantile neuronal ceroid lipofuschinosis, 32 cases, mitochondrial cytopathy, 30 cases, and Rett syndrome, 29 cases. Notably, geographical foci of these disorders were also found and correlate with high rate of consanguinity in some local populations.