Chapter 148 - Metabolic neuropathies and myopathies
Introduction
Among the heterogeneous group of disorders that involve the peripheral nerve, the concept of metabolic peripheral neuropathies is linked in most conditions to the association of a peripheral neuropathy with a more complex sequence of symptoms and signs that go with a metabolic disorder. By metabolic disorder, we currently wish to indicate a disorder in which a metabolic marker is available to clarify physiological and pathogenic mechanisms and is useful for proper definition. Moreover the presence or absence of a peripheral neuropathy in a given disorder can help to better address the correct diagnosis. For example, when faced with a neuroimaging diagnosis of a leukodystrophy, the presence of a peripheral neuropathy together with the neuroimaging distribution and quality of white matter anomalies is useful to restrict the number of conditions to take into consideration for further evaluation and genetic confirmation. Conversely, isolated neuropathy may be the first symptom of an inborn error of metabolism which needs to be identified.
In contrast, metabolic myopathies can generally be distinguished by characteristic clinical signs that help to tackle the proper diagnostic algorithm, such as muscle weakness with myalgia, exercise intolerance, rhabdomyolysis, or ptosis with limitation of eye movements. Additionally, while the morphology of the sural nerve biopsy is not particularly useful in the assessment of a metabolic disorder, muscle biopsy is frequently of paramount importance for evaluating and defining a metabolic myopathy.
Section snippets
Metabolic peripheral neuropathies
A neuropathy is suspected in the presence of distal motor deficit, proprioceptive ataxia, associated with orthopedic deformities. Where there are familial antecedents of Charcot–Marie–Tooth disease, genetic investigations after nerve conduction velocity studies should be the first line. If the neuropathy has an early onset and there is no family history, metabolic investigations are required. Nerve conduction velocity studies contribute to characterize the type of neuropathy: axonal or
Metabolic myopathies
Metabolic myopathies are a heterogeneous group of hereditary muscle disorders caused by specific enzymatic defects of muscle energy metabolism resulting in skeletal muscle dysfunction. Traditionally, these diseases are grouped into abnormalities of glycogen, lipid, purine, or mitochondrial biochemistry. The exact incidence and prevalence of metabolic myopathies is uncertain. Carnitine palmitoyl transferase deficiency is the most commonly identified metabolic cause of recurrent myoglobulinemia
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