Nerve excitability study – a method to assess the pathophysiology of peripheral nervous system disorders

Abstract

Nerve excitability study is an interesting extension of classical neurography. The principle of the test is to stimulate the nerve with a long-term depolarising or hyperpolarising stimulus, followed by a test stimulus that induces a complex motor or sensory potential of a given amplitude. The analysis of the intensity of the test stimulus used in chronic depolarisation or hyperpolarisation of the nerve fibre allows for the assessment of changes in nerve excitability. The median nerve is stimulated, and it is possible to assess the excitability of sensory or motor fibres. Nerve excitability study is the only method to evaluate the function of neuronal membrane ion channels in vivo. Test results are impacted by the following membrane-related factors: voltage-gated sodium and potassium ion channels, sodium and potassium ATPase, as well as the degree of fibre myelination and the levels of extracellular and intracellular electrolytes. Nerve excitability study yields a result in the form of a series of complex peripheral nerve excitability parameters, such as the relationship between the stimulus and the excitation threshold, rheobase, fibre chronaxie, threshold electrotonus, and reactivation after action potential. Nerve excitability study helps assess the pathophysiology of nerve fibre damage in various disorders of the peripheral nervous system, such as hereditary polyneuropathies, neurodegenerative or metabolic diseases. The test is highly sensitive, e.g. it shows pre-symptomatic excitability changes. It can also be used to monitor treatment outcomes in metabolic and drug-induced polyneuropathies and to search for peripheral markers of central nervous system channelopathies.