Peripheral Nerve Hyperexcitability Syndromes
Peripheral nerve hyperexcitability (PNH) syndromes are an uncommon group of conditions characterised by spontaneous discharges from motor nerve fibres. These are largely immunological (sometimes secondary to neoplasm like thymoma), and some forms are know
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26.1 Introduction PNH syndromes are caused by spontaneous discharges originating from the motor fibres, which lead to increased activity of the muscles. The term is broadly interchangeable with cramp-fasciculation syndrome (CFS), myokymia, neuromyotonia (NMT), Isaacs’ syndrome and Morvan’s syndrome. Antibodies directed against voltage-gated potassium channels (VGKC) located in the terminal segments of the peripheral nerves seem to be pathogenic for many of these diseases. A proportion of patients experience sensory nerve involvement and autonomic dysfunction, e.g. hyperhidrosis, and develop central nervous system (CNS) features such as mood changes, sleep disorders and hallucinations. Immune-mediated PNH can be associated with other autoimmune diseases and also occur as a part of paraneoplastic syndrome secondary to malignancies. Some features of PNH can be seen secondary to peripheral neuropathy and degeneration of anterior horn cells. A small subgroup of PNH is known to be genetic in origin and hence familial. It is important for clinicians to recognise PNH, as these patients can require long-term immunosuppression and screening for underlying tumours (Skeie 2010; Hart et al. 2002; Küçükali et al. 2015; Maddison 2006). Based on aetiology, PNH syndromes can be classified into following subtypes as mentioned in Table 26.1.
26.2 Epidemiology The syndrome of continuous muscle fibre activity was first described by Isaac in 1961. In 1965, Mertens and Zschocke described the term ‘neuromyotonia’ (NMT). In 1993, Newsom–Davis and Mills described the immunological associations of NMT. Morvan described a syndrome similar to that observed by Isaac, but these patients had additional features of encephalopathy. He used the term ‘fibrillary chorea’, but due to electrophysiological evidence of spontaneous discharges, this terminology became unpopular (Gutmann and Gutmann 2004). Overall, PNH syndromes © Springer Nature Singapore Pte Ltd. 2018 S.V. Khadilkar et al., Neuromuscular Disorders, https://doi.org/10.1007/978-981-10-5361-0_26
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26 Peripheral Nerve Hyperexcitability Syndromes
Table 26.1 Etiological classification of PNH syndromes PNH primary, immune mediated (often VGKC) or paraneoplastic
Secondary PNH – PNH is associated with changes of denervation
Genetic and hereditary diseases
Primary/idiopathic: CFS, Isaacs’ syndrome and Morvan’s syndrome Associated with other autoimmune disorders: MG, SLE, CIDP, autoimmune thyroid disease and rheumatoid arthritis Paraneoplastic syndromes: Thymoma, small-cell lung carcinoma, lymphoma and other haematological malignancies [Note: Satoyoshi syndrome is an idiopathic autoimmune disorder that can be included in PNH spectrum] Peripheral neuropathy: CIDP, GBS, entrapment neuropathy like CTS, radiation plexopathies/neuropathies, radiculopathy Anterior horn cell degeneration: ALS, SMA Toxins: gold, alcohol, snake venom, insecticide [Note: facial myokymia can occur secondary to brainstem neoplasm or demyelination] VGKC (KCNA1) mutations Familial episodic ataxia 1(EA1) Here
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