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THE BARE ESSENTIALS
Myasthenia gravis and other neuromuscular junction disorders
  1. Saiju Jacob1,
  2. Stuart Viegas2,
  3. Daniel Lashley3,
  4. David Hilton-Jones4
  1. 1
    Specialist Registrar in Neurology, Queen Elizabeth Neurosciences Centre, Birmingham, UK
  2. 2
    Specialist Registrar in Neurology, Wessex Neurological Centre, Southampton, UK
  3. 3
    Clinical Research Fellow, Department of Clinical Neurology, John Radcliffe Hospital, Oxford, UK
  4. 4
    Consultant Neurologist, Department of Clinical Neurology, John Radcliffe Hospital, Oxford
  1. Correspondence to Dr Saiju Jacob, Department of Neurology, Queen Elizabeth Neurosciences Centre, University Hospitals of Birmingham, B15 2TH, UK; saiju.jacob{at}nhs.net

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Myasthenia gravis is the most common autoimmune disease affecting the neuromuscular junction and is characterised by painless fatigable muscle weakness. It is caused by autoantibodies against neuromuscular junction proteins, either the nicotinic acetylcholine receptor (AChR) or the muscle specific tyrosine kinase (MuSK). Mutations in neuromuscular junction proteins cause congenital myasthenic syndromes. Other antibody mediated conditions affecting the neuromuscular junction include Lambert Eaton myasthenic syndrome and neuromyotonia.

Epidemiology of myasthenia gravis

  • Myasthenia gravis affects approximately 100 patients per million population.

  • It has a bimodal age of onset—early and late onset myasthenia gravis.

  • Early onset myasthenia gravis typically affects women less than 40 years of age whereas the later onset form is more common in older men.

  • With improved diagnosis and survival, the prevalence is increasing, especially in the elderly.

Neuromuscular physiology (fig 1)

Figure 1

The neuromuscular junction and the proteins involved in neuromuscular transmission. Several of the proteins at the neuromuscular junction are targets for autoimmune disorders (AChR and MuSK in myasthenia gravis, VGCC in Lambert–Eaton myasthenic syndrome and VGKC in neuromyotonia). Genetic mutations can affect several of these proteins (AChR, Rapysn, MuSK, Dok-7, etc) causing congenital myasthenic syndromes. ACh, acetylcholine; AChE, acetylcholinesterase; AChR, acetylcholine receptor; MuSK, muscle specific tyrosine kinase; VGCC, voltage gated calcium channel; VGKC, voltage gated potassium channel; VGSC voltage gated sodium channel.

  • Acetylcholine is the key neurotransmitter at the interface between the presynaptic nerve terminal and postsynaptic muscle membrane.

  • Nerve impulses reaching the synapse open voltage gated calcium channels (VGCC) causing influx of calcium ions. This induces fusion of the acetylcholine (ACh) vesicles to the synaptic membrane and release of neurotransmitter.

  • Binding of acetylcholine to nicotinic AChRs at the postsynaptic membrane leads to short lived openings of their intrinsic ion channels. The resulting cation entry depolarises the muscle membrane locally; if that reaches a critical threshold, it opens voltage gated sodium channels, generating an action potential …

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