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Something for the weak-end? Electromyography appearances of myasthenia gravis
  1. F Britton,
  2. A Osei-Lah,
  3. MP Tighe
  1. Paediatric Department, Poole Hospital NHS Trust, Poole, UK
  1. Correspondence to Dr Mark P Tighe, Paediatric Department, Poole Hospital NHS Trust, Longfleet Rd, Poole, BH15 2JB, UK; mpt195{at}hotmail.com

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Case history

A 13-year-old girl, with a background of stage 4 thoracic neuroblastoma (diagnosed 2003) and residual spastic paraplegia with neuropathic bladder and irregular bowel habit, presented with increasing muscle fatigue and decreased exercise ability over the past 6 months. She reported symptoms including periodic slurring of speech, difficulty swallowing and the inability to smile or blink. These episodes were reported to last between 10 and 90 min. She also presented with ongoing nausea and a reduced appetite.

On inspection, she had an indistinct smile and loss of the left nasolabial fold. Examination of the cranial nerves elucidated a bilateral facial weakness, decrease palate movement and bilateral reduced corneal reflexes. She had normal eye movements, no ptosis and her pupils were equal and reactive to light. Neurological examination of the upper limbs demonstrated weakness in C4–5 but otherwise she had normal power in her arms and hands. Tone, reflexes, coordination and sensation were normal.

In light of her history of neuroblastoma, an urgent MRI was arranged which showed no evidence of tumour recurrence or other structural abnormality to explain the new onset of neurological symptoms. Other investigations included a normal blood count, renal and liver function, and tumour markers and Lyme serology were negative.

Following this, the patient had an electromyography including repetitive nerve stimulation (RNS) which showed evidence of generalised neuromuscular junction dysfunction suggestive of myasthenia gravis (MG) (figure 1).

Figure 1

Three-hertz repetitive nerve stimulation in myasthenia gravis. Right ulnar nerve at the wrist, recording from the right abductor digiti minimi. 31% decrement (white arrow) of compound muscle action potential (CMAP) at rest. The area/amplitude of the fourth CMAP response is 31% less than that of the first CMAP in the train of 10. Decrement of 10% or more in this muscle is abnormal.

Questions

  1. A decremental response on RNS is an indicator of MG. What is the most likely cause for this?

    1. Antibodies blocking the acetylcholine receptors on the postsynaptic membrane

    2. Antibodies blocking the calcium receptors on the presynaptic membrane

    3. Depleted acetylcholine stores in presynaptic vesicles

    4. Overproduction of acetyl cholinesterase in the postsynaptic membrane

  2. In addition to electrophysiological tests, which of these would be of least diagnostic use for MG?

    1. Tensilon test

    2. Muscle biopsy

    3. CT/MRI of thorax

    4. Anti-acetylcholine receptor (AChR) and anti-muscle-specific tyrosine kinase (MuSK) antibody level

  3. Which of the following is used as first line in the symptomatic management of a child newly diagnosed with MG?

    1. Immunosuppression

    2. Thymectomy

    3. Acetylcholinesterase inhibitors

    4. Plasmapheresis±immunoglobulins

  4. Which of the following is not a recognised form of MG in children?

    1. Transient neonatal MG

    2. Congenital MG

    3. Juvenile MG

    4. Relapsing-remitting MG

Answers are on page ▪▪▪

ANSWERS TO THE QUESTIONS ON PAGE ▪▪

Question 1: The correct answer is A

MG is an acquired autoimmune disease with antibodies to the nicotinic AChR at the neuromuscular junction and less commonly to MuSK.1 This leads to muscular weakness with easy ‘fatigability’, which is worse on exercise and improves with rest.

Question 2: The correct answer is B

Approximately 85% of patients with generalised MG have antibodies to the AChR, as with this case, and 40%–70% to MuSK.2 The Tensilon test is highly sensitive (95%) although rarely carried out as it has the potential to cause life-threatening bradycardia.3 A CT/MRI scan is recommended in all patients diagnosed with MG, to assess for an underlying thymoma.3 A muscle biopsy is used in the diagnosis of myopathies and not in neuromuscular junction disorders such as MG.

Question 3: The correct answer is C

Acetylcholinesterase inhibitors (AChI), such as pyridostigmine, provide symptomatic relief and are first-line treatment, although they should be used with caution in patients who have antibodies to MuSK as they may show a hypersensitivity response.4 Corticosteroids or steroid-sparing agents may be used in more severe MG or in those who have failed to respond to AChI, whereas intravenous immunoglobulin and plasma exchange are reserved for rapidly deteriorating MG and for the treatment of a myasthenic crisis.4 A thymectomy is indicated if a thymoma is present; however, it has also been shown to be beneficial in the absence of thymoma. An evidence-based review of 21 cohort studies identified 18 studies showing clinical improvement in patients with MG who underwent thymectomy compared with conservative management.5

Question 4: The correct answer is D

Approximately 10% of all cases of MG begin as juvenile myasthenia, which is a lifelong condition.6 Transient neonatal MG occurs in 10%–15% of babies born to mothers with MG and disappears without recurrence in the first few weeks of life.6 The most rare form is congenital MG involving specific genetic mutations that are inherited recessively; symptoms appear in the first few years of childhood and they do not go into remission.7

References

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Footnotes

  • Contributors All authors contributed equally to the final report.

  • Competing interests None declared.

  • Patient consent Parental/guardian consent obtained.

  • Provenance and peer review Not commissioned; internally peer reviewed.

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