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How to demonstrate fatigueability in myasthenia gravis on neurological examination

How to demonstrate muscle fatigueability (e.g. in Myasthenia Gravis) for doctors clinical exams, medical student finals, OSCES, PACES and USMLE

 

  • In an neurological examination, if you feel the patient may have myasthenia gravis there are various ways of demonstrating this. The techniques below are adjuncts to the full neurological examination which can be found by clicking here. Demonstrating fatigueability on examination will slightly depend on the muscles involved:

 

Ocular myasthenia

  • Patients may have diplopia at rest or it may only come out with use of the eyes
  • Sustained upgaze and repeated blinking will use these muscles. Repeating eye movement testing after this sustained upgaze or repeated blinking will exacerbate any weakness and bring out diplopia and ptosis respectively from use of the relevant muscles

 

Limb  weakness

  • Ask the patient to lift their arms to 90 degrees and check their shoulder abduction and adduction. If this is equal, then ask them to move ONE ARM up and down 20 times to fatigue these muscles
  • Following this, retest shoulder abduction and adduction and compare sides
  • Both abduction and adduction should now be demonstrably weaker than in the unused arm

 

Central/bulbar involvement

  • Test the power of head/neck flexion and extension (fatigue this movement as above)
  • Ask the patient to take a deep breath and count out loud as many numbers as they can. Compare this to your own (i.e. count at the same time)
    • NB. This is considered an indication of respiratory muscle strength but is NOT a substitute for forced vital capacity (FVC) monitoring

 

TOP TIPS

  • In an exam situation if you identify weakness, especially of eyes/face, make sure you look at the chest for the scar of a previous thymectomy
    • If you see a thymectomy scar, always consider MG and test for fatiguability as above
  • Always remember to say you would test the patient’s FVC in suspected myasthenia

 

Now click here for how to diagnosis and manage Myasthenia Gravis or click here for Myasthenia Gravis exam questions

Perfect exam revision for medical student finals, OSCES, MRCP PACES and USMLE