NHS Greater Glasgow & Clyde Area Drug and Therapeutics Committee
Greater Glasgow and Clyde Medicines
Medicines Update Acute

Prescribing in Myasthenia Gravis

Key messages

  • Delay or omission of Myasthenia Gravis (MG) medicines can very quickly result in a significant worsening of the patient’s myasthenic status.  Patients’ regular MG medicines should therefore be prescribed without delay.
  • MG medicines must be prescribed and administered at the correct frequency and time.
  • A detailed pyridostigmine Q&A document on StaffNet includes advice on supply and management of Nil by Mouth patients.
  • Gentamicin can cause clinically significant muscle weakness in myasthenia patients resulting in respiratory depression.  It is therefore contraindicated in MG.
  • StaffNet provides a list of medicines which are contraindicated or cautioned in MG.
  • Magnesium containing preparations (e.g. some antacids) should be avoided in MG, unless being prescribed specifically to manage hypomagnesaemia. 
  • It is advisable to discuss any significant changes to corticosteroid doses with neurology, as increased or decreased doses may have an impact on MG. 

Myasthenia Gravis (MG) is a condition characterised by fatigable muscle weakness occurring as a result of an autoimmune response that targets acetylcholine (ACh) receptors at the neuromuscular junction. Medicines to manage MG aim to strengthen muscle contractions either by increasing the amount of ACh available at the ACh receptors (e.g. pyridostigmine, brand name Mestinon®) or by dampening the autoimmune response (e.g. prednisolone and azathioprine).

Medication incidents resulting in clinically significant exacerbations of MG, reported both in NHSGGC and other healthboards, include the following categories:

1) Delayed or missed doses of MG medicines

Delay or omission of MG medicines can very quickly result in a significant worsening of the patient’s myasthenic status; it is therefore vital that such medicines are prescribed and administered without delay.

Pyridostigmine
Pyridostigmine (normally prescribed four times a day) is quickly cleared from the body and omitting just one dose can result in significant muscle weakness within a matter of hours.   Refer to the pyridostigmine Q&A on StaffNet for advice on supply and management of Nil by Mouth (NBM) patients.

Case example 1:  A NBM hospitalised patient missed 3 days of corticosteroid and pyridostigmine doses, resulting in deterioration of his MG; the pyridostigmine Q&A on StaffNet provides guidance on managing this clinical scenario.

Special care: Patients with MG in A&E or acute admission wards.
Patients awaiting medical review in A&E, or an acute admission ward, must continue to take their MG medicines at the times and frequency they are scheduled to do so, without any delay.  Staff must therefore ensure that such medicines are available on the ward, prescribed and administered at the appropriate time(s).

 
2) Medicines known to exacerbate MG

There is a list of medicines (on StaffNet) that patients with MG should not receive, unless under specialist advice in life saving circumstances, since they can potentially cause life threatening worsening of their symptoms.

If there is any doubt about starting a patient with MG on a new medicine, a referral should be made to the clinical pharmacist or to an appropriate member of the neurology team.

Aminoglycosides e.g. gentamicin
Aminoglycoside antibiotics impair neuromuscular transmission and can cause clinically significant muscle weakness, resulting in respiratory depression in myasthenia patients. Gentamicin is therefore contraindicated in MG; amikacin, tobramycin, neomycin and streptomycin are also contraindicated.

Case example 2:  A patient with MG was given a ‘stat’ dose of gentamicin in A&E, without reference to it being contraindicated in MG. Administration of this contraindicated antibiotic potentially contributed to the deterioration of the patient’s condition. 

Magnesium
Magnesium can inhibit the release of ACh from the neuromuscular junction and hypermagnesaemia (even in patients without MG) can therefore cause symptoms resembling MG.

Typically, MG symptoms worsen with parenteral and sometimes oral magnesium administration. Magnesium containing preparations should be avoided in MG, unless being used specifically to manage hypomagnesaemia. 

Some indigestion remedies contain magnesium (e.g. Maalox® and Mucogel®) and should not be used in MG patients. Peptac® liquid does not contain magnesium and is therefore safe to use.

3) Corticosteroid dose changes

MG patients will often be stabilised on a maintenance dose of prednisolone.  In addition, a steroid-sparing immunosuppressant such as azathioprine may also be prescribed. These medicines are essential for the management of MG and should not be stopped or amended unless under specialist neurology advice.

Prednisolone regimens in patients with MG are established over a prolonged period to achieve a stable maintenance dose.  Sudden and large dose increases can cause a direct inhibitory effect on neuromuscular function with a subacute worsening of MG status. Likewise sudden drops in steroid doses will have a deleterious effect on a patient's MG status. Inappropriate corticosteroid dose changes (increases or decreases) could therefore potentially precipitate a myasthenic crisis. Where the dose needs to be altered significantly for any reason, it would always be advisable to discuss with a neurologist first to set the most appropriate dose regimen for the individual patient.

 

 

Published 15/01/2016