Myasthenia Gravis

Key Messages

1. Delay or omission of Myasthenia Gravis (MG) medicines can very quickly result in a significant worsening of the patient’s myasthenic status. 
2. 2MG medicines must be prescribed and administered at the correct frequency and time.
3. Gentamicin can cause clinically significant muscle weakness in myasthenia patients resulting in respiratory depression.  It is therefore contraindicated in MG.
4. Magnesium containing preparations (e.g. some antacids) should be avoided in MG, unless being prescribed specifically to manage hypomagnesaemia. 
5. Discuss any significant changes to corticosteroid doses with neurology, as increased or decreased doses may have an impact on MG. 
6. Pyridostigmine and prednisolone (not enteric coated) tablets can be crushed and mixed with water for patients requiring nasogastric feeding tube.

Myasthenia Gravis

1. 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.
2. Clinical features of MG include difficulty swallowing, speaking or chewing; double vision and ptosis; neck and limb weakness; type 2 respiratory failure.
3. Medicines to manage MG aim to strengthen muscle contractions either by increasing the amount of ACh available at the ACh receptors eg pyridostigmine (Mestinon®) or by dampening the autoimmune response eg prednisolone and azathioprine.
4. Medication incidents resulting in clinically significant exacerbations of MG include both myasthenia and cholinergic crises (see below).

Pyridostigmine

1. It is important that pyridostigmine is administered at exact times. Ensure no missed doses.
2. Where a patient does not have an individual supply of pyridostigmine this must be obtained via pharmacy or on call pharmacist via switchboard if out of hours.
3. 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 (myasthenic crisis)
4. By contrast overdose of pyridostigmine can lead to a cholinergic crisis  
5. Patients awaiting medical review in ED or CAU must continue to take their  pyridostigmine at the times and frequency they are scheduled to do so.  Staff must therefore ensure that such medicines are available on the ward, prescribed and administered at the appropriate time(s).
6. Administration of medication via nasogastric feeding tube is the preferred option in patients who are unable to swallow. Pyridostigmine and prednisolone (not enteric coated) tablets can be crushed and mixed with water.
7. Any delays in passing NG tube, or if NG insertion is not possible, seek senior medical and neurology advice as soon as possible.

Differentiating Myasthenic Crisis from Cholinergic Crises

Medicines Known to Exacerbate MG

1. A number of medicines can potentially cause life threatening worsening of symptoms in patients with MG and should not be given, unless under specialist advice in life saving circumstances.
2. 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.
3. Click here for list of medications that should be avoided or used with caution in MG. Our thanks go to NHS Greater Glasgow & Clyde for allowing us to link to their content.

Aminoglycosides

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

Magnesium

1. Magnesium can inhibit the release of ACh from the neuromuscular junction and hypermagnesaemia (even in patients without MG) can therefore cause symptoms resembling MG. 
2. Magnesium containing preparations should be avoided in MG, unless being used specifically to manage hypomagnesaemia. 
3. Some indigestion remedies contain magnesium, eg Maalox® and Mucogel®, and should not be used in MG patients.
4. Peptac® liquid does not contain magnesium and is therefore safe to use.

Corticosteroids

1. MG patients will often be stabilised on a maintenance dose of prednisolone.  In addition, a steroid-sparing immunosuppressant such as azathioprine may be prescribed. These medicines are essential for the management of MG and should not be stopped or amended unless under specialist neurology advice.
2. Prednisolone regimens in patients with MG are established over a prolonged period to achieve a stable maintenance dose. 
3. Sudden and large increases or decreases in steroid dose can worsen muscle weakness (myasthenic crisis)
4. When the dose needs to be altered significantly for any reason, discuss with a neurologist first to set the most appropriate dose regimen for the individual patient.

Minor Infection

1. Continue regular immunosuppressant drugs.
2. Stable MG patients may not require any change in steroid dose, so do not automatically double steroid dose in the context of infection.

Severe Infection or Sepsis

1. Withhold regular immunosuppressant drugs (and discuss with neurology).
2. Consider increasing steroid dose after consulting with neurology.
3. Patients with stable MG can usually revert to baseline steroid dose once infection is treated (provided <3 weeks on higher dose).
4. Patients with active MG may require slower reduction in steroid dose (consult neurology).

Patients Undergoing Surgical Procedures

1. Patients should continue with regular medication for MG pre- and post-operatively.
2. Discuss on a case by case basis with an anaesthetist and neurology.

Content by Dr Sharon Irvine