It may be rare, but myasthenia gravis can have a profound impact on quality of life, and rates are rising, as neurologist Dr Fiona Chan explains.

Dr Fiona Chan, a neurologist at Inner West Neurology and Concord Hospital in Sydney says it’s important for GPs to have myasthenia gravis (MG) on their radar as there’s been a 3-fold increase in cases since the 1950s.

In 2009 a study looking at the PBS data on prescription rates for medications for myasthenia gravis (MG) concluded that the incidence was 24.9 per million person years and the prevalence was about 10 to 20 per 100,000.

MG is a chronic disease that causes significant morbidity, and mortality in severe cases if it’s not managed. It can have a major impact on quality of life, and although rare, it has a high economic burden.

“It has implications at an individual and societal level, where it costs patients time away from work. It costs society in terms of lost productivity and finally the treatment costs to the health care system,” Dr Chan says.

Myasthenia gravis has a bimodal distribution and is generally seen in young females and in older males, she says.

There are two different forms – congenital/inherited and acquired/immune mediated. The congenital version is extremely rare so GPs are more likely to see immune mediated cases.

Dr Chan says this form is an “autoimmune disorder of the post synaptic neuromuscular junction, which interrupts the transmission of messages from nerves to muscles, patients present with weakness and fatigability. I tell my patients it’s a serious disorder because if the nerves don’t speak to the respiratory muscles, then they may require a ventilator to assist with breathing”.

When might you suspect MG?

Clinical presentations can range from mild to severe, and traditionally MG is divided into two broad categories – purely ocular versus generalised, which means it involves the limbs, the bulbar (swallowing and speech) muscles and the respiratory system.

“There is a wide variety of clinical features and the distinction is not absolute, but there are a few clinical features that should make you think about myasthenia gravis”.

These include:

• Fluctuating ptosis, can be symmetric or asymmetric
• Fatiguing power of the limbs
• Head drop
• Bulbar speech and swallowing difficulties.

Examination tips and tricks

Ocular

• Ask the patient to face you but look up (without lifting their head) for one minute. If the ptosis is worse then this could be a sign of fatigable ptosis
• MG should never affect the pupils. If the pupils are affected, consider alternative causes of ocular palsies (e.g Horner’s syndrome).
• Perform the ice test : place some ice (either an ice pack or within a surgical glove). and place it over the ptotic eye. In MG, this test may improve the ptosis.

Limb

• Do the ‘chicken wing test’ where the elbow is flexed and the patients flaps that arm up and down, approximately 50 times and you compare their pre and post exercise. strength. The patient may be tired, but the strength should not change.

Red flags

• A patient with shortness of breath or difficulty swallowing, with previous presentation of ptosis and diplopia.
• Cardiac and respiratory causes excluded.

Diagnostic tests in GP setting:

• • Auto-antibodies: acetylcholine-receptor antibodies, muscle specific tyrosine kinase antibody. Between 50% and 70% of purely ocular MG (with no systemic involvement) can be seronegative, returning negative results for AcHr and MuSK antibodies.
  • If antibodies return positive, request for CT chest to review for a thymoma while awaiting neurologist review. Approximately 10% of patients with MG have a thymoma (note that removing the thymoma will not resolve myasthenea gravis).
  • Some patients may have other autoimmune conditions like thyroid disorders, so thyroid function tests and thyroid autoantibodies may be required.

Urgency of referrals

Dr Chan says there are red flags that require emergent intervention, such as head drop with weak neck muscles, or breathing difficulties. These patients should be advised to present to the Emergency Department, highlighting your concerns of MG.

In other cases, it is reasonable to refer to a neurologist with the case triaged accordingly.

However, even patients with purely ocular MG should be monitored closely after the initial presentation to ensure there is no systemic involvement, in particular shortness of breath or dysphagia.

It is important to note on the referral if symptoms are affecting the head and neck so the neurologist is aware.

Myasthenia Gravis (MG), Multiple Sclerosis (MS) or Motor Neurone Disease (MND)?

When it comes to telling myasthenia gravis apart from MS or MND, Dr Chan says these are three quite distinct conditions. “MG is a neuromuscular junction disorder, MND an anterior horn cell disorder and MS is a disorder purely of the central nervous system.”

Diagnosis depends on the presenting history and clinical examination.

“If you have a young female patient, who has a history of a relapsing remitting clinical symptoms such as ‘I woke up one day and my whole right leg was weak and after five days, it improved,’ and then a few months later, a discrete neurological episode involving sensory, motor or visual deficits occurs, then you would consider Multiple Sclerosis. An MRI brain or spine scan will be able to clarify that.”

“If a patient presents with a sub-acute onset of a foot drop, for example, that has been persisting for a few months and it doesn’t wax and wane and you find a mixture of both upper motor neuron features and lower motor neuron features, then you start thinking, okay, perhaps we’re dealing with motor neuron disease.”

By contrast myasthenia gravis is a bilateral disorder which can be asymmetric with a history of fluctuations, Dr Chan says.

Treatment

Dr Chan recommends patients remain active within the limits of their condition. There are no supplements that neurologists would routinely recommend.

A number of medications can exacerbate myasthenia and should be largely avoided in MG patients unless discussed with their treating specialists and then carefully monitored. These are described in detail here.

Neurologists often use immunosuppressive medications that can be delivered orally or via infusion. The mainstay of treatment is with initial instigation of oral prednisolone.

“GPs need to be aware that MG patients do not react well to sudden and significant changes to their steroid regime, be it an increase or decrease. In patients who are newly commenced on oral prednisolone, if not done carefully with low doses, this can precipitate a myasthenic crisis where patients develop severe breathing and swallowing difficulties culminating in an intensive care admission. If there is any need to alter the steroid dose, it is best to discuss this with the treating neurologist.”