Myasthenia gravis: Symptoms, diagnosis and similarities with other conditions

What is myasthenia gravis? 

The prevalence of the condition is estimated at 50 to 200 cases per million inhabitants in 2015. It can occur at any age, between 6 months and 80 years old. In 60% of cases, myasthenia gravis occurs in young adults under 40.

Myasthenia gravis is a rare autoimmune disease that affects neuromuscular junctions. It causes muscle weakness which gets worse with physical effort. It is also called autoimmune myasthenia gravis.

The symptoms result from an unexplained reaction of the immune system turning against the body and targeting neuromuscular junctions.

>> To learn more about myasthenia gravis, its symptoms and treatments, take a look at the fact sheet! <<

How is myasthenia gravis diagnosed?

The diagnosis of myasthenia gravis is established after various examinations, including:

• A clinical examination during which the doctor can assess the symptoms: visual disturbances, chewing disorders, weakness in the limbs (arms, legs). Weakening of the upper eyelid with limited eye movement and partial paralysis of the face, or the association of dysphonia (hoarse voice) with weakness of the neck muscles are often signs of the disease;
  
• Physical tests to assess muscle strength and detect worsening of the symptoms during exercise;
  
• Observation of the variability of symptoms with periods of aggravations lasting from a few weeks to a few months, and periods of improvement;
  
• Blood tests, detecting the presence of anti-acetylcholine receptor antibodies (molecules produced by the immune system which bind to the acetylcholine receptors and prevent acetylcholine from carrying the message from the nerve to the muscle) which provoke the symptoms;
  
• The improvement of ocular symptoms following the injection of an anticholinesterase, allowing to counteract the impact of the immune system.

To establish the diagnosis, the doctor usually performs a differential diagnosis. It is a way of differentiating a disease from other conditions which have similar symptoms or characteristics.

What other conditions should be considered during a differential diagnosis of myasthenia gravis? 

There are two main types of conditions to be considered when diagnosing myasthenia gravis: conditions mimicking autoimmune myasthenia gravis (causing similar symptoms) and other myasthenia gravis syndromes (of a different origin than myasthenia gravis).

Conditions causing symptoms similar to those of myasthenia gravis 

Weakness in the eye muscles can be present in different diseases, so carrying out a differential diagnosis may be useful. Drooping of the eyelid can also suggest a blepharospasm which provokes facial dystonia (lowering of the eyebrow) occuring in both eyes, or a hemispasm, which affects only one side of the face.

It is also important to differentiate myasthenia gravis from Claude Bernard Horner syndrome, which is characterised by the combination of ptosis (weak upper eyelid) and mysosis (raised lower eyelid).  

In Basedow's disease (increase in the size of the thyroid), the ocular involvement is similar to that present in myasthenia gravis. It is differentiated by the presence of reddening, inflammation and deformation of the eye, which seems larger and more open (due to inflammation of the muscles).

It may also be necessary to make a differential diagnosis of Guillain Barré syndrome. The symptoms appear later in patients with myasthenia gravis. The two diseases can be differentiated by performing an electroneuromyogram to detect the type of disturbance in the electrical activity of nerves and muscles.

Drooping eyelid may also simply be due to the patient's advanced age.

In the case of myasthenia gravis, in the absence of anti-acetylcholine receptor antibodies, the main differential diagnosis to be made is that of ocular myopathy. Its main characteristic is that the development of the disease is progressive, without fluctuation, and lasts for several years.

Other myasthenic syndroms 

Myasthenia gravis is also to be distinguished from Lambert-Eaton myasthenic syndrome (LEMS). This condition provokes weakness in the lower limbs, as well as ocular symptoms, such as a slight weakening of the upper eyelid, which can suggest myasthenia gravis. The diagnosis leans towards LEMS when the patient also has a dry mouth. An electromyogram (examination of the electrical activity of the muscles) shows a characteristic abnormality of LEMS.

Myasthenia gravis should also be distinguished from congenital myasthenic syndromes which appear in children, the origins of which can be various: genetic, toxic, iatrogenic (side-effects from one or several medications).

The diagnosis of botulism should also be ruled out. This disease is caused by a bacterium named “clostridium botulinum”. The onset of the symptoms is rapid, between 12 and 24 hours, unlike that of myasthenia gravis. It also causes digestive disorders, as well as the presence of toxins in the blood.

Myasthenia gravis can be caused by taking certain medicines, such as penicillamine (anti-rheumatic), chloroquine, or hydroxychloroquine (an anti-malaria drug also used to treat lupus). In more than 70% of cases, myasthenia gravis resolves when treatment is over. In other cases, myasthenia gravis continues to progress.

The diagnosis of myasthenia gravis requires the opinion of a neurologist or other specialists.

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