Myasthenia Gravis Due to Thymoma
Myasthenia gravis is a neuromuscular condition that can be caused by a thymoma - a tumour on the thymus gland.
In this article we explore the treatment for myasthenia gravis in more detail, explaining when medical mistakes might amount to medical negligence.
What is myasthenia gravis?
Myasthenia gravis can cause double vision (known medically as diplopia), drooping eyelids (known medically as ptosis) and swaying whilst walking. There can also be some weakness of the jaw, something which is often noticed when eating.
Diagnosing myasthenia gravis
If myasthenia gravis is expected, the patient should undergo an edrophonium test (Tensilon test). If myasthenia gravis is present, the test will be positive.
Myasthenia gravis is a neuromuscular condition where the neurotransmitter acetylcholine cannot function properly because of the presence of antibodies, which block the acetylcholine receptor in the neuromuscular junction. This leads to muscle weakness.
Edrophonium is an acetylcholinesterase inhibitor which prevents the breakdown of the neurotransmitter acetylcholine, thus attenuating the effects of myasthenia gravis. A positive edrophonium test is therefore suggestive of myasthenia gravis.
If a patient's test is positive, blood tests must be taken to check for acetylcholine receptor antibodies. A CT scan of the thorax must also be carried out to see if there is a problem with the thymus gland.
The thymus gland is linked to the production of antibodies. If the gland becomes enlarged, or a tumour develops on the gland, it can lead to myasthenia gravis. A tumour on the thymus gland is a condition known as thymoma. It is often benign, although in some cases the mass will be cancerous.
Removing the thymus gland
If an enlarged thymus is found and the test results are positive, medical practitioners can be sure of a diagnosis of myasthenia gravis.
The patient may then be recommended for a thymectomy, where the thymus gland is surgically removed. The patient should be properly consented for the procedure, during which he/she must be told of all the risks. Consent should only be obtained by a medical practitioner competent to perform the procedure to which consent is being obtained.
The serious or frequently occurring risks of a thymectomy are:
- Infection
- Bleeding
- Deep vein thrombosis
- Pulmonary embolism
- Incomplete removal of the thymus gland
- Possibility of requiring a blood transfusion
- Nerve injury
It is important to note that a thymectomy may not improve a patient's myasthenia gravis, particularly if a thymoma is to blame. However, a thymectomy may be required anyway to stop the spread of the tumour to other parts of the body. For patients who cannot have surgery, either because of other illness of the extent of the thymoma, consideration may be given to radiotherapy and/or chemotherapy.
Phrenic nerve injury
Some of the nerves that can be injured are the phrenic nerves. The phrenic nerves supply the nerve supply to the diaphragm. Damage to these nerves can reduce the diaphragm function, which can cause significant impairment to breathing capacity, particularly when lying down.
With thymoma patients, it is important to obtain pre-operative pulmonary function tests, as thymic tumours can extend along the phrenic nerves, which can be damaged during the removal (or 'resection').
There should be very close pre-operative multidisciplinary care, especially between the cardiothoracic surgeon and the neurologist. If it is felt there is a clinical need for pre-operative anti-myasthenia treatment this must be carried out. Otherwise the patient will be at risk of respiratory failure after the operation, which will require long-term mechanical ventilation.
Recognising phrenic nerve injury
If the phrenic nerves are injured during a thymectomy, the patient will struggle to breathe when he/she comes round from the anaesthetic. The problem will be notably worse when the patient lies down. The patient will therefore depend upon intubation (where a tube is inserted into the trachea) or non-invasive ventilation (NIV), where breathing support is provided through a mask or other device.
Without ventilation support, the patient will experience rapid oxygen desaturation, a high respiratory rate and abnormal gas levels in the blood. Specifically, the carbon dioxide and oxygen levels will be unusual, representing profound alkalosis with severe hypoxaemia. In plain English, this means the lungs are working extremely hard to keep the oxygen level up, and even then are failing to do that. This indicates severe respiratory dysfunction.
If a patient is suffering such problems after a thymectomy, phrenic nerve damage should be immediately suspected. To verify the injury, medical practitioners should order a fluoroscopy, which is an x-ray examination that evaluates whether or not the movement of the diaphragm is normal. In the meantime, the patient should be treated with non-invasive ventilation.
Medical negligence
Phrenic nerve damage can be highly distressing as it causes difficulty breathing. This can continue after being discharged from hospital, and the patient may depend upon ventilation support in the long-term. This can hinder tasks such as walking, sleeping and gardening.
But could this outcome have been avoided, or does phrenic nerve damage amount to medical negligence? The answer is dependent upon the circumstances.
Firstly, the patient should have been advised about the risk before having surgery. If there was a failure to obtain informed consent, there could be a case of negligence.
Secondly, medical practitioners should perform a neurological assessment prior to a thymectomy. If the phrenic nerves are thought to be in danger, treatment should be given before the operation. If there is a failure to assess the patient's respiratory issues before the operation, there could be a breach of duty.
If patient's phrenic nerve damage is not diagnosed straight away, he/she will endure unnecessary suffering due to their breathlessness. However, this may not have any effect on the long-term outcome, and so may not amount to negligence.
There may also be other examples of negligence throughout the course of treatment. For instance, a patient who presents with neurological symptoms of myasthenia gravis should be quickly investigated for thymoma. If there is a delay in diagnosis, the tumour may spread across the body, making it harder to treat.
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