MG - Definition
An autoimmune disease mediated by antibodies to nicotinic acetylcholine receptors interfering with neuromuscular transmission via depletion of working post-synaptic receptor sites.
MG - Presentation
Increasing muscular fatigue – muscle groups are affected in the following order – extra-ocular, bulbar (for swallowing and chewing), face, neck, limb girdle and finally the trunk.
Initially you should look for ptosis, diplopia, myasthenic snarl on smiling and the peek sign (after brief opposition to gentle sustained lid closure, the lids separate (peek) to show white sclera).
In addition when the patient is asked to count for 50 the voice gradually deteriorates (although rarely the first presentation of MG).
MG - Weakness Worsened by . . .
Pregnancy, hypokalaemia, infection, over-treatment, change in climate, emotion, exercise, gentamicin, opiates, tetracycline’s, quinine, procainamide and β blockers.
MG - Associations
If under 50 years of age it is more common in women and associated with other autoimmune diseases (e.g. rheumatoid arthritis or lupus) and thymic hyperplasia.
If over 50 years of age it is more common in men and is associated with thymic atrophy or a thymic tumour.
MG - Differential
Polymyositis, systemic lupus erythematous, Takayasu’s arteritis (fatigability of the extremities), botulism or other myopathies.
MG - Investigations
Tensilon test – prepare 2 syringes – 1 with 10mL edrophonium and 1 with 0.9% saline. Give 20% of each IV as a test dose and wait 30 seconds before giving the rest of each syringe. The test is positive if edrophonium improves muscle power in around a minute.
Antibodies – anti-acetylcholine receptor antibodies will be present in 90% of patients.
Neurophysiology – decremental muscle response to repetitive stimulation ± single fibre jitter.
Imaging – CT of the thymus to look for either hyperplasia (under 50’s) or atrophy (over 50’s).
MG - Management
Symptom control – anticholinesterase e.g. 60-120mg Pyridostigmine PO up to 6 times daily. Causes cholinergic side effects – increased salivation, lacrimation, sweats, vomiting or miosis.
Immunosuppression – treat relapses with Prednisalone. Give osteoporosis prophylaxis with long term corticosteroid use. Other therapies can be added to prednisolone e.g. Azathioprine or weekly Methotrexate.
Thymectomy – consider if onset before 50 years of age and disease not controlled by drugs. Expect remission in 25% and worthwhile benefit in a further 50%.
Weakness of the respiratory muscles during a relapse can be life-threatening. Monitor forced vital capacity and if declines ventilator support may be required.
Treat the relapse with plasmapheresis or IV IgG and identify and treat the cause e.g. infection or medications.
Lambert-Eaton Myasthenic Syndrome
Can be paraneoplastic (from small cell lung malignancies) or autoimmune.
Unlike myasthenia gravis it presents with gait abnormality before eye signs, autonomic dysfunction (dry mouth, constipation or impotence), hyporeflexia and weakness that improves with exercise, less response to edrophonium, antibodies to the pre-synaptic calcium channels.
Treatment – IV immunoglobulin. Do regular CXRs as may precede cancer by >4 years.
Type 1 Neurofibromatosis
An autosomal dominant disorder with a prevalence of 1 in 2500, a male to female ratio of 1:1 and no racial predilection.
Type 1 NFM - Clinical Features
Café au lait spots – flat, coffee coloured patches of skin seen in the first year of life increasing in size and number with age (do not predispose to malignancy).
Freckling – typically in skin folds presents by the age of 10.
Dermal neurofibromas – small, violaceous nodules, gelatinous in texture which appear at puberty and may become papillomatous.
Nodular neurofibromas – arise from nerve trunks, firm and clearly demarcated and if pressed can give rise to paraesthesia.
Lisch nodules – small, harmless regular brown lesions on the iris.
Type 1 NFM - Diagnosis
Requires 2 of the following - >6 café au lait spots, >2 neurofibromas, frecking in the axillary or inguinal regions, optic glioma, >2 Lisch nodules, distinctive osseous lesions typical of neurofibromatosis type 1 or a first degree relative with the disorder.
Type 1 NFM - Complications
Occur in 30% - mild learning disability, local effects of neurofibromas - nerve root compression, GI bleeds or obstruction, bone cystic lesions, scoliosis, pseudoarthritis, hypertension from renal artery stenosis or phaeochromocytoma.
Malignancy occurs in 5% - optic glioma or sarcomatous change in neurofibromas. The risk of epilepsy is also increased.
Type 1 NFM - Management
Yearly blood pressure measurement and cutaneous survey. Dermal neurofibromas are unsightly and can catch on clothing so if troublesome a few can be excised.
Type 2 Neurofibromatosis
Café au lait spots – but fewer than in NF1.
Bilateral vestibular schwannomas (aka acoustic neuromas) are characteristic and cause sensorineural hearing loss by 20 years of age (they may also cause tinnitus and vertigo). The rate of tumour growth is variable and unpredictable. The tumours are benign but cause problems by pressing on nearby structures or by raising ICP.
Type 2 Neurofibromatosis
Hearing tests from an early age in affected families with MRI brain if an abnormality is detected.
A clear MRI at 30 years indicates the gene has not been inherited.