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Polymyositis is an inflammatory myopathy of skeletal muscle. Cardiac involvement in the disease was first described in 18991 but has only been studied in detail over the past 20 years. About 10–15% of all patients with polymyositis have a cardiac abnormality as their initial presenting feature, while up to 70% of all those with polymyositis will have some cardiac involvement diagnosed non-invasively during the course of their illness.2 These can manifest either as cardiac failure; electrocardiographic changes, including non-specific ST changes, and varying forms of AV block; myocarditis; valve disease and myocardial ischaemia in the presence of normal coronary vasculature.3
Myasthenia gravis is thought to be an autoimmune disease characterised by the presence of antibodies to the acetylcholine receptor of the motor end plate. Idiopathic forms are rarely associated with polymyositis,4 but a much stronger correlation exists in the presence of a thymoma, particularly in those with myocardial involvement, when anti-titin antibodies are usually present.5 However, thymoma is often less than 1 mm in size and therefore not visible on computed tomographic (CT) scanning.
We describe a 56 year old woman with polymyositis, who initially presented with myocardial involvement and was also found to have antibodies consistent with myasthenia gravis.
A 56 year old woman was admitted to this hospital for insertion of a permanent pacemaker after an episode of dizziness and collapse, secondary to complete heart block.
She had a past history of anterolateral myocardial infarction 10 months previously for which she received thrombolytic treatment in a district general hospital. Treatment was started with aspirin, atenolol, ramipril, and pravastatin after the infarction. Subsequent coronary angiography was normal. At outpatient follow up five months later, she was found to have a creatine kinase (CK) level of greater than 3000 U/l associated with significant leg weakness. Electromyographic (EMG) analysis showed changes consistent with old polio, which had affected the patient as a child. Attempts at muscle biopsy were unsuccessful. A statin-induced myositis was thought likely, and pravastatin was discontinued.
Six months later she presented in complete heart block and at the time of transfer to cardiology in this hospital for pacing, examination showed significant proximal muscle weakness with 3/5 power in both arms, and 2/5 power in the legs; distal muscle power was normal. Neurological examination demonstrated normal sensation but absent knee, bicep and ankle jerks, with bilaterally downgoing plantars.
Investigations showed a CK of 22 800 U/l (CK MB fraction <0.1). The erythrocyte sedimentation rate was mildly raised at 35 mm/1st h; thyroid function was normal; and rheumatoid factor, antinuclear antibodies, antineutrophil cytoplasmic antibodies, and viral screens were negative. Echocardiography showed good LV function with trivial MR; ultrasound of the abdomen was normal; and EMG analysis again showed features consistent with previous poliomyelitis, in addition to severe myositis. Magnetic resonance imaging could not be performed because of the pacemaker. A diagnosis of polymyositis was made and treatment was started with 60 mg/day prednisolone and intravenous immunoglobulin. A muscle biopsy, after 13 days of prednisolone, showed an inflammatory infiltrate with myonecrosis consistent with inflammatory myopathy.
Anti-acetylcholine receptor antibodies and anti-titin antibodies were also positive but CT of the thorax showed no evidence of thymoma and single fibre EMG studies were not supportive of a diagnosis of myasthenia gravis.
The patient was discharged after 20 days on 55 mg/day prednisolone with repeat admissions organised for immunoglobulin every six weeks, and neurological review for her possible myasthenia gravis. Upon discharge, she had 4/5 muscle power with a CK of 1113 U/l.
Subsequent follow up two months after her initial admission showed a CK of <200 U/l and complete restoration of muscle power. She is currently receiving 35 mg/day prednisolone, reducing by 2.5 mg/month.
Our case demonstrates several of the important cardiac features in polymyositis. The initial infarction, which occurred in the presence of only two significant cardiovascular risk factors (hypertension and hypercholesterolaemia), and the subsequent normal coronary angiography suggest the presence of myocardial ischaemia with normal coronary vasculature as described previously.6 It is thought that the ischaemia may be related to small vessel myocardial disease, which would not be evident on angiography.7 However, myocardial infarction in the presence of normal coronary vasculature is not uncommon, and alternative explanations in this example include infarction with clot lysis; infarction secondary to coronary artery spasm; or myocarditis.
Fibrosis of the conducting system of the heart, particularly the AV node, has been previously characterised8 and might have explained the patient's subsequent presentation in complete heart block. As there was no evidence of significant myocardial ischaemia in this patient (CK MB fraction <0.1 and only mildly raised increase in troponin), fibrosis was felt to be the most likely pathological mechanism in this instance.
Arriving at a diagnosis of myasthenia gravis in a patient with muscle inflammation is difficult, as standard investigations such as tensilon tests are inaccurate in the presence of pre-existing muscle weakness, and a negative EMG analysis does not necessarily dismiss the diagnosis.9 The presence of old polio in this patient was a further confounding factor. Equally, symptoms may be disguised by both the pre-existing polymyositis and its treatment, as immunosuppression is a well recognised treatment for both. However, anti-acetylcholine and anti-titin antibodies are highly specific,10 and the association between myasthenia gravis, polymyositis, and cardiac disease, particularly with microscopic thymoma, is well characterised. Although we do not feel we have enough evidence to support a diagnosis of myasthenia gravis at present, we have referred this patient for neurological follow up and review.
This case demonstrates several of the cardiac manifestations of polymyositis. It also highlights the possible neurological associations with the disease, and the difficulties in subsequent diagnosis.
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