057 Neuromyotonia (Isaac's syndrome): a mimic of myositis

Abstract

Background: We present a case of a 49 year old male who presented to the pain team with severe bilateral calf pain and difficulty in walking. This had developed over approximately four months. He had a past medical history of moderately controlled type II diabetes mellitus, hypertension, hyperlipidemia and symptoms of depression and anxiety. Despite the discontinuation of both fibrate and statin and an increase in his analgesia, which included opiates and gabapentin, there was no resolution in his pain. He was referred to the rheumatology department for a further opinion. Method(s): Examination revealed myokimia, mild weakness of knee extension and unexpectedly, some hypertrophy of the gastrocnemius muscles, reduced knee but brisk ankle reflexes. Blood tests revealed an erythrocyte sedimentation rate of 2, C-reactive protein of 2 and creatine kinase of 326. Anti nuclear and extractable nuclear antibodies were negative. An extended myositis antibody panel showed positive Mi-2 antibodies. Voltage gated sodium and potassium channel antibodies were negative. Magnetic resolution imaging of his lower limbs demonstrated bilateral fatty infiltration. Electomyography showed patchy changes in keeping with myopathic change. A PET scan did not show any evidence of malignancy, but did show some diffuse tracer uptake in both lower and upper limbs. Muscle biopsy showed increased variation of fibre size, increased numbers of internalised nuclei and several split fibres and vacuolated fibres - in keeping with myopathic changes. Immunohistochemical staining was negative with no signs of active inflammation. Further staining undertaken at a specialist muscular dystrophy centre showed no further changes with normal dysferlin levels. Result(s): A diagnosis of polymyositis was made based upon the clinical findings, MRI, biopsy, Mi-2 antibody positivity. He was treated with 30mg of prednisolone, 25mg of methotrexate and intravenous immunoglobulin with no improvement in symptoms. He was then referred to neurology who reviewed the findings and suggested a trial of carbamezapine which was discontinued secondary to adverse effects. He was then commenced on lamotrigine, with a resolution in symptoms and reduction in analgesic use. On review of the case, a diagnosis of Isaac's syndrome was made. Conclusion(s): Isaac's syndrome (neuromyotonia) is a rare condition. It presents with muscle stiffness, hyperexcitability of nerve endings, myokimia, excessive sweating and reduced reflexes. The aetiology of the disease remains unascertained, although hereditary and acquired forms have been described. An autoimmune form does exist, associated with Caspr2 antibodies which affect voltage gated potassium channels, although many patients are antibody negative. There is one report of the Isaac's syndrome coexisiting with dermatomyositis. Rheumatologists, pain physicians and neurologists should be aware of this condition as a mimic of polymyositis, particuarly in those with weakness and pain refractory to standard therapies.