63F presents with a two week history of progressive weakness and dysphagia. There is no diurnal variation. On examination she has a violaceous rash over the eyelids and papules over the knuckles. Tone is normal. There is a proximal pattern of weakness bilaterally. Reflexes are normal. A muscle biopsy has been sent off, with your working diagnosis of dermatomyositis. What additional investigation should you consider at this stage?