A Case of Thyrotoxic Myopathy with Extreme Type 2 Fiber Predominance
A 55-year-old woman was admitted to our hospital with two years of progressive weakness of both legs. Initially, she had difficulty in climbing stairs and rising from a chair, but she felt no problem when she walked on flatland. Both legs weakness was progressed and she lifted up her both legs when going up stairs and walking on flatland at three months before visiting our clinic. Over the previous 1 year she had lost 2 kg in weight and become increasingly nervous, vulnerable to heat and shortness of breath on exertion. She had noticed tremor of the outstretched fingers and hyperhidrosis over the previous four months. However, there were no weakness of upper extremities, sensory symptom, myalgia, dysarthria and dysphagia. Physical examination revealed a mild degree of bilateral exophthalmos and diffuse enlargement of thyroid gland. The arterial blood pressure was 189/93 and the pulse was regular tachycardia and the frequency 118 per minute. Motor examination showed proximal muscle atrophy with MRC grade 4/5 in proximal limbs and grade 4+/5 in distal limbs. She showed Gower sign when trying to stand up from squatting but toe gait and heel gait were intact. Motor power of both upper extremities was normal and deep tendon reflexes were normal. Cranial nerve examination was intact and there was no sensory loss. No fasciculation or myotonic reactions were present.\nThe following laboratory test showed normal results: complete blood cell count, hemoglobin, C- reactive protein, erythrocyte sedimentation rate, serum electrolyte, calcium, urinalysis, blood urea nitrogen, creatinine, chest x-ray and liver function test. Serum creatine phosphokinase (CPK), lactate dehydrogenase (LDH), fluorescent antinuclear antibody (FANA) and rheumatoid factor were also normal. She was diagnosed with hyperthyroidism by thyroid function studies which showed free T4 9.02 (0.7-1.8 ng/dl), T3 680 (87-184 ng/dl) and thyroid stimulating hormone (TSH) 0.05 (0.4-4.1 µIU/ml). In thyroid autoantibodies studies, TSH receptor antibody 31.69 (0-1 IU/L) and antithyroid microsomal antibody 1,478 (0-60 U/ml) were increased. Tc-99m thyroid scan showed diffuse enlargement with evenly increased uptake (40.8%).\nElectromyography showed motor unit potentials of reduced amplitude and short duration (30%) in left vastus lateralis, tibialis anterior, peroneous longus and gastrocnemius muscles with early recruitment, but no spontaneous activity. These findings suggested a myopathic pattern. Muscle specimens were obtained from the vastus lateralis. Sections were stained with the modified Gomori trichrome and processed for the nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR) reaction. Muscle fiber typing was done using the pH 9.4 myofibrillar adenosine triphosphatase (ATPase) reaction and cytochrome C oxidase (COX-2) immunohistochemistry staining. When examined by light microscopy and electron microscopy, size of myofibers was slightly variable and there were a few atrophic myofibers. There were no inflammatory infiltrates or blood vessel abnormalities (). There was extreme predominance of type 2 fibers (fast twitch) in the analysis of NADH-TR reaction, ATPase reaction and COX-2 immunohistochemistry staining ().\nFor treatment of hyperthyroidism and thyrotoxic myopathy, methimazole 15 mg/day was administered orally. After 1 month, hands tremor, shortness of breath, nervous and weakness of both legs were improved. She has continued methimazole treatment presently.