Potassium
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Potassium Aggravation of myotonia: case report
A 31-year-old woman developed aggravation of myotonia during treatment with potassium in status myotonicus. The woman had a history of painful, intermittent flares of lower extremity contractions, several times a year. She presented with fatigue after a similar attack, one week before presentation. Her neurological examination was normal. However, she showed low levels of potassium, and received potassium 40mEq [route not stated], resulting in potassium levels of 4.5 mmol/L from an initial 3.2 mmol/L. However, at the time, she complained of severe painful muscle cramps and contractions of lower leg, prominent on the right side. She could not move her leg. Due to the severe and disabling symptoms, she was admitted. Upon admission, a clinical exam revealed inversion and supination of her bilateral feet consistent with dystonic posturing and forceful plantar flexion. Owing to severe pain, a motor exam was not performed; however, she showed normal overall strength. Neurological findings were absent. Subsequent laboratory examinations showed elevated levels of creatine kinase during the first seven days of hospitalisation. An MRI of the right lower leg revealed diffuse mild oedema in the deep and superficial portions of the posterior and anterior compartment musculature. Additionally, relative sparing of the lateral compartment muscles was noted. Slight intermuscular oedema was noted between the medial deep and superficial posterior compartments. She also showed increased T2 signal in the flexor hallucis longus tendon, suggestive of mild reactive tenosynovitis. Mild soft tissue oedema in the superficial adipose layer around the ankle was also noted. An electromyography revealed constant myotonic discharges in the distal leg muscles, corresponding to her painful leg posture. Continuous myotonic discharges were noted in the right medial and lateral gastrocnemius, and right tibialis anterior. The myotonia on the left side was lesser. She underwent cooling and exercise protocols to monitory the right peroneal motor nerve conduction, which did not show changes. Interictal electromyography revealed subclinical myotonic potentials in the right gastrocnemius and tibialis anterior muscles. Further, it was noted that her father, paternal aunt, brother and niece all experienced similar paroxysmal attacks described with stiffness, cramping and weakness. Subsequent testing revealed a Nav1.4-M1592V mutation in her as well as other affected family members. She was thus diagnosed with ’status myotonicus’ [aetiology not stated], along with potassium therapy-related aggravation of myotonia. The woman was treated with acetazolamide and salbutamol [albuterol] for lowering of potassium levels. However, she did not show improvement and on day 5 of admission, mexiletine and lamotrigine were added to her therapy. She was also tried on IV infusion fosphenytoin, however, it was discontinued due to treatment related generalised paraesthesias and tachycardia; her lamotrigine was thus up-titrated
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