Isaacs症候群の1例—持続性筋線維活動症候群

A CASE OF ISAACS' SYNDROME:CONTINUOUS MUSCLE FIBER ACTIVITY SYNDROME

　抄録　症例は歩行障害，手挙開排困難を主訴とした34歳女性。全身の筋肉硬直，発汗過多，ミオキミアを認め，筋由来の酵素の上昇，基礎代謝率の亢進，CPK分画にてextrabandにIgA-λの結合を認めた。筋電図所見にて安静時に自発放電を認め，さらに正中神経のブロックで自発放電の消失を示した。上記よりIsaacs症候群の診断にて，carbamazepine 200mgの投与で症状は著明に改善した。治療前の終夜睡眠ポリグラフ法（PSG）ではREM睡眠期でも筋活動電位の抑制はみられなかったが，睡眠構築は正常であった。治療前後でのPSGを比較すると，治療後，筋活動電位の抑制および％stage IVの増加が認められ，全睡眠stageでの持続性の筋活動が睡眠深度へ影響を与えたと考えられた。本症の原因はほとんど解明されていないが，CPKが正常値になっても，extrabandにIgA-λの結合が認められ，原因として免疫の関与も示唆された。

A 34-year-old woman noted difficulty of gait initiation, then delated finger opening and hyper-hidrosis appeared. Her stature was a muscular habitus, and muscle stiffness and myokymia were found in all muscles of the extremities. Her stiffness persisted during sleep. Her calf muscles were large and a contracture was noticed in ankle joints. There was no evidence of wasting and weakness. A remarkable delay in voluntary relaxation of the contracted muscles without percussion myotonia was recognized. Tendon reflexes of lower extrimi-ties were absent.

Laboratory examination revealed elevation of CPK, LDH, myoglobulin, aldolase and basal meta-bolic rate (BMR). An extraband of CPK isoenzyme between MB and MM fraction was observed. The thin layer gel filtration technique and immunofixa-tion technique showed that this extraband was complexes of CPK and IgA, and light chain of the CPK linked IgA was lamda type. All other laboratory tests were normal for the following: urinalysis, ESR, a blood count, liver function, kidney function, glucose, rheumatoid factor, CRP, thyroid function, parathyroid function, serum elect-rolytes, ECG, EEG, cranial CT, without slight ele-vation of IgA, and CSF protein.

In needle EMG and surfase EMG spontaneous discharges were recorded at rest. These discharges consist of normal motor unit potentials, doublets, and triplets in needle EMG. The discharges were markedly reduced after the median nerve block with xylocaine. In needle EMG, myotonic dischage was not observed. Nerve conduction velocities were within normal ranges.

According to these data, she was diagnosed as having Issacs' syndrome (continuous muscle fiber activity syndrome). Carbamazepine, 200mg daily was administrated and showed a dramatic reversal of the symptoms. In laboratory data, CPK, LDH, aldolase, myoglobulin and BMR returned to the normal range. But, we recognized the existence of CPK-linked IgA-lamda. Phenytoin and dantro-lene were administrated, but not so effective as compared to carbamazepine.

Polysomnography was examined before and after treatment. Before treatment, suppression of muscle fiber discharges was not recognized during REM sleep. After treatment, suppression of muscle fiber discharges appeared and % stage was elevated (% stage IV, 3.6%→11.4%). It was suspected that continuous muscle fiber dischages throughout the sleep had an influence on the depth of the sleep stage.

There have been reports of some 40 cases of syndrome of continuous muscle fiber activity. A number of studies using EMG and nerve block tests have indicated the originate in the motor nerve terminals or more proximally. However, the pathogenesis of this disease is unkown. From the existence of CPK-linked IgA-lamda, it was suggest-ed that an immunological abnormality might play a role in this pathogenesis.