In a series of 48 patients with myasthenia gravis the authors have determined in 14 instances muscle LDH activity, isoenzyme pattern, the MDH/LDH ratio, the percentual ratio of M subunits, and in 60 instances total serum LDH activity and isoenzyme pattern. Slight increase of serum LDH activity, decreased muscular LDH activity (with increased MDH/LDH ratio), increased serum LDH1, LDH2 and LDH3, the reduction of muscular LDH5 and M subunits are assumed to be the consequence of muscle atrophy accompanying myasthenia in some cases, or of myositis and/or myopathy complicated by myasthenia. In pure myasthenia without muscle damage, investigations did not show any difference, except that an increase of serum LDH3 isoenzyme was observed in over a third of the cases; the authors state their hypothesis concerning the origin of this increase.
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