Isaacs syndrome associated with GABA_B and AChR antibodies in sarcomatoid carcinoma

Isaacs syndrome is an acquired peripheral nerve hyperexcitability syndrome characterized by twitching, painful cramps, stiffness, delayed muscle relaxation, and sweating.¹ Needle EMG findings include continuous single motor unit discharges occurring as doublets, triplets, and multiplet single unit discharges firing at higher rates (150–300 Hz; neuromyotonic discharges) and at lower rates (less than 60 Hz; myokymic discharges).² Isaacs syndrome has been reported in patients with thymoma, small-cell lung carcinoma (SCLC), or lymphoma.² Antibodies specific for voltage-gated potassium channels (VGKC) have been reported as a serologic marker of Isaacs syndrome. Antibodies specific for the neuronal ganglionic acetylcholine receptor (AChR) are sometimes encountered in some patients.¹ It has not been previously associated with sarcomatoid carcinoma (SC) or antibodies against γ-aminobutyric acid B (GABA_B) receptors, and the latter often present in limbic encephalitis and other uncommon clinical syndromes (such as cerebellar ataxia).³ Here we describe a case of Isaacs syndrome with a positive test for anti-GABA_B and anti-AChR antibodies and broad tumor screening for this case of SC.