Purpose. Imatinib treatment causes muscle cramps in up to40% of patients, but their pathogenesis is unknown. We presenta case series illustrating an association between imatinib,relative hypocalcaemia, and the development of cramps.Patients. The index patient developed muscle spasms andcramps after receiving imatinib for gastrointestinal stromal tumour(GIST) for 5 months. The adjusted serum calcium had dropped to thelower limit of normal. The low serum calcium and muscle crampsimproved on stopping imatinib and recurred on rechallenge. Wereviewed the medical records of 16 further patients.Results. Two patients reported muscle cramps (12%).There was a rapid and sustained reduction in adjusted serumcalcium in the first 6 months from 2.45 ± 0.11 mmol/L(mean ± SD) to 2.30 ± 0.08 mmol/L (p = 0.025).Conclusion. Imatinib treatment of GIST is associated withreduction in serum calcium which may explain the development ofneuromuscular symptoms. In patients receiving imatinib, serumelectrolytes should be monitored and muscle cramps treated bycorrection of serum calcium, or an empirical trial of quininesulphate.
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