Dantrolene-Responsive Muscle Stiffness in a Patient With a Normal Neurologic Exam and EMG: A Case Report.

Muscle stiffness or rigidity is a common problem yet is addressed in few studies. Patients with muscle rigidity/spasticity due to injury of upper motor neurons or genetic muscle diseases are sometimes treated with dantrolene. It is not widely used to treat muscle tightness in patients with a negative workup. Here, we present a 62-year-old neuromuscular physician with no family history of hereditary neuromuscular disease who presented with prolonged, episodic muscle rigidity causing significant functional limitations. Next-generation sequencing identified a heterozygous calpain 3 (CAPN3) variant [NM_000070.3(CAPN3):c.2393C > A (p.Ala798Glu)] categorized as pathogenic for autosomal-recessive CAPN3-related limb girdle muscular dystrophy type 1 (LGMD R1), which is of unclear significance. He was treated with dantrolene and showed marked functional gains that were lost with attempts to wean off medication. This case suggests that there may be a subset of patients suffering from muscle tightness who benefit from dantrolene.