Agenesis of Pectoralis Major Muscle in Late-Onset -Related Congenital Myasthenic Syndrome: A Case Report.

Objectives: The objective of this study was to expand the phenotypic spectrum of glutamine-fructose-6-phosphate transaminase 1 ()-related congenital myasthenia syndrome (CMS).

Methods: A 61-year-old man with agenesis of the left pectoralis major muscle presented with progressive muscle weakness for a decade that transiently improved after exertion.

Results: His examination revealed proximal and distal muscle weakness in upper extremities and proximal muscle weakness in lower extremities. Muscle enzymes were elevated. An electromyogram revealed a myopathic pattern; however, a muscle biopsy of deltoid muscle and genetic testing for limb-girdle muscular dystrophies were nondiagnostic. A 3-Hz repetitive nerve stimulation of the spinal accessory nerve recording from trapezius muscle demonstrated a >20% drop in amplitude of the 5th compound motor action potential relative to 1st at both baseline and after 45-second exercise. Acetylcholine receptor binding, lipoprotein-related protein 4, muscle-specific kinase, and voltage-gated calcium channel P/Q antibodies were negative. Genetic testing targeting CMS revealed 2 likely pathogenic variants within : novel c.7+2T>G (intron 1) that was predicted to result in a null allele and known c*22 C>A (exon 19) associated with reduced expression. His muscle strength dramatically improved after pyridostigmine initiation.

Discussion: In addition to other reported neurodevelopmental abnormalities, pectoralis major muscle agenesis (or Poland syndrome) may be a clinical manifestation of -related CMS.