Background: Myasthenia gravis (MG) is an acquired autoimmune disorder characterized by autoantibodies targeting the neuromuscular junction of skeletal muscles. In contrast, congenital myasthenic syndromes (CMSs) represent a clinically diverse group of genetic disorders affecting the neuromuscular junctions, with early onset and autosomal recessive inheritance. CMSs are particularly prevalent in Devon Rex and Sphynx cats. The gold standard for diagnosing MG in cats involves detecting neuromuscular junction autoantibodies by measuring acetylcholine receptor autoantibodies using radioimmunoassay. For CMS, a definitive diagnosis requires the identification of a causative genetic mutation in addition to clinical signs of skeletal muscle weakness and fatigue. With the Sphynx and Devon Rex breeds, data collected have identified a candidate disease region on the feline C2 chromosome, discovered by employing a genome-wide single-nucleotide polymorphism-based homozygosity mapping strategy. Given that MG is an autoimmune condition, it is treated with steroids, immunosuppressive drugs, and sometimes a thymectomy (surgical removal of the thymus gland). CMS is a set of genetic conditions that do not respond to these treatments. Hence, accurate differential diagnosis is critical. Case presentation: Presented in this case study was a British Shorthair feline which was anamnetically, clinically, paraclinically, and pharmacologically assessed. Genetic testing revealed a positive result for the COLQ gene mutation. Conclusion: This case study clarified and added criteria to the differential diagnosis between MC and CMS, allowing for more accurate prognostic evaluations and appropriate treatment planning. It also underscores the importance of genetic testing in British Shorthair cats to differentiate between these conditions.