First Reported Coexistence of Acetylcholine Receptor-Positive Myasthenia Gravis and Genetically Confirmed Spinocerebellar Ataxia Type 6

When a patient with a long-standing neurological diagnosis develops new symptoms, those symptoms are frequently attributed to the disease that is already known, and a second, unrelated disorder may remain unrecognized for years.We describe a patient in whom myasthenia gravis (MG) and spinocerebellar ataxia type 6 (SCA6) occurred together, a combination that has not, to our knowledge, been previously reported.A 57-year-old man with acetylcholine receptor antibody-positive MG diagnosed in 1983 presented with progressive imbalance despite largely stable MG symptoms.His MG remained well controlled on low-dose prednisone and pyridostigmine following thymectomy, with intermittent azathioprine therapy.Neurological examination demonstrated cerebellar signs including dysmetria, dysdiadochokinesia, gaze-evoked nystagmus, and dysarthria.Brain MRI showed atrophy of the cerebellar hemispheres and vermis with associated brainstem volume loss.Extensive evaluation for acquired causes of cerebellar ataxia/degeneration, including paraneoplastic and autoimmune antibody testing, was unrevealing.Progressive ataxia continued despite largely stable neuromuscular symptoms.Family history revealed ataxia in his father and a similar disorder in his identical twin brother.Several years before the diagnosis was established, he experienced an MG exacerbation with bulbar and generalized weakness that responded to plasmapheresis.Genetic testing subsequently identified a pathogenic CACNA1A CAG repeat expansion of 22 repeats, confirming SCA6, just over a decade after the initial onset of his ataxia symptoms and establishing the coexistence of two distinct and rare neurological disorders.This case highlights the diagnostic challenge of evaluating new neurological symptoms in patients with established chronic, rare neurological disease.Progressive cerebellar dysfunction was initially overshadowed by the patient's longstanding MG, resulting in delayed recognition of a hereditary ataxia.Clinicians should maintain a broad differential diagnosis when symptoms evolve in a manner that is atypical for the known disease process.Recognition of coexisting neurological disorders is essential for accurate diagnosis, prognostication, genetic counseling, and multidisciplinary management.Coexistence of an acquired neuromuscular disorder and a hereditary cerebellar ataxia is possible; antibody testing to exclude an acquired ataxia and genetic confirmation of the inherited disorder are both required, and recognition of both diagnoses directly informs immunotherapy decisions, rehabilitation, and family counselling.