IgLON5 masquerading as Myasthenia Gravis

<h3></h3> A 43-year-old male with no past medical history presented in 2013 with a two year history of fatigable ptosis and a one year history of diplopia (worse in the afternoon), dysphagia and exertional dyspnoea. An initial NCS demonstrated a decremental response supporting a diagnosis of Myasthenia Gravis (MG). AChR, MuSK, Clustered AChR and LRP4 antibodies were negative. He was treated as seronegative MG with pyridostigmine, prednisolone and immunosuppressants. He had recurrent hospital admissions requiring multiple treatments with IVIg and plasma exchange (PLEX) with variable benefit. Multiple immunosuppressants including Azathioprine, Methotrexate, Mycophenolate Mofetil were subsequently trialled and later withdrawn in view of partial effect or side effects. Further investigations for mitochondrial diseases (with a muscle biopsy and analysis of mitochondrial cytopathies), LEMS and congenital myasthenia mutations were negative. He had a positive AChR antibody test in 2018 which was negative on repeat testing. Repeat neurophysiological studies were also normal. Given a limited response to previous treatments, he was started on Rituximab in addition to regular PLEX. Nine years after the onset, he developed sleep disturbances, a predominantly choreiform movement disorder, worsening dysphagia and recurrent confusional states necessitating hospital admission. The CSF and serum antibody screen was positive for IgLON5.