Dropped Head Syndrome: The Importance of Neurophysiology in Distinguishing Myasthenia Gravis from Parkinson's Disease

Dropped head syndrome (DHS) is characterized by severe forward flexion of the cervical spine due to an imbalance in neck muscle tone. This condition can be linked to various neuromuscular diseases, including myasthenia gravis (MG). On the other hand, Parkinson's disease (PD) patients may show a clinically indistinguishable picture named antecollis, which is caused by increased axial tone, but without muscle weakness. Differentiating between DHS and antecollis is crucial due to their distinct treatment requirements. We present the case of a 71-year-old White male with a one-month history of severe neck flexion, mild dysphagia, and dysphonia. His medical history included diabetes mellitus, coronary artery disease, arterial hypertension, and mild cervical spondylosis. Neurological examination revealed features of Parkinsonism, including hypomimia, asymmetric rigidity, and reduced arm swing. There was significant weakness in his neck extensor muscles, with no signs of ptosis or diplopia. Brain/spine MRI scans were unremarkable, but electromyography showed a reduced compound muscle action potentials amplitude in repetitive nerve stimulation, consistent with MG. High-titer acetylcholine receptor antibodies confirmed the diagnosis. Treatment with pyridostigmine (60 to 120 mg/day) and plasma exchange (daily, for five consecutive days) improved the patient's general condition and neck posture. Concurrently, the patient was diagnosed with PD based on established clinical criteria and improved with carbidopa/levodopa therapy (up to 150/600 mg/daily). This case highlights the rare co-occurrence of MG and PD, emphasizing the need for thorough clinical, neurophysiological, and laboratory evaluations in complex DHS presentations. Managing MG's life-threatening aspects and addressing PD symptoms requires a tailored approach, showcasing the critical role of neurophysiology in accurate diagnosis and effective treatment.