Differential Diagnosis
- Single most likely diagnosis
- Myasthenia Gravis (MG): The patient's significant improvement with the Mestinon trial (a cholinesterase inhibitor used to diagnose MG) suggests a strong likelihood of MG, despite negative acetylcholine receptor (AChR) antibodies. Some patients with MG may have seronegative MG, where AChR antibodies are not detected.
- Other Likely diagnoses
- Seronegative Myasthenia Gravis: As mentioned, the patient's response to Mestinon and the presence of some symptoms suggestive of MG (improved with warming up, which can be seen in some MG patients due to improved neuromuscular transmission with increased temperature) support this diagnosis, even with negative AChR and MuSK antibodies.
- Congenital Myasthenic Syndrome (CMS): Although less common, CMS could present with variable symptoms and might not always have a clear antibody profile. The patient's response to Mestinon could be indicative of a form of CMS, though typically CMS presents at a younger age.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Botulism: Although rare, botulism can mimic myasthenic symptoms and has a high mortality rate if not treated promptly. The lack of forehead weakness and the specific EMG findings might not fully align with botulism, but it's crucial to consider due to its severity.
- Lambert-Eaton Myasthenic Syndrome (LEMS): This condition can present with similar symptoms to MG but often includes autonomic features like Raynaud's phenomenon, as hinted at in the patient's need to be warmed up. LEMS is associated with a different antibody profile and can be life-threatening if not recognized and treated.
- Rare diagnoses
- Neuromyotonia: Characterized by muscle stiffness and delayed relaxation, it might not fully explain the patient's response to Mestinon but could be considered in the differential for rare neuromuscular disorders.
- Stiff Person Syndrome: Although primarily involving stiffness and rigidity, some forms might respond to treatments that enhance neuromuscular transmission, making it a rare consideration in this context.