Differential Diagnosis

• Single most likely diagnosis:
  • Myasthenia Gravis (MG) - The AChR blocking percentage of 14% is below the threshold of 25%, but still suggests some level of antibody presence against acetylcholine receptors, which is a hallmark of MG. The normal SFEMG (single-fiber electromyography) on a muscle that doesn't typically get weak could be seen in early or mild cases of MG, where the disease might not have progressed to significantly affect all muscles.
• Other Likely diagnoses:
  • Seronegative Myasthenia Gravis - This condition presents similarly to MG but with negative tests for AChR antibodies. The clinical presentation and the borderline AChR blocking percentage could suggest this diagnosis, especially if the patient's symptoms are suggestive of myasthenia.
  • Congenital Myasthenic Syndromes - These are a group of rare genetic disorders that affect the neuromuscular junction. While the AChR blocking test is not typically used for diagnosis, the normal EMG and borderline antibody test could lead to consideration of these syndromes, especially in younger patients or those with a family history.
• Do Not Miss diagnoses:
  • Lambert-Eaton Myasthenic Syndrome (LEMS) - This is an autoimmune disorder characterized by muscle weakness, fatigue, and autonomic dysfunction. It is crucial not to miss LEMS because it can be associated with small cell lung cancer, and early diagnosis can lead to treatment of the underlying cancer. The normal SFEMG and borderline AChR blocking do not rule out LEMS, as it is diagnosed through different criteria, including symptoms and specific EMG findings.
  • Botulism - While less likely, botulism can present with muscle weakness and could potentially have a normal SFEMG if the affected muscles are not tested. The clinical presentation and exposure history are critical for diagnosing botulism.
• Rare diagnoses:
  • Neuromyotonia - This condition involves muscle stiffness and delayed relaxation after muscle contraction. It could be considered if the patient's symptoms include muscle stiffness or cramps, although it does not directly explain the borderline AChR blocking percentage.
  • Acquired Neuromyotonia - Similar to neuromyotonia, this condition could be considered with symptoms of muscle stiffness, but it is less likely given the specific test results provided.