Differential Diagnosis for a Man with Small Cell Lung Cancer Presenting with Ptosis, Diplopia, and Proximal Muscle Weakness

• Single Most Likely Diagnosis

  • B. Eaton Lambert syndrome: This is the most likely diagnosis given the patient's symptoms of ptosis, diplopia, and proximal muscle weakness in the context of small cell lung cancer. Eaton-Lambert syndrome, also known as Lambert-Eaton myasthenic syndrome (LEMS), is a rare autoimmune disorder characterized by muscle weakness, fatigue, and autonomic dysfunction. It is often associated with small cell lung cancer, where antibodies against voltage-gated calcium channels (VGCCs) at the neuromuscular junction are produced, leading to the symptoms observed.

• Other Likely Diagnoses

  • A. GBS (Guillain-Barré Syndrome): While less likely than Eaton-Lambert syndrome in this specific context, GBS is an autoimmune disorder that can cause muscle weakness and sometimes cranial nerve involvement leading to symptoms like diplopia. However, GBS typically presents with ascending paralysis and reduced or absent deep tendon reflexes, which might not perfectly align with the described symptoms but should still be considered.
  • C. Multiple sclerosis: Although multiple sclerosis (MS) can cause a wide range of neurological symptoms including vision problems and muscle weakness, its presentation is usually more heterogeneous and less directly linked to cancer. MS is an autoimmune disease affecting the central nervous system, and while it could potentially cause some of the symptoms described, it is less directly associated with small cell lung cancer compared to Eaton-Lambert syndrome.

• Do Not Miss Diagnoses

  • Paraneoplastic syndromes: These are conditions caused by the immune response to a cancer, but not due to direct local effects of the tumor cells. In the context of small cell lung cancer, various paraneoplastic syndromes could present with neurological symptoms, including those described. Missing a paraneoplastic syndrome could lead to delayed diagnosis and treatment of the underlying cancer.
  • Metastatic disease: Direct metastasis to the brain or spinal cord from small cell lung cancer could cause a variety of neurological symptoms. Although the specific combination of ptosis, diplopia, and proximal muscle weakness might be less common for metastatic disease, it is crucial not to miss this potentially life-threatening condition.

• Rare Diagnoses

  • Botulism: This is a rare condition caused by a toxin produced by Clostridium botulinum, which can lead to muscle weakness, ptosis, and diplopia among other symptoms. While extremely unlikely in this context, botulism is a medical emergency and thus is mentioned for completeness.
  • Myasthenia Gravis: An autoimmune disease that leads to variable weakness of the voluntary muscle groups, with a particular tendency to affect the muscles of the face, eyes, and swallowing. It could present with similar symptoms but is less directly associated with small cell lung cancer compared to Eaton-Lambert syndrome.