Differential Diagnosis
- Single most likely diagnosis
- Lambert-Eaton Myasthenic Syndrome (LEMS): This condition is often associated with small cell lung cancer and presents with progressive weakness, particularly in the lower legs, and improvement in deep tendon reflexes after exercise. The symptoms described, such as initial absence of deep tendon reflexes that become present following muscle exercise, are characteristic of LEMS, which involves abnormal presynaptic ion channels leading to decreased release of acetylcholine at the synapse.
- Other Likely diagnoses
- Myasthenia Gravis: Characterized by decreased acetylcholine receptors due to autoimmune destruction, leading to fluctuating muscle weakness that worsens with activity and improves with rest. While it's a possibility, the specific pattern of weakness and reflex response to exercise points more towards LEMS in this context.
- Decreased metabolism of acetylcholine at synapse: This could theoretically lead to an accumulation of acetylcholine, causing initial overstimulation followed by desensitization of the postsynaptic receptors. However, this is less directly related to the patient's symptoms and cancer history.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Metastatic disease affecting the spinal cord or brain: Direct involvement of the central nervous system by metastatic small cell lung cancer could cause progressive weakness and gait disturbances. Early detection is crucial for potential intervention.
- Paraneoplastic syndromes: Besides LEMS, other paraneoplastic neurological disorders could present with similar symptoms, emphasizing the need for a broad differential diagnosis in cancer patients.
- Rare diagnoses
- Excitation-contraction decoupling in muscle fibers: This would involve a defect in the process by which muscle action potentials lead to muscle contraction, which is less commonly associated with the clinical presentation described.
- Degeneration of muscle fibers: While muscle fiber degeneration could lead to weakness, the specific pattern of weakness, improvement with exercise, and association with small cell lung cancer makes this a less likely primary diagnosis.
- Abnormal postsynaptic ion channels: This could be considered in the context of myasthenic syndromes but is less directly implicated by the patient's history and physical examination findings compared to presynaptic abnormalities in LEMS.