Differential Diagnosis for Left Shoulder Weakness
Single Most Likely Diagnosis
- C6 Radiculopathy: This is the most likely diagnosis due to the involvement of muscles primarily innervated by the C6 nerve root, including the biceps (elbow flexion), brachioradialis, and extensor carpi radialis (wrist extension). The pattern of weakness in these muscles strongly suggests a lesion affecting the C6 nerve root.
Other Likely Diagnoses
- C7 Radiculopathy: Although less likely than C6 radiculopathy given the specific muscle groups mentioned, C7 radiculopathy could also present with weakness in the triceps and potentially some overlap with C6 innervated muscles, making it a plausible alternative.
- Brachial Plexopathy: A lesion affecting the upper trunk of the brachial plexus could result in similar patterns of weakness, particularly affecting the C5 and C6 nerve roots' distributions. This would include muscles like the biceps, brachioradialis, and potentially some forearm extensors.
Do Not Miss Diagnoses
- Acute Stroke or Transient Ischemic Attack (TIA): Although the pattern of weakness might not perfectly align with a typical stroke syndrome, any new onset of focal weakness warrants consideration of a vascular event, especially given the potential for atypical presentations.
- Spinal Cord Injury or Compression: Conditions like cervical spondylotic myelopathy or a spinal cord tumor could present with focal weakness, among other symptoms, and are critical not to miss due to their potential for significant and irreversible neurological damage.
Rare Diagnoses
- Neuromuscular Junction Disorders (e.g., Myasthenia Gravis): These conditions can present with fluctuating weakness that might seem localized at times but typically involve more widespread muscle groups and have distinctive features like fatigability.
- Mononeuritis Multiplex: A form of peripheral neuropathy that can cause asymmetric weakness due to damage to two or more named peripheral nerves. It's less likely given the specific pattern of muscle involvement but remains a consideration in the differential diagnosis of localized weakness.