Differential Diagnosis for Arm Weakness without Leg Weakness

Single Most Likely Diagnosis

• Stroke or Transient Ischemic Attack (TIA): The most common cause of sudden arm weakness without leg weakness is a stroke or TIA, particularly if it involves the cerebral cortex or subcortical areas responsible for motor function of the arm. This is due to the specific vascular supply to these areas and the high prevalence of cerebrovascular disease.

Other Likely Diagnoses

• Radial Neuropathy: Compression or injury to the radial nerve can cause weakness or paralysis of the muscles it supplies, leading to arm weakness. This condition might not affect the leg.
• Brachial Plexus Injury: Trauma to the brachial plexus, which is a network of nerves that conducts signals from the spinal cord to the shoulder, arm, and hand, can result in arm weakness without affecting the legs.
• Cervical Spondylosis or Disc Herniation: Compression of the cervical spinal nerves or spinal cord due to spondylosis or disc herniation can lead to arm weakness. The pattern of weakness depends on the level and nature of the compression.

Do Not Miss Diagnoses

• Spinal Cord Infarction or Compression: Although less common, spinal cord infarction or compression (e.g., due to a tumor or abscess) at the cervical level can cause arm weakness. Missing this diagnosis could lead to severe and irreversible neurological damage.
• Multiple Sclerosis: An episode of arm weakness could be the presenting symptom of multiple sclerosis, especially if there are other neurological symptoms or signs. Early diagnosis and treatment can significantly impact the disease course.
• Amyotrophic Lateral Sclerosis (ALS): ALS is a progressive neurological disease that affects nerve cells in the brain and the spinal cord, causing loss of muscle control. Arm weakness without leg weakness could be an early manifestation.

Rare Diagnoses

• Neuralgic Amyotrophy: Also known as brachial neuritis, this condition involves inflammation of the nerves that control the muscles of the arm, leading to severe pain and arm weakness.
• Plexiform Neurofibroma: A type of tumor that arises from the nerve sheath and can compress or infiltrate the brachial plexus, causing arm weakness.
• Inflammatory Myopathies: Conditions like polymyositis or dermatomyositis can cause muscle weakness, which might initially be more pronounced in the arms.