Cervical Spondylosis: This condition is common in the elderly due to wear and tear of the cervical spine, leading to compression of the nerves that supply the arm, resulting in numbness.

Stroke or Transient Ischemic Attack (TIA): Although less likely than cervical spondylosis given the presentation of isolated numbness, it's still a consideration, especially in the elderly who are at higher risk for vascular events.
Peripheral Neuropathy: Diabetes, vitamin deficiencies, or other metabolic issues can cause peripheral neuropathy, leading to numbness in the limbs.
Brachial Plexus Injury: Though less common, a fall or trauma could result in a brachial plexus injury, causing numbness in the arm.

Acute Stroke: While mentioned earlier, it's crucial to reiterate the importance of not missing an acute stroke, as timely intervention can significantly impact outcomes.
Spinal Cord Compression: Conditions like metastatic cancer or a herniated disk can compress the spinal cord, leading to numbness and potentially severe consequences if not promptly addressed.
Vascular Occlusion: Conditions such as subclavian artery stenosis or occlusion can lead to arm numbness and are critical to diagnose to prevent further complications.

Multiple Sclerosis: Although rare in the elderly, especially as a new diagnosis, multiple sclerosis can cause numbness and should be considered if other symptoms are present.
Neurosyphilis: A rare condition that can cause a variety of neurological symptoms, including numbness, and is important to consider in patients with a history of syphilis.
Lyme Disease: Neurological manifestations of Lyme disease can include numbness, but this would be less common and typically associated with other symptoms such as a rash or systemic illness.