Cervical Radiculopathy: This condition, often due to a herniated disc or cervical spondylosis, can cause weakness and paresthesia in the upper extremity by compressing or irritating the nerve roots. The symptoms typically follow a dermatomal distribution, which can help in identifying the affected nerve root.

Peripheral Neuropathy: Conditions such as diabetic neuropathy, alcohol-related neuropathy, or neuropathy due to vitamin deficiencies can cause weakness and paresthesia in the upper extremities. The pattern of symptoms can vary but often includes sensory changes.
Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It can cause pain, weakness, and paresthesia in the arm and hand.
Stroke or Transient Ischemic Attack (TIA): Although less common as a cause for isolated upper extremity weakness and paresthesia without other symptoms, a stroke or TIA should be considered, especially if there are risk factors or accompanying symptoms like facial weakness or speech changes.

Acute Stroke: Even if less likely, missing an acute stroke can have devastating consequences. The presence of sudden onset of symptoms, especially if accompanied by other neurological deficits, warrants immediate imaging and possible intervention.
Spinal Cord Compression: Conditions like spinal stenosis, epidural abscess, or metastatic spinal cord compression can present with progressive weakness and sensory changes in the upper extremities. Prompt diagnosis is crucial to prevent irreversible damage.
Multiple Sclerosis: An episode of optic neuritis, transverse myelitis, or brainstem syndrome can sometimes present with focal neurological deficits like upper extremity weakness and paresthesia. Early diagnosis can significantly impact treatment and prognosis.

Brachial Plexitis (Neuralgic Amyotrophy): An inflammatory condition of the brachial plexus that can cause severe pain, weakness, and atrophy of the muscles of the shoulder and upper arm.
Tumors Affecting the Brachial Plexus or Spinal Cord: Although rare, tumors (either primary or metastatic) can compress or infiltrate the nerves of the brachial plexus or the spinal cord, leading to progressive weakness and sensory changes.
Vasculitis Affecting the Nerves: Conditions like giant cell arteritis or polyarteritis nodosa can cause ischemic neuropathy leading to weakness and paresthesia, though this is less common in the upper extremities compared to other presentations.