What is the cause of a 1-week history of right arm and hand pain, without a known recent injury, presenting with grip-strength weakness, tingling, and slightly diminished sensation in the thumb, but intact reflexes and no evidence of compartment syndrome or infection, in a patient with a history of a forearm injury years ago?

Cervical Radiculopathy: This condition is characterized by pain, numbness, or weakness in the arm or hand due to a pinched nerve in the cervical spine. The symptoms of arm and hand pain, difficulty picking up items, and grip-strength weakness without a recent injury, along with slightly diminished sensation at the thumb, are consistent with cervical radiculopathy, particularly affecting the C6 or C7 nerve roots.

Thoracic Outlet Syndrome (TOS): TOS involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). Symptoms can include pain, numbness, and weakness in the arm and hand, which align with the patient's complaints. The absence of a recent injury and the nature of the symptoms make TOS a plausible diagnosis.
Carpal Tunnel Syndrome: Although the patient's symptoms are not limited to the typical carpal tunnel distribution (median nerve), the complaint of difficulty picking up items and grip-strength weakness, along with slightly diminished sensation at the thumb, could suggest carpal tunnel syndrome, especially if the symptoms are predominantly in the median nerve distribution.
Tendinitis or Tenosynovitis: Inflammation of the tendons or their surrounding sheaths in the forearm or hand could lead to pain and weakness, especially with activities involving gripping. The history of a past forearm injury could potentially contribute to chronic inflammation or tendon issues.

Acute Compartment Syndrome: Although the patient does not currently show signs of compartment syndrome (such as severe pain out of proportion to what is expected, pallor, poikilothermia, paresthesias, paralysis, and pulselessness), this condition is a medical emergency. Any delay in diagnosis can lead to permanent damage, so it's crucial to monitor for these signs closely.
Brachial Plexus Injury: Given the patient's history of a past injury involving being trapped in a machine, there's a possibility of a brachial plexus injury, which could cause arm and hand pain, weakness, and sensory changes. This diagnosis is critical to consider due to the potential for long-term disability.
Vascular Occlusion or Embolism: Although less likely given the gradual onset and the absence of other symptoms like coldness or discoloration, vascular occlusion or embolism could cause arm and hand pain and weakness. This would be a medical emergency requiring immediate intervention.

Pancoast Tumor: A rare type of lung cancer that can cause shoulder and arm pain due to invasion of the brachial plexus. It's an unlikely diagnosis without other symptoms like weight loss or cough but is included due to its potential severity.
Amyotrophic Lateral Sclerosis (ALS): A progressive neurological disease that affects nerve cells in the brain and the spinal cord, causing loss of muscle control. ALS could present with weakness and atrophy, but the absence of other symptoms like bulbar signs or widespread weakness makes this a less likely diagnosis at this stage.
Multifocal Motor Neuropathy: A rare condition characterized by asymmetric muscle weakness and atrophy, often with cramps and fasciculations, due to immune-mediated damage to motor nerves. This diagnosis would be considered if there were more widespread or asymmetric neurological findings.