Differential Diagnosis for Inability to Flex 4th Finger with Decreased Active Range of Motion

• Single Most Likely Diagnosis
  • Ulnar Nerve Injury or Compression: This is the most likely diagnosis because the ulnar nerve controls the movement of the 4th and 5th fingers, including flexion. Injury or compression of this nerve can lead to weakness or paralysis of these muscles, resulting in the inability to flex the 4th finger.
• Other Likely Diagnoses
  • Flexor Tendon Injury: An injury to the flexor tendon of the 4th finger can cause a decrease in active range of motion and an inability to flex the finger.
  • Rheumatoid Arthritis: This condition can cause inflammation and stiffness in the joints, leading to decreased range of motion and potentially affecting the ability to flex the fingers.
  • Dupuytren's Contracture: A condition where one or more fingers become permanently bent in a flexed position, but in early stages, it can cause decreased active range of motion.
• Do Not Miss Diagnoses
  • Acute Compartment Syndrome: Although less likely, this is a medical emergency that requires immediate attention. It can cause pain, numbness, and decreased mobility in the affected limb, including the inability to flex fingers.
  • Infection (e.g., Osteomyelitis or Septic Arthritis): Infections in the bone or joint can cause severe pain, swelling, and decreased mobility, including an inability to flex the fingers.
• Rare Diagnoses
  • Tumors (e.g., Soft Tissue Sarcoma): Rarely, tumors in the hand or forearm can compress nerves or tendons, leading to decreased mobility.
  • Neurological Disorders (e.g., Amyotrophic Lateral Sclerosis): Certain neurological conditions can cause progressive muscle weakness, including the muscles responsible for finger flexion.