What nerve injury causes numbness of the fourth and fifth fingers (digits)?

Ulnar Nerve Injury: The ulnar nerve is responsible for sensation in the fourth and fifth digits. Injury to this nerve, which can occur at various points along its path (e.g., at the elbow or wrist), is the most direct cause of numbness in these areas.

C8 Radiculopathy: Compression or injury to the C8 nerve root can lead to numbness in the fourth and fifth digits, among other symptoms. This condition often results from cervical spine issues such as herniated discs or spinal stenosis.
Brachial Plexus Injury: While less specific, injuries to the brachial plexus, particularly those affecting the lower trunk, can result in numbness of the fourth and fifth digits, along with other arm and hand symptoms.

Thoracic Outlet Syndrome: This condition, involving compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib), can lead to numbness in the fourth and fifth digits, among other symptoms. It's crucial not to miss this diagnosis due to its potential for serious complications if left untreated.
Spinal Cord Injury: High-level spinal cord injuries can result in numbness and other sensory deficits in the hands, including the fourth and fifth digits. Early recognition and treatment are critical to prevent further damage.

Syringomyelia: A rare condition characterized by the development of a fluid-filled cavity or syrinx within the spinal cord, which can expand and damage the spinal cord from within. It may cause numbness in a "cape-like" distribution, including the fourth and fifth digits, but its presentation can be highly variable.
Tumors Affecting the Brachial Plexus or Spinal Cord: Rarely, tumors (either benign or malignant) can compress or infiltrate the nerves of the brachial plexus or the spinal cord, leading to numbness and other neurological deficits in the affected areas, including the fourth and fifth digits.