Numbness in the little finger of the left hand is an injury that occurred in the ulnar nerve. The ulnar nerve is composed of C8-T1 nerve root fibers, which are divided into motor and sensory branches. The sensory branch is mainly located on the ulnar side of the hand below the wrist, and on the skin of the ulnar half of the little finger and ring finger. Because the course of the ulnar nerve is relatively superficial compared to other nerves, it is most prone to trauma or impingement injury, which is triggered in many patients by sleeping on their stomach somewhere and resting their head on their upper arm. Other times it is caused by improper posture when walking on crutches. Physical examination may reveal superficial sensory deficits in the ulnar side of the arm, the lesser interosseous muscle, and the ulnar half of the little finger and ring finger, manifesting as hyperalgesia of pins and needles. Complete electromyography conduction velocity examination can reveal decreased conduction velocity of the ulnar nerve.