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Digital Cutaneous Branches of Ulnar Nerve

OVERVIEW
Key Points:
• Assessing injury to these nerves requires a high index of clinical suspicion based on injury/lacerations and clinical findings.  Testing of both the medial/ulnar and lateral/radial aspect of the fingers should be done for finger level lacerations.  Failure to repair lacerations often leads to neuroma formation and pain in addition to loss of normal function.
Sensory Innervation:
• Provides sensation to the anterior/volar and medial/ulnar cutaneous aspect of the hand including the ulnar half of the ring finger and small finger.
• Originates from roots C8 and branches from the superficial branch of the ulnar nerve.

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EXAMINATION
Sensory examination includes testing the following two components innervated by the digital cutaneous branches of the ulnar nerve:
1. Volar and medial/ulnar aspect of the hand.
2. Volar small finger and medial/ulnar half of the ring finger.
3. Distal dorsal aspects of the entire small finger and medial/ulnar half of the ring finger.
Sensory Examination:
• Ask patients to draw area of diminished sensation/numbness on a hand diagram.
• Test the skin along the territories described above for reduced/absent sensation by use of light touch or other sensory examination modalities.
• A quick test includes testing the autonomous zone of the ulnar nerve--the distal volar aspect of the small finger. 
• The autonomous zone for a single digital nerve is the ulnar or radial side of the digit at the level of the middle phalanx.  The distal phalanx level is not an autonomous zone because of overlap from the opposite normal digital nerve.  This is especially important for detecting digital nerve injury of the radial or ulnar digital nerve to the affected finger but not both.
Clinical Relevance:
• Provides sensation to the territories described above.
• Patients exhibiting symptoms of ulnar nerve compression at Guyon’s canal or proximally will include reduced sensation along this sensory territory.
• The superficial branch of the ulnar nerve branches into two sensory nerves that innervate the described territory. Injury to any of these branches can occur and produces sensory loss to their corresponding territories. It is critical in understanding the ulnar sensory nerve anatomy within the hand when examining this nerve.
• See above regarding the specific autonomous zone exam to detect individual radial or ulnar digital nerve injuries due to trauma to the fingers.
Adjacent Sensory Distribution:

VOLAR/PROXIMAL – Palmar cutaneous branch of the ulnar nerve and medial antebrachial cutaneous nerve.
VOLAR/LATERAL – Palmar and digital branches of the median nerve.
 
DORSAL – Dorsal cutaneous branch of the ulnar nerve.
 
Relevant Anatomy:
Innervation
• Root: C8
• Nerve: Ulnar nerve.
• Sensory Territory: Volar and medial aspect of the hand; 4th webspace.
• Innervation Route: C8 → ulnar nerve → superficial branch → palmar digital cutaneous branches of ulnar nerve.
Course: Branches of the digital cutaneous nerves include the common and proper digital cutaneous nerves. The superficial branch of the ulnar nerve branches into two sensory nerves within the hand and are composed of a common and a proper cutaneous branch (lateral to medial, anteriorly). The common branch innervates the 4th webspace (and goes to the proper digital nerves that in turn innervate the ulnar aspect of the ring finger and the radial aspect of the small finger). The proper branch innervates the medial/radial aspect of the small finger.
 
Communicating Branch from the Ulnar Nerve: There exists a communicating branch that originates from the ulnar nerve. This branch originates from the digital cutaneous branch of the ulnar nerve that innervates the medial/ulnar aspect of the ring finger. The branch then combines with the proper digital cutaneous branch of the median nerve that innervates the lateral/radial aspect of the ring finger. This variation occurs in (46%) of cases. A second variation exists where the communicating branch combines with two proper digital cutaneous branches of the median nerve innervating the 3rd webspace. This variation occurs in (20%) of cases. The third variation is that no communicating branch between the median and ulnar nerve exists. This occurs in the other (20%) of cases. This communicating branch is often referenced as the Berretini anastomosis for the ulnar and median nerve anastomosis in the palm.