Median nerve

• The muscular branch (also known as
  flexor pollicis brevis)^[3]
• Digital cutaneous to the
  common palmar digital branch: The proper palmar digital branch gives out three digital branches to the lateral one and a half digits (two digital branches to the thumb, one digital branch to the lateral side of the index finger). The digital branch to the index finger also supplies the first lumbrical. The common palmar digital branch divides further into two branches. Both the medial and lateral branches supply the second and third interdigital clefts with adjoining index, middle, and lateral half of ring finger. The lateral branch also supplies the second lumbrical.^[3]

• Bifurcation of the median nerve typically occurs after the nerve exits the carpal tunnel; however, in a small percentage (5-10%) of individuals, the median nerve bifurcates more proximal in the carpal tunnel, wrist, or forearm.[5]
• During gestation, a median artery that serves the hand retracts. However, in some individuals, the median artery does not retract and follows the course next to the median nerve into the hand.^[5]
• Martin-Gruber anastomoses can occur when branches of the median nerve cross-over in the forearm and merge with the ulnar nerve to innervate portions of the forehand.^[6]
• Riche-Cannieu anastomosis can occur when a connection exists between recurrent branch of the median nerve and deep branch of the ulnar nerve of the hand.^[7]

Function

The median nerve is the main nerve of the front of the forearm. It supplies the muscles of the front of the forearm and muscles of the thenar eminence, thus controlling the coarse movements of the hand. Therefore, it is also called "labourer's nerve".^[3]

Arm

The median nerve has no voluntary motor or cutaneous function in the brachium. It gives vascular branches to the wall of the brachial artery. These vascular branches carry sympathetic fibers.

Forearm

It innervates all of the

• Pronator teres^[2]
• Flexor carpi radialis^[2]
• Palmaris longus^[2]

• Flexor digitorum superficialis muscle^[2]

The anterior interosseus branch of the median nerve supplies these muscles:

Deep group:

• Flexor digitorum profundus (only the lateral half)^[2]
• Flexor pollicis longus^[2]
• Pronator quadratus^[2]

In the hand, the median nerve supplies motor innervation to the first and second lumbrical muscles. It also supplies the muscles of the thenar eminence by a recurrent thenar branch.^[8] The rest of the intrinsic muscles of the hand are supplied by the ulnar nerve.

The median nerve innervates the skin of the palmar (volar) side of the index finger, thumb, middle finger, and half the ring finger, and the nail bed. The radial aspect of the palm is supplied by the palmar cutaneous branch of the median nerve, which leaves the nerve proximal to the wrist creases. This palmar cutaneous branch travels in a separate fascial groove adjacent to the flexor carpi radialis and then superficial to the flexor retinaculum. It is, therefore, spared in carpal tunnel syndrome.

Clinical significance

Injury

Injury of median nerve at different levels causes different syndromes with varying motor and sensory deficits.

At the shoulder

• Injury can occur at the brachial plexus^[9]

Above the elbow

• Common mechanism of injury: a supracondylar humerus fracture
• Motor deficit:
  • Loss of
    pronation
    of forearm, weakness in flexion of the hand at the wrist, loss of flexion of radial half of digits and thumb, loss of abduction and opposition of thumb.
  • Presence of an ape hand deformity when the hand is at rest, due to an hyperextension of index finger and thumb, and an adducted thumb
  • Presence of benediction sign when attempting to form a fist, due to loss of flexion of radial half of digits
• Sensory deficit: loss of sensation in lateral 3+1⁄2 digits including their nail beds, and the thenar area

At the elbow

• Entrapment at the level of the elbow or the proximal forearm could be due to the pronator teres syndrome.

Within the proximal forearm: anterior interosseous syndrome

• Injury to the anterior interosseous branch in the forearm causes the anterior interosseous syndrome^[4]
• Common mechanisms: tight cast, forearm bone fracture
• Motor deficit: loss of
  pronation
  of forearm, loss of flexion of radial half of digits and thumb
• Sensory deficit: none

At the wrist

• Common mechanism: wrist laceration
• Motor deficit:
  • Weakness in flexion of radial half of digits and thumb, loss of abduction and opposition of thumb
  • Presence of an ape hand deformity when the hand is at rest may be likely, due to an hyperextension of index finger and thumb, and an adducted thumb. Nevertheless, an ape hand deformity is not a requirement for a carpal tunnel syndrome diagnosis.
  • Presence of a benediction sign when attempting to form a fist, due to weakness in flexion of radial half of digits
• Sensory deficit: loss of sensation in lateral 3+1⁄2 digits including their nail beds, and the thenar area

Within the wrist: carpal tunnel syndrome

• Common mechanism: carpal tunnel syndrome, an injury by compression in the carpal tunnel, without transection of the median nerve, due to overuse by activities such as keyboard typing and cooking
• Motor deficit:
  • Weakness in flexion of radial half of digits and thumb, weakness in abduction and opposition of thumb
  • Presence of an ape hand deformity or when attempting to form a fist, the benediction sign, due to compression of the median nerve, as opposed to complete median nerve palsy
• Sensory deficit: numbness and tingling in lateral 3+1⁄2 digits including their nail beds, but excluding the thenar eminence which is supplied by the palmar cutaneous branch of the median nerve^[10] Unlike in wrist laceration, sensation still occurs in the area of the central palm. Sensation is not lost because the palmar cutaneous branch runs above the flexor retinaculum, and is not affected in compression in carpal tunnel syndrome.

Assessment

When symptoms of tingling, numbness, pain, or muscle weakness occur, various assessments are conducted, including patient-reported subjective surveys, provocative tests meant to elicit or exacerbate symptoms, sensory and motor function tests, and diagnostic equipment.

• Common patient-reported measures of symptoms and function are the Boston Carpal Tunnel Questionnaire (BCTQ)and, its short version, the CTS-6,^[11] the Disabilities of the Arm, Shoulder, and Hand (DASH),^[12] and the Upper Limb Functional Index.^[13]
• Provocative testing to examine free movement and gliding for the entire length of the median nerve is completed using an upper limb tension test. Phalen's, Tinel's, and Durkan's tests are used to assess compression at the wrist in the case of carpal tunnel syndrome.
• Sensory testing is conducted across the dermatomes associated with sensory functioning of the median nerve using monofilaments or two-point discrimination instruments. Motor function tests focus on examining the use of the thenar muscles within the hand most affected by median nerve injury. Common functional tests include the evaluation of grip and pinch strength using dynamometry and gross and fine motor coordination ability using the Purdue pegboard, 9-hole peg test, and other assessment tools.
• Electrodiagnostic testing using surface or needle electromyography is a widely used diagnostic assessment that can identify a specific location of nerve compression/injury or rule out other central nervous system conditions that could mimic a median nerve injury. Sonographic imaging can evaluate the median nerve's size, shape, and movement for comparison to normative values^[14] and identify locations or causes of nerve compression to support intervention selection, particularly within the carpal tunnel.^[15]

Combinations of these assessments are useful for ruling out other conditions, identifying the location of nerve injury or compression, uncovering underlying causes of symptoms (such as tendonitis), determining the severity of the nerve injury, and developing individualized treatment plans based on patient characteristics.^[16] Such combinations of median nerve evaluations have been most widely validated in the assessment of carpal tunnel syndrome.^[17][18]

Additional images

• 
  Brachium seen anterolaterally, showing nerves and vasculature
• 
  Antebrachium

  seen anteriorly, showing nerves and vasculature
• 
  Cross-section through the middle of upper arm (left), and middle of the forearm (right)
• 
  Transverse section across distal ends of radius and ulna
• 
  Transverse section across the wrist and digits
• 
  The right brachial plexus (infraclavicular portion) in the axillary fossa, viewed from below and in front
• 
  Diagram of segmental distribution of the cutaneous nerves of the right upper extremity.
• 
  Superficial palmar nerves
• 
  Deep palmar nerves
• 
  Front of right upper extremity, showing surface markings for bones, arteries, and nerves
• 
  Median nerve
• 
  Median nerve

See also

References

This article incorporates text in the public domain from page 938 of the 20th edition of Gray's Anatomy (1918)

External links

Wikimedia Commons has media related to Median nerve.

• Median nerve at the Duke University Health System's Orthopedics program
• Median+Nerve at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Hand kinesiology at the University of Kansas Medical Center
• Atlas image: hand_plexus at the University of Michigan Health System - "Axilla, dissection, anterior view"