Carpal Tunnel Syndrome Case File

Carpal Tunnel Syndrome Case File

Eugene C. Toy, MD, Lawrence M. Ross, MD, PhD, Han Zhang, MD, Cristo Papasakelariou, MD, FACOG

CASE 4

A 34-year-old pregnant woman complains of tingling of her right index and middle fingers over a 2-month duration. She notes some weakness of her right hand and has begun to drop items such as her coffee cup. She has otherwise been healthy and denies any trauma or neck pain.

⯈ What is the most likely diagnosis?

⯈ What is the anatomic mechanism for this condition?

ANSWERS TO CASE 4:

Carpal Tunnel Syndrome

Summary: A pregnant woman experiences tingling and weakness of her right index and middle fingers.

• Most likely diagnosis: Carpal tunnel syndrome

• Anatomical mechanism: Compression of the medial nerve as it passes through the carpal tunnel of the wrist

CLINICAL CORRELATION

The most likely cause for this individual’s symptoms is carpal tunnel syndrome. the carpal tunnel is a confined, rigid space at the wrist that contains nine tendons with their synovial sheaths and the median nerve. Any condition that further reduces the available space within the tunnel may compress the median nerve, producing numbness and pain in the areas of cutaneous distribution, muscle weakness (especially in the thumb), and muscle atrophy after long-term compression. However, we are not given the distribution of neuropathy of this case. The median nerve may be compressed in several sites along its length between the brachial plexus and the hand, but the carpal tunnel is the most common site. Carpal tunnel syndrome has been associated with endocrine conditions such as diabetes, hypothyroidism, hyperthyroidism, acromegaly, and pregnancy. Other causes include autoimmune disease, lipomas within the canal, hematomas, and carpal bone abnormalities. Females are more commonly affected than males in a ratio of 3:1

Initial treatment is a nighttime splint of the wrist and avoidance of excessive activity with the hand. If symptoms do not decrease, division of the flexor retinaculum (carpal tunnel release) may be necessary.

APPROACH TO:

The Carpal Tunnel

OBJECTIVES

1. Be able to describe the structures that form and pass through the carpal tunnel

2. Be able to describe the course, branches, and muscles innervated by the median nerve in the forearm and hand

3. Be able to describe the skin areas supplied by the median nerve in the hand

4. Be able to describe the course of the ulnar nerve at the wrist as it relates to the carpal tunnel

DEFINITIONS

NEUROPATHY: Any disease or disorder of the peripheral nervous system

CARPAL TUNNEL SYNDROME: Entrapment of the median nerve within the carpal tunnel, resulting in pain, sensory paresthesia, and muscle weakness

MUSCLE ATROPHY: Wasting of muscle tissue, often the result of disuse secondary to interference with its motor innervation

DISCUSSION

The carpal tunnel is formed posteriorly by the concave surfaces of the carpal bones (see Case 3 for their anatomic arrangement). The anterior boundary of the tunnel is formed by a thickening of the deep fascia, the flexor retinaculum (transverse carpal ligament). The flexor retinaculum is attached laterally to the tubercles of the scaphoid and trapezium and medially to the pisiform and hook of the hamate. The carpal tunnel is a passageway for the nine tendons and their investing synovial sheaths of the flexor muscles of the thumb and fingers: four tendons each of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), the tendon of the flexor pollicis longus (FPL), and the median nerve. The flexor retinaculum (and the extensor retinaculum dorsally) prevent “bowstringing” of the tendons of the extrinsic hand muscles at the wrist (Figure 4-1).

The median nerve (C6 through T1) is formed by contributions from the lateral and medial cords. It passes distally along the arm with the brachial artery and enters the cubital fossa medial to that artery. The nerve is at some risk in the cubital fossa region. It enters the forearm by passing between the heads of the pronator teres muscle and then descends in the forearm between the FDS and the FDP. In the forearm, the nerve innervates all the muscles of the anterior compartment except the flexor carpi ulnaris and the bellies of the FDP to the ring and little fingers. As it approaches the carpal tunnel at the wrist, the median nerve lies just medial to the tendon of the flexor carpi radialis muscle and slightly posterior to the tendon of the palmaris longus muscle, if it is present. The median nerve enters the hand through the carpal tunnel together with the tendons of the FDS, FDP, and FPL and is at risk for laceration at the wrist and compression within the carpal tunnel, deep to the flexor retinaculum (transverse carpal ligament). Typically, the recurrent branch of the median nerve arises distal to the flexor retinaculum and the tunnel to innervate the three thenar muscles: flexor pollicis brevis, abductor pollicis brevis, and the opponens pollicis. The lumbrical muscles of the index and middle fingers receive their motor branches from adjacent common palmar digital branches.

The remainder of the median nerve divides into the common palmar digital nerves to the skin of the thumb and the index, middle, and radial side of the ring fingers, including their dorsal nail beds. The skin of the palm of the hand and thenar eminence is supplied by the palmar cutaneous branch of the median nerve, which typically arises from the median nerve in the distal forearm and does not traverse the carpal tunnel. Intact skin sensation in the palm of the hand suggests carpal tunnel entrapment of the median nerve, whereas loss of palmar skin sensation suggests a higher nerve lesion.

Figure 4-1. Carpal bones in cross section: 1 = pisiform, 2 = hamate, 3 = capitate, 4 = trapezoid, 5 = trapezium, 6 = palmaris longus, 7 = ulnar artery and vein, 8 = flexor carpi radialis, 9 = median nerve, 10 = flexor pollicis longus, 11 = flexor superficialis, 12 = flexor profundus, 13 = extensor carpi ulnaris, 14 = extensor digit minimi, 15 = extensor digitorum, 16 = extensor indicis, 17 = extensor carpi radialis brevis, 18 = extensor carpi radialis, 19 = extensor pollicis longus, 20 = radial artery, 21 = extensor pollicis brevis, 22 = abductor pollicis longus. (Reproduced, with permission, from the University of Texas Health Science Center in Houston Medical School.)

Damage to the median nerve in the upper forearm results in loss of pronation, weakness in flexion at the wrist, and medial (ulnar) deviation. There will also be loss of flexion at the proximal interphalangeal joint of the index, middle, ring, and little fingers and loss of flexion at the distal interphalangeal joints of the index and middle fingers. Damage to the median nerve in the upper forearm or at the wrist will also result in loss of flexion, abduction and opposition of the thumb, and flexion at the metacarpal phalangeal joints of the index and middle fingers. Loss of the function of the median nerve results in the “hand of benediction” (a condition in which index and middle fingers are extended, with the ring and small fingers flexed) when the patient is asked to make a fist and an “ape hand” (MP joint extended, PIP and DIP joints flexed) due to longstanding injury with atrophy of the thenar muscles (Figure 4-2).

The ulnar nerve, which innervates all the other intrinsic hand muscles not noted above, enters the hand anterior to the flexor retinaculum and medial to the ulnar artery. The artery and the nerve are covered anteriorly by a condensation of the fascia of the forearm, called the volar carpal ligament. Thus the ulnar nerve and artery come to lie in the Guyon canal, bounded anteriorly by the volar carpal ligament, posteriorly by the flexor retinaculum, medially by the pisiform, and laterally by the hook of the hamate.

Figure 4-2. The median nerve. (Reproduced, with permission, from Waxman SG. Clinical Neuroanatomy, 25th ed. New York: McGraw-Hill, 2003:352.)

COMPREHENSION QUESTIONS

4.1 You are examining an axial (cross-sectional) magnetic resonance imaging (MRI) scan of the wrist and have identified the carpal tunnel. Which of the following is the structure that forms the anterior wall of the tunnel?

A. Palmar aponeurosis

B. Volar carpal ligament

C. Flexor retinaculum

D. Extensor retinaculum

E. Deep fascia

4.2 As you are explaining carpal tunnel syndrome to a woman who has the condition, you show her where the median nerve is located just as it is about to enter the tunnel. Where is the median nerve located?

A. Just lateral to the flexor carpi radialis tendon

B. Just medial to the flexor carpi radialis tendon

C. Just medial to the flexor palmaris longus tendon

D. Just lateral to the flexor carpi ulnaris tendon

E. Just medial to the flexor carpi ulnaris tendon

4.3 If the median nerve were severed in an industrial accident at the wrist, which of the following muscles would still retain their function?

A. Flexor pollicis brevis

B. Abductor pollicis brevis

C. Opponens pollicis

D. Lumbricals of the index and middle fingers

E. Lumbricals of the ring and little fingers

Answers

4.1 C. The flexor retinaculum or transverse carpal ligament forms the anterior boundary of the carpal tunnel.

4.2 B. The median nerve lies just medial to the tendon of the flexor carpi radialis at the wrist.

4.3 E. The lumbricals to the ring and little finger are innervated by the ulnar nerve.

ANATOMY PEARLS

⯈ All muscles of the anterior compartment of the forearm are supplied by the median nerve except for the flexor carpi ulnaris and the medial half of the flexor digitorum profundus, which are innervated by the ulnar nerve.

⯈ Injury to the median nerve results in the hand of benediction when one attempts to make a fist.

⯈ The carpal tunnel is formed by the flexor retinaculum and the eight carpal bones.

⯈ The carpal tunnel contains nine tendons (four for the FDS, four for the FDP, and one for the FPL) and the median nerve.

⯈ The median nerve supplies five muscles in the hand (flexor pollicis brevis, abductor pollicis brevis, opponens pollicis, lumbricals 1 and 2); the skin of the thumb; and the index, middle, and lateral ring fingers.

⯈ The palmar cutaneous branch of the median nerve does not traverse the carpal tunnel.

⯈ The ulnar nerve does not traverse the carpal tunnel; it enters the hand anterior to the flexor retinaculum in Guyon canal.

References

Gilroy AM, MacPherson BR, Ross LM. Atlas of Anatomy, 2nd ed. New York, NY: Thieme Medical Publishers; 2012:370−371. 

Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy, 7th ed. Baltimore, MD: Lippincott Williams & Wilkins, 2014:761−764, 786, 790−792. 

Netter FH. Atlas of Human Anatomy, 6th ed. Philadelphia, PA: Saunders, 2014: plates 447, 449−450.

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