The median nerve also called the 'eye of the hand,' is a mixed nerve with a role of primary importance in the functionality of the hand. It innervates the group of flexor-pronator muscles in the forearm and most of the musculature present in the radial portion of the hand, controlling abduction of the thumb, flexion of the hand at the wrist, flexion of the digital phalanx of the fingers. Again the nerve allows the sensory innervation to the flying face of the thumb, index, middle and radial side of the ring finger and the entire palmar region of the radial half of the hand. It also provides sensitivity to the dorsal skin of the last two phalanges of the index and middle fingers.

The nerve forms in the cervical area of the spinal cord from the medial and lateral cord of the brachial plexus. These cords form from the ventral primary rami of cervical nerve roots five to eight, as well as, the first thoracic spinal segment. The median nerve descends medially to the brachial artery at the level of the humerus and enters the forearm between the two heads of pronator teres. The nerve is very superficial in the cubital fossa and lies deep to bicipital aponeurosis. In the forearm, the median nerve lies deep to the flexor digitorum superficialis and superficial to flexor digitorum profundus. It then enters the palm under the flexor retinaculum lateral to the tendon of flexor digitorum superficialis and posterior to the tendon of palmaris longus. Pathology and injury to the median nerve can occur anywhere along the length of the median nerve.

Of note, in the arm, there are no muscles innervated by the median nerve. Although a branch to pronator teres is innervated proximal to the elbow joint, there are a few vascular branches of the median nerve that supply to the brachial artery and articular branches of the median nerve innervates the elbow joint. In the forearm, the median nerve innervates the flexor digitorum superficialis, pronator teres, the medial half of the pronator quadratus, the palmaris longus, flexor carpi ulnaris, and flexor carpi radialis. Furthermore, in the hand, the flexor pollicis longus and flexor digitorum profundus are innervated by the anterior interossei branch of the median nerve. Articular branches of the median nerve feed the carpal joints, distal radioulnar, and radiocarpal joint. Multiple communicating branches of the median nerve connect to the ulnar nerve. The median nerve innervates the muscles of the thenar compartments of the palm, flexor pollicis longus, abductor pollicis brevis, opponens pollicis, and adductor pollicis. Also, the palmar cutaneous branch of the median nerve innervates the skin over the thenar eminences and lateral two and a half fingers on the palmar aspect of the hand and the skin over the two and a half fingers over the dorsum of the hand.

The median nerve can be affected by acute traumatic, chronic micro traumatic, and compressive lesions. The nerve can also become damaged during multiple-cause degenerative processes and neuropathies. The different types of lesions can affect the median nerve at various levels along its long path from the brachial plexus and axilla to the hand. Neuropathies mainly concern the distal territory. Peripherally, the median nerve can become compressed under the fascial sheath of the flexor retinaculum, which often causes burning pain, numbness, and tingling (neuropathic pain). This condition is known as entrapment syndrome or carpal tunnel syndrome. The carpal tunnel syndrome pain is explainable as a needle and pin sensation, along with the distribution of the median nerve. The condition is idiopathic and is also associated with hypothyroidism, pregnancy, and diabetes. Decreased sensation over a patient's thenar eminence is an indication of a medial nerve injury that is proximal to the carpal tunnel. The sensation of the thenar eminence receives its nerve supply by a branch of the median nerve, which is proximal to the carpal tunnel, the palmar cutaneous branch of the median nerve.

Clinically, symptoms can be intermittent with flares and remissions.

14) An 18-year-old boy is brought to the ER after suffering a gunshot wound to his back. Radiographic imaging reveals extensive damage to the neural arches of the L1 and L2 vertebrae, with bone and bullet fragments lodged in the vertebral canal at these levels. Comprehensive neurologic examination indicates destruction of the sacral segments of the spinal cord. Which of the following functional outcomes is most likely present?

Destruction of the sacral spinal cord will eliminate parasympathetic outfl ow to the hindgut, pelvic organs, and perineum as well as somatic innervation to much of the pelvis and lower limbs. Because it stimulates gut motility and tone, loss of parasympathetic input will result in relaxation and inactivity of the teniae coli in the descending colon. Choice A (Reduced sweat gland secretion in the abdominal wall) is incorrect. Sweat gland secretion is controlled by the sympathetic system, without parasympathetic balance. Sympathetic outfl ow is from the thoracolumbar spinal cord (T1-L2), and it would not be affected by loss of the sacral spinal cord segments. Choice B (Decreased motility in the duodenum) is incorrect. Decreased gut motility may be the result of loss of parasympathetic input. However, the duodenum is innervated by the vagus nerves, not the sacral spinal cord. Choice C (Increased motility in the ileum) is incorrect. Increased gut motility may result from the interruption of sympathetic input. However, the ileum receives its sympathetic input from the thoracolumbar spinal cord (T1-L2/3), not the sacral spinal cord. Choice D (Paralysis of the psoas major muscle) is incorrect. The psoas major is a large muscle in the posterior abdominal wall. It is a skeletal muscle supplied by somatic motor fi bers via spinal nerves L2-3. Thus, the psoas major is not affected by loss of the sacral spinal cord segments.

15) A 68-year-old man was choking on a piece of steak at a family restaurant. Despite attempts to dislodge the food via abdominal thrusts (or the Heimlich maneuver), his upper airway remained blocked. An emergency medical technician (EMT), eating at the scene, performed an emergency tracheotomy to enable the man to breathe. Which subcutaneous structure was most likely cut during this procedure?

In an emergency tracheotomy, the cricothyroid membrane is incised in order to establish a direct airway for the patient. This procedure is also called a cricothyrotomy or cricothyroidotomy, and it is used as a last resort to circumvent upper airway obstructions. The cricothyroid membrane is an important component of the conus elasticus, which is composed of the vocal ligaments, median cricothyroid membrane, and lateral cricothyroid membranes. The cricothyroid membrane is the perfect location to perform an emergency tracheotomy because of several nearby palpable landmarks, and it is located below the (true) vocal folds, which serve as the main inspiratory sphincter of the larynx. Do not confuse the emergency tracheotomy with a tracheostomy, which is a procedure performed in a hospital setting and involves surgically creating a hole in the cartilaginous rings of the trachea. Choice A (Cricoid cartilage) is incorrect. Due to the thickness of the cricoid cartilage, incising through this cartilage would be diffi cult outside the hospital setting. Moreover, an incised cricoid cartilage would need surgical intervention to heal due to its lack of blood supply. Moreover, damaging the cricoid cartilage would be detrimental to the integrity of the larynx and the laryngeal skeleton. Choice B (Thyrohyoid membrane) is incorrect. Though it is easily palpated due to its position above the laryngeal prominence (or the “Adam’s apple”), the thyrohyoid membrane is located between vertebral levels C3 and C4 and may not establish a direct airway. To combat against an upper airway obstruction, the cricothyroid membrane (between vertebral levels C5 and C6) would be a better option. Damage to the thyrohyoid membrane could also compromise the superior laryngeal artery and the internal branch of the superior laryngeal nerve, which pierce this membrane to enter the larynx. Choice D (Tracheal rings) is incorrect. In a tracheostomy, a hole is created surgically in the cartilaginous rings of the trachea. However, this procedure is usually performed in a hospital setting under sterile conditions. Damage to the thyroid gland and infrahyoid muscles can easily occur if a tracheostomy is not performed correctly. Incising the cricothyroid membrane would be a much easier means of establishing an airway, especially considering the emergency conditions surrounding this patient’s choking incident. Choice E (Isthmus of the thyroid gland) is incorrect. Cutting through the isthmus of the thyroid gland would not establish an airway for this patient, so this option can be easily eliminated. Due to its location at the seventh cervical vertebra, this glandular tissue is often transected or resected during a tracheostomy, when the tracheal rings are incised. However, a tracheostomy is performed in a hospital setting under sterile conditions.

16) A pregnant woman is in a car accident and goes into premature labor. Her fetus is approximately 24 weeks in gestation. Her physicians administer her steroids and try to delay the birth of her baby. The survival rate of a premature baby rises signifi cantly if the baby can reach 28 weeks gestation mainly due to the maturation of the lungs. What stage of lung maturation are the doctors hoping to reach, in which the blood-air barrier is beginning to be established?

The simple cuboidal epithelium within the terminal sacs differentiates into pneumocytes within the terminal sac period. The rapidly proliferating capillary network makes intimate contact with the terminal sacs, and the blood-air barrier is established with Type I pneumocytes (or alveolar epithelial cells). These events take place in the terminal sac period, which runs from embryonic week 24 until birth and are crucial for the survival of an infant born prematurely. Administration of steroids to the infant in utero can promote lung maturation by increasing the proliferation of Type II pneumocytes (or alveolar epithelial cells), which secrete surfactant, a fl uid capable of decreasing the surface tension at the air-alveolar interface. Due to the terminal sac period of lung maturation, a premature infant’s survival rate will increase considerably from week 24 to 28 as the bloodair barrier continues to be established. Choice A (Embryonic period) is incorrect. At approximately 4 weeks of gestation, the respiratory diverticulum, or lung bud, begins to arise from the ventral wall of the foregut. Two longitudinal tracheoesophageal ridges fuse in the midline to form the tracheoesophageal septum, which separates the lung bud away from the foregut, specifi cally the esophagus, forming the trachea. The distal end of the trachea divides into two lateral bronchial buds. At the beginning of week 5, the bronchial buds elongate to form the primary (left and right main stem) bronchi, which further divide into the secondary bronchi. Choice B (Pseudoglandular period) is incorrect. The pseudoglandular period occurs from week 5 to 16 of gestation. During this period of lung maturation, the terminal bronchioles, which were established in the embryonic period of lung development, continue to divide to establish respiratory bronchioles and alveolar ducts. The vascular supply to the lungs also increases; however, respiration is not possible until there are enough capillaries within the lungs to enable suffi cient gas exchange. Adequate gas exchange is not possible until the terminal sac period is reached at approximately the seventh month of gestation. Choice C (Canalicular period) is incorrect. The canalicular period of lung maturation occurs from week 16 to 26 of gestation. During this period of lung development, the terminal sacs (primitive alveoli) form, and capillaries within the lung tissue begins to establish close contact with the primitive alveoli. However, respiration is not possible until the terminal sac period when an adequate supply of capillaries exist to enable suffi cient gas exchange. Adequate gas exchange is not possible until the seventh month of gestation. Choice E (Alveolar period) is incorrect. The alveolar period of lung maturation continues from month 8 of gestation and/ or birth until well into childhood. During this period of lung development, the number of mature alveoli within the lungs continues to increase in number and the blood-air barrier is now well established. However, it is during the terminal sac period of lung development (week 26–birth) when adequate gas exchange is established, making respiration possible in a premature infant.

17) A 12-year-old girl is examined by a school nurse who notices the girl’s right scapula is more prominent than the left, her head is not centered directly over the pelvis, and her right hip is raised and more prominent. When the girl is asked to bend forward at the waist, the nurse observes asymmetry of the trunk. Which of the following diagnoses is most likely?

Scoliosis (G: crookedness) is abnormal lateral and rotational curvature of the spine that may present with uneven hips, shoulders, and rib cage, a head that is not centered over the pelvis, the entire body leaning to one side, back pain, and/or fatigue. The given AnteriorPosterior (AP) X-ray shows an S-shaped curvature of the spine or vertebral column in this patient. In this X-ray, no apparent vertebral anomaly is apparent, so the diagnosis is most likely adolescent idiopathic scoliosis, which has an onset of 10 to 18 years of age and has no known cause. Most cases of adolescent idiopathic scoliosis (with curvatures of less than 20 degrees) require no treatment; however, if the curvature goes above 25 degrees, a back brace can be implemented to slow the progression of scoliosis. Choice B (Lordosis) is incorrect. Lordosis (G: bending backward) is an anteriorly convex curvature of the vertebral column (spine). The cervical and lumbar vertebral curvatures are normally lordotic; however, excessive lordotic curvature (also called hollow back, swayback, and saddleback) can be caused by tight lower back muscles, excessive abdominal fat, and pregnancy. Excessive lordosis can lead to lower back pain and can be treated with strengthening of the abdominal muscles and hamstrings. Choice C (Kyphosis) is incorrect. Kyphosis (G: hump-back) is an anteriorly concave curvature of the vertebral column (spine). The thoracic and sacral vertebral curvatures are normally kyphotic; however, deformities of the spine, due to degenerative arthritis, osteoporosis with compression fractures of the vertebrae, trauma, and developmental problems, can lead to excessive kyphotic curvature (or hunchback). Excessive kyphosis can cause pain and breathing diffi culties. The given lateral X-ray shows an example of abnormal kyphotic curvature of the lumbar spine due to benign compression fractures of the L1 and L3 vertebrae secondary to osteoporosis. This disease is characterized by compromised bone strength and decreased bone mass. The affected L1 and L3 vertebrae depict an anterior wedging deformity, which would cause the patient to appear shorter (lose height). Choice D (Osteoporosis) is incorrect. Osteoporosis is a disease characterized by compromised bone strength and decreased bone mass, which can lead to an increased rate of fracture in the vertebral column, the mid-forearm, and more frequently the proximal femur resulting in hip fractures. Women are four times more likely to receive the prognosis of osteoporosis, with approximately 25% of women between 65 and 85 years old being diagnosed with osteoporosis. Due to the age of this 14-year-old patient and the fi ndings of the nurse, osteoporosis is not likely. Choice E (Osteoarthritis) is incorrect. Osteoarthritis (or degenerative arthritis) erodes the articular cartilage in primarily weight-bearing joints. Because this type of arthritis is found in older populations, this diagnosis is not likely in this 14-year-old patient, especially due to the symptoms noted by the nurse.

This plain fi lm of the lateral knee indicates an avulsion fracture of the tibial tuberosity (white arrowhead), which is the insertion site for the quadriceps femoris muscles (rectus femoris and the vastus muscles [lateralis, medialis, and intermedius]) via the patellar ligament. Sudden, powerful muscle actions may cause avulsion fractures of the attachments of muscles. In this case, the push out of the starting blocks involves sudden, powerful extension of the right knee, which is the main action of the quadriceps. Because of the insertion onto the tibial tuberosity (via the patella), the force of the contraction of the quadriceps muscles may avulse the tibial tuberosity off the anterior surface of the tibia, resulting in the fragmented tibial tuberosity (white arrow), being pulled superiorly above the knee joint into the anterior thigh. The inability to straighten the leg refl ects the loss of extension of the knee from detachment of the quadriceps. Choice A (Gastrocnemius) is incorrect. The gastrocnemius is included in the posterior superfi cial compartment of the leg. It lies across the posterior aspect of the knee and contributes to fl exion of that joint. Because it inserts into the calcaneal tendon (tendo calcaneus; Achilles tendon), contraction of the gastrocnemius may cause an avulsion fracture of the calcaneal tuberosity. However, an avulsion fracture of the tibial tuberosity was shown on the given radiograph. Choice B (Tibialis anterior) is incorrect. The tibialis anterior is a member of the anterior compartment of the leg. It has an extensive origin from the lateral condyle and upper half of the lateral surface of the tibia and the interosseous membrane of the leg, and it crosses the anterior aspect of the ankle to insert onto the dorsum of the foot, thus producing dorsifl exion and inversion of the foot. It is commonly involved in the painful condition of shin splits. Choice C (Adductor magnus) is incorrect. This muscle is contained in the medial (adductor) compartment of the thigh. It runs from the ischial tuberosity to an extensive insertion along the linea aspera, medial supracondylar line, and adductor tubercle of the femur. It acts to adduct and fl ex the hip. Choice E (Semitendinosus) is incorrect. This hamstring muscle is part of the posterior compartment of the thigh. It originates from the ischial tuberosity and inserts via its long cord-like tendon onto the medial side of the proximal tibia, acting to extend the hip and flex the knee.

19) A 17-year-old boy comes to the emergency room after a hard fall onto the lateral aspect of his left shoulder during a high school basketball game. He complains of generalized pain during shoulder motion. On physical examination, the distal end of the clavicle is prominent and distinctly palpable. Radiological fi ndings confi rm the diagnosis of a severe (grade 3) shoulder separation. Which of the following features is a component of this condition?

“Shoulder separation” describes a dislocation of the acromioclavicular joint. In its most severe form (grade 3), the condition includes a tearing of both the intrinsic acromioclavicular ligament and the extrinsic coracoclavicular ligament. As a result, the scapula separates from the clavicle and falls away due to the weight of the upper limb. Thus, the distal end of the clavicle is prominent. Choice A (Dislocated head of the humerus) is incorrect. Dislocations of the GH joint easily occur inferiorly due to its lack of muscular and ligamentous support. Thus, damage to the axillary nerve often occurs following inferior displacment of the head of humerus from the GH joint. However, the acromioclavicular joint, which is more proximal, was injured in this patient. Dislocations of the glenohumeral joint in other directions are more diffi cult (but not impossible) because of the support of the rotator cuff muscles (anteriorly and posteriorly) and the coracoacromial arch (superiorly). Choice C (Fractured clavicle) is incorrect. Radiological imaging would have detected a fractured clavicle, but these tests confi rmed a shoulder separation and not a fractured clavicle. Choice D (Dislocated sternal end of the clavicle) is incorrect. Due to its intrinsic strength, dislocation of the sternoclavicular (SC) joint is rare. Most dislocations of the SC joint occur in persons less than 25 years of age following a fracture of the epiphysial plate of the clavicle. The epiphysis at the proximal end of the clavicle does not close until approximately age 25. Though this patient was under the age of 25, his injury was localized to the acromioclavicular joint. Choice E (Torn anterior glenohumeral [GH] ligament) is incorrect. Three GH ligaments reinforce the anterior part of the joint capsule; however, the GH joint was not involved in this patient.

20) A 1-year-old girl is brought to the clinic for a routine checkup. The child appears normal except for a dimpling of the skin in the lumbar region with a tuft of hair growing over the dimple. You reassure the mother that this condition is seen in 10% to 25% of births and normally has no ill effects. What is this relatively common condition that results from incomplete embryologic development?