A 39-year-old male who recently presented with acetaminophen overdose was admitted to the MICU, where several attempts were made at obtaining intravenous access without success. The decision was made to place a right axillary arterial line, which became infected and was removed by the medical student while the patient was still intubated. It was later noticed that he had substantial swelling and bruising of the upper extremity. Given his sedation, a proper neuro exam was not performed at that time. Several days later, after the patient's liver function improved, he was successfully extubated. On exam, he complained of lack of sensation over the palmar and dorsal surface of the small finger and half of the ring finger, as well as weak digit abduction, weak thumb adduction, and weak thumb-index finger pinch of the affected extremity. What is the most likely cause and corresponding location of the injury?

Compression of ulnar nerve secondary to coagulopathy

Compression of median nerve secondary to coagulopathy

Stretch injury to ulnar nerve secondary to frequent repositioning

Needle injury to ulnar nerve secondary to blind line placement

Needle injury to median nerve secondary to blind line placement

A 32-year-old man comes to the physician because of a 1-month history of intermittent tingling of his hand. He is an avid cyclist and has recently started training for a cycle marathon. Physical examination shows decreased grip strength in the right hand and wasting of the hypothenar eminence. On asking the patient to grasp a piece of paper between his right thumb and right index finger in the first web space, there is hyperflexion of the right thumb interphalangeal joint. Which of the following additional findings is most likely in this patient?

Inability to extend the little finger at the proximal interphalangeal joints

Loss of sensation over the dorsum of the medial half of the hand

Inability to extend the ring finger at the metacarpophalangeal joint

Inability to flex the index finger at the interphalangeal joints

Loss of sensation over the palmar aspect of the middle finger

Nerves and blood supply of upper limb for USMLE Step 3: High-Yield Anatomy Lessons

Brachial Plexus - The Nerve Highway

Structure: A network of nerves originating from spinal roots C5-T1.
Mnemonic: 📌 Really Tired Drink Coffee Black (Roots, Trunks, Divisions, Cords, Branches).

Brachial Plexus Structure

Key Injuries:
- Upper Trunk (C5-C6): Erb-Duchenne palsy ("waiter's tip" hand).
- Lower Trunk (C8-T1): Klumpke's palsy (total claw hand).

⭐ The long thoracic nerve, arising from roots C5, C6, and C7, innervates the serratus anterior. Injury causes a "winged scapula," a classic exam finding.

Major Nerves - The 5 Big Wires

📌 My Aunt Runs Makes Uncles Nervous → Musculocutaneous, Axillary, Radial, Median, Ulnar (from lateral to medial).

Brachial Plexus: Roots, Trunks, Divisions, Cords, Nerves

Musculocutaneous (C5-C7)
- Motor: Biceps, brachialis, coracobrachialis (flexes elbow).
- Sensory: Lateral forearm.
Axillary (C5-C6)
- Motor: Deltoid (abducts arm), teres minor.
- Sensory: Skin over deltoid.
Radial (C5-T1)
- Motor: Extensors of wrist/fingers ("wrist drop").
- Sensory: Posterior arm/forearm, dorsal hand (lateral 3.5 digits).
Median (C5-T1)
- Motor: Forearm flexors, thenar eminence.
- Sensory: Palmar hand (lateral 3.5 digits).
Ulnar (C8-T1)
- Motor: Intrinsic hand muscles (except thenar/lumbricals 1-2).
- Sensory: Medial 1.5 digits.

⭐ Ulnar Claw vs. Hand of Benediction: Ulnar claw is seen at rest with a distal ulnar lesion. Hand of Benediction is seen when asking a patient with a proximal median nerve lesion to make a fist.

Arterial Supply - The Red River

Pathway: Subclavian → Axillary (at lateral 1st rib) → Brachial (at teres major) → bifurcates into Radial & Ulnar arteries in the cubital fossa.

Key Branches:
- Brachial: Profunda brachii (supplies posterior compartment).
- Axillary: Ant/Post circumflex humeral (supply humeral head).
Anastomoses: Rich networks around the scapula and elbow provide crucial collateral circulation, vital in occlusions.
📌 Mnemonic (Axillary Artery): "Screw The Lawyer, Save A Patient" (Superior thoracic, Thoracoacromial, Lateral thoracic, Subscapular, Ant/Post circumflex humeral).

⭐ Allen's Test: Assesses collateral circulation by checking patency of radial and ulnar arteries before cannulation (e.g., ABG). Essential to prevent hand ischemia.

Arterial supply and ulnar nerve of the forearm and hand

Clinical Correlates - When Wires Go Wrong

Surgical exposure of median nerve and artery in the wrist

Upper Trunk (C5-C6) → Erb-Duchenne Palsy: "Waiter's tip" hand from birth trauma or falls.
Lower Trunk (C8-T1) → Klumpke's Palsy: Total claw hand from upward arm pull.
Long Thoracic n. → Winged Scapula: Post-mastectomy; serratus anterior paralysis.
Axillary n. → Deltoid Paralysis: Surgical neck of humerus fracture.
Radial n. → Wrist Drop: Mid-shaft humerus fracture; crutch palsy.
Median n. → "Ape Hand" / "Hand of Benediction": Carpal tunnel, supracondylar fracture.
Ulnar n. → "Ulnar Claw" Hand: Medial epicondyle fracture.

⭐ Supracondylar Humerus Fracture: A common pediatric injury. Can damage the brachial artery (→ Volkmann's ischemic contracture) and the median nerve, causing significant neurovascular compromise.

Vascular Compromise:
- Volkmann's Contracture: Post-ischemic forearm muscle fibrosis.

Brachial plexus roots are C5-T1; its cords supply all major upper limb nerves.

Axillary nerve injury (surgical neck of humerus fracture) causes deltoid paralysis and sensory loss over the shoulder.

Radial nerve palsy (mid-shaft humerus fracture) leads to wrist drop due to extensor muscle paralysis.

Median nerve injury results in an “Ape Hand” and loss of thumb opposition.

Ulnar nerve damage causes a “Claw Hand” affecting the 4th and 5th digits.

The axillary artery continues as the brachial artery, the principal arterial supply.

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