DIP Episode 148 - The Why’s of Anatomy For The USMLE Step 1 (Upper Limbs 2, Brachial Plexus)

Source / episode info

**Episode:**148
**Title:**Divine Intervention Episode 148 – The Why’s of Anatomy For The USMLE Step 1 (Upper Limbs 2, Brachial Plexus)
**Published:**2019-09-10
Source:Episode page

One-liner

This episode provides an advanced review of upper limb anatomy, focusing on the functional consequences of injuries to the brachial plexus (Upper/Lower Trunk), and detailing specific deficits associated with Median, Ulnar, and Radial nerve palsies at various levels.

High-yield summary

Brachial Plexus Injury Gradient: Deficits are more pronounced when the injury is proximal (e.g., Upper Trunk C5-C6) than when it is distal (e.g., wrist).
Upper Trunk Palsy (C5-C6): Leads to weakness in shoulder abduction and external rotation, resulting in a characteristic "Adducted/Internally Rotated" posture (Writers Tip Sign).
Median Nerve Deficits: Loss of function includes thumb opposition, flexion of the index/middle fingers, and loss of sensation over the palmar surfaces of digits 1-3.
Ulnar Nerve Deficits: Characterized by "Claw Hand" affecting digits 4 and 5 (interossei paralysis) and sensory loss on the medial half of digits 4 and 5.
Radial Nerve Deficits: Loss of function affects all wrist, elbow, and finger extensors; a classic sign is wrist drop.

Learning objectives

Identify the specific motor and sensory deficits resulting from injuries to the Median, Ulnar, and Radial nerves at various levels (wrist, elbow).
Differentiate between upper trunk (C5-C6) and lower trunk (C8-T1) brachial plexus injury patterns.
Apply the principle that more proximal nerve lesions result in a greater number of functional deficits.
Recognize classic clinical signs associated with specific peripheral nerve palsies (e.g., wrist drop, claw hand).

Board exam buzzwords

Condition	Key Finding	Association	Board Exam Tip
Median Nerve Injury	Loss of thumb opposition; flattening of the thenar eminence.	Median Claw (Digits 2-3); Flexor pollicis longus/brevis paralysis.	If loss of thumb flexion is present, suspect an injury at the elbow or proximal to it.
Ulnar Nerve Injury	Interossei paralysis; clawing of digits 4 and 5.	Guyon's canal (wrist); Hook fracture of the hamate bone.	The classic "claw" pattern is specific to ulnar nerve deficits in the hand.
Radial Nerve Palsy	Wrist drop; inability to extend wrist/digits.	Fracture of the radial head or mid-shaft spiral groove.	Always test for extension weakness when suspecting a radial nerve deficit.
Upper Trunk Injury (C5-C6)	Adduction and internal rotation of the arm ("Writers Tip Sign").	Birth trauma; forceful separation of head from shoulder.	The "SABER" mnemonic helps recall key muscles/actions lost in this injury pattern.