A 43-year-old woman comes to the physician because of tingling and weakness in her left arm for the past 2 days. An image of the brachial plexus is shown. Nerve conduction study shows decreased transmission of electrical impulses in the labeled structure. Physical examination is most likely to show impairment of which of the following movements?

Extension of the wrist and fingers

Abduction of the shoulder above 100 degrees

Flexion of the metacarpophalangeal joints

A 47-year-old woman presents to her primary care provider because of numbness and tingling on the palmar aspects of both hands. She denies any symptoms at the base of her thumbs. The symptoms are worse on the right (dominant hand) and are increased with activities such as driving or brushing her hair. She frequently wakes up with pain and has to shake her hand for pain relief. She has had rheumatoid arthritis for 9 years, for which she takes methotrexate. Her blood pressure is 124/76 mm Hg, the heart rate is 75/min, and the respiratory rate is 15/min. Lightly tapping over the middle of the anterior aspect of the right wrist leads to a tingling sensation in the palm. In this patient, electromyography (EMG) will most likely show which of the following results?

Focal slowing of conduction velocity in the median nerve in the carpal tunnel

Neuropathic changes in the palmar branch of the median nerve

Denervation in C7 innervated paraspinal, arms, and shoulder muscles

Widespread symmetrical neuropathic changes without focal abnormalities

Widespread denervation in proximal muscles with normal sensory nerves

A 35-year-old man is brought to the trauma bay by ambulance after sustaining a gunshot wound to the right arm. The patient is in excruciating pain and states that he can't move or feel his hand. The patient states that he has no other medical conditions. On exam, the patient's temperature is 98.4°F (36.9°C), blood pressure is 140/86 mmHg, pulse is 112/min, and respirations are 14/min. The patient is alert and his Glasgow coma scale is 15. On exam, he has a single wound on his right forearm without continued bleeding. The patient has preserved motor and sensation in his right elbow; however, he is unable to extend his wrist or extend his fingers further. He is able to clench his hand, but this is limited by pain. On sensory exam, the patient has no sensation to the first dorsal web space but has preserved sensation on most of the volar surface. Which of the following structures is most likely injured?

Recurrent motor branch of the median nerve

A 35-year-old woman presents with progressive vision loss and severe headache. MRI shows cavernous sinus thrombosis with extension into the superior ophthalmic vein. Blood cultures grow Staphylococcus aureus. History reveals she had squeezed a facial pustule near her upper lip 5 days prior. Evaluate the anatomical explanation and risk stratification for this complication.

Valveless facial venous system allowing retrograde flow from danger triangle to cavernous sinus

Direct lymphatic spread from facial infection due to rich subcutaneous lymphatic network

Contiguous spread through cribriform plate from nasal cavity involvement

Hematogenous seeding via internal jugular vein and sigmoid sinus

Extension through pterygoid venous plexus communicating with middle meningeal vein

Upper limb fascial compartments for USMLE Step 1: High-Yield Anatomy Lessons

Upper Limb Fascia - The Body's Tupperware

Axillary Fascia: Forms the axilla floor, blending with pectoral and brachial fascia.
Brachial Fascia (Arm): A deep fascial sleeve creating two compartments via intermuscular septa:
- Anterior: Flexors (Biceps, Brachialis, Coracobrachialis).
- Posterior: Extensors (Triceps).
Antebrachial Fascia (Forearm): Encases forearm muscles; continuous with brachial fascia.

⭐ Compartment Syndrome: A surgical emergency common in the forearm due to its tight fascial boundaries, risking neurovascular compromise from swelling.

Axial cross-section of arm showing fascial compartments

Arm Compartments - Flex vs. Extend

Cross-section of arm showing fascial compartments

Anterior (Flexor) Compartment
- Muscles: Biceps brachii, Brachialis, Coracobrachialis.
- 📌 Mnemonic: Buy Big Cars.
- Innervation: Musculocutaneous nerve (C5-C7).
- Artery: Brachial artery.
- Function: Flexes elbow & supinates forearm.
Posterior (Extensor) Compartment
- Muscles: Triceps brachii.
- Innervation: Radial nerve (C5-T1).
- Artery: Profunda brachii artery (deep brachial).
- Function: Extends elbow.

⭐ Clinical Pearl: Mid-shaft humeral fractures risk injuring the radial nerve in the radial groove, leading to paralysis of extensors ("wrist drop").

Forearm Compartments - Grip & Go

Two primary fascial compartments separated by the interosseous membrane and lateral intermuscular septum.

Feature	Anterior (Flexor-Pronator)	Posterior (Extensor-Supinator)
Muscles	Superficial & deep groups. Flexors of wrist/digits, pronators.	Superficial & deep groups. Extensors of wrist/digits, supinator.
Innervation	Median & Ulnar Nerves	Radial Nerve (deep branch)
Artery	Ulnar & Radial Arteries	Posterior Interosseous Artery
Function	Wrist flexion, finger flexion, pronation.	Wrist extension, finger extension, supination.

-   Superficial: Pronator Teres, Flexor Carpi Radialis, Palmaris Longus, Flexor Carpi Ulnaris.
-   📌 Mnemonic: **P**ass **F**ail **P**ass **F**ail (from lateral to medial).
-   Intermediate: Flexor Digitorum Superficialis.
-   Deep: Flexor Digitorum Profundus, Flexor Pollicis Longus, Pronator Quadratus.

⭐ Compartment Syndrome: The earliest and most common sign is pain disproportionate to injury, especially with passive stretch of the digits. Paresthesia is a later sign. Pulselessness is a very late finding.

Hand & Clinical Spaces - Danger Zones

Hand Fascial Spaces & Synovial Sheaths

Key Spaces & Infection Routes:
- Thenar Space: Lateral aspect; infections from index finger.
- Midpalmar Space: Central; infections from middle/ring fingers.
- Space of Parona: Deep wrist space; potential for proximal spread to forearm.
Infectious Tenosynovitis:
- 📌 Kanavel's Signs (FLEX):
  - Finger held in slight flexion.
  - Length of tendon sheath is tender.
  - Enlarged finger (fusiform swelling).
  - Xtension (passive) causes pain.

⭐ Horseshoe Abscess: Infection can spread from the little finger (ulnar bursa) to the thumb (radial bursa) via the communicating Space of Parona.

High‑Yield Points - ⚡ Biggest Takeaways

Fascial compartments of the upper limb separate anterior (flexor) and posterior (extensor) muscle groups.

Compartment syndrome, a surgical emergency, results from increased intracompartmental pressure, leading to ischemia.

Arm: The anterior compartment is innervated by the musculocutaneous nerve; the posterior by the radial nerve.

Forearm: The anterior compartment is primarily supplied by the median nerve; the posterior by the radial nerve.

Severe forearm compartment syndrome can cause Volkmann's ischemic contracture, a permanent flexion deformity.

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