A 74-year-old male is brought to the emergency department 1 hour after he fell from the top of the staircase at home. He reports pain in his neck as well as weakness of his upper extremities. He is alert and immobilized in a cervical collar. He has hypertension treated with hydrochlorothiazide. His pulse is 90/min and regular, respirations are 18/min, and blood pressure is 140/70 mmHg. Examination shows bruising and midline cervical tenderness. Neurologic examination shows diminished strength and sensation to pain and temperature in the upper extremities, particularly in the hands. Upper extremity deep tendon reflexes are absent. Strength, sensation, and reflexes in the lower extremities are intact. Anal sensation and tone are present. Babinski's sign is absent bilaterally. Which of the following is most likely to confirm the cause of this patient's neurologic examination findings?

MRI of the cervical spine without contrast

CT of the cervical spine with contrast

A 29-year-old woman presents with progressive vision loss in her right eye and periorbital pain for 5 days. She says that she has also noticed weakness, numbness, and tingling in her left leg. Her vital signs are within normal limits. Neurological examination shows gait imbalance, positive Babinski reflexes, bilateral spasticity, and exaggerated deep tendon reflexes in the lower extremities bilaterally. FLAIR MRI is obtained and is shown in the image. Which of the following is the most likely cause of this patient’s condition?

Acute disseminated encephalomyelitis

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL)

A 65-year-old man is referred by his primary care provider to a neurologist for leg pain. He reports a 6-month history of progressive bilateral lower extremity pain that is worse in his left leg. The pain is 5/10 in severity at its worst and is described as a "burning" pain. He has noticed that the pain is acutely worse when he walks downhill. He has started riding his stationary bike more often as it relieves his pain. His past medical history is notable for hypertension, diabetes mellitus, and a prior myocardial infarction. He also sustained a distal radius fracture the previous year after falling on his outstretched hand. He takes aspirin, atorvastatin, metformin, glyburide, enalapril, and metoprolol. He has a 30-pack-year smoking history and drinks 2-3 glasses of wine with dinner every night. His temperature is 99°F (37.2°C), blood pressure is 145/85 mmHg, pulse is 91/min, and respirations are 18/min. On exam, he is well-appearing and in no acute distress. A straight leg raise is negative. A valsalva maneuver does not worsen his pain. Which of the following is the most appropriate test to confirm this patient's diagnosis?

Computerized tomography myelography

A 29-year-old woman presents to the primary care office for a recent history of falls. She has fallen 5 times over the last year. These falls are not associated with any preceding symptoms; she specifically denies dizziness, lightheadedness, or visual changes. However, she has started noticing that both of her legs feel weak. She's also noticed that her carpet feels strange beneath her bare feet. Her mother and grandmother have a history of similar problems. On physical exam, she has notable leg and foot muscular atrophy and 4/5 strength throughout her bilateral lower extremities. Sensation to light touch and pinprick is decreased up to the mid-calf. Ankle jerk reflex is absent bilaterally. Which of the following is the next best diagnostic test for this patient?

Electromyography (including nerve conduction studies)

Peripheral nerve surgery basics for USMLE Step 1: High-Yield Surgery Lessons

🧬 Anatomy - Nerve Network Blueprint

Epineurium: Outermost dense connective tissue sheath enclosing the entire nerve; contains the vasa nervorum.
Perineurium: Surrounds each nerve fascicle (a bundle of axons).
- Forms the blood-nerve barrier, regulating the endoneurial microenvironment.
Endoneurium: Delicate connective tissue surrounding individual nerve fibers (axons) and their associated Schwann cells.

Peripheral nerve cross-section and neuron diagram

Schwann Cells: Glial cells of the PNS that produce the myelin sheath. One Schwann cell myelinates one segment of a single axon.

⭐ The Perineurium is the key layer for surgical coaptation (nerve repair). Aligning fascicles using perineurial sutures is critical for successful axonal regeneration and functional recovery.

⚡ Pathology - When Nerves Snap

Wallerian Degeneration: An active process of anterograde degeneration affecting the axon and myelin sheath distal to an injury site (occurs in axonotmesis/neurotmesis). Begins within 24-72 hours.
- Cleanup: Axonal cytoskeleton disintegrates; myelin breaks into ovoids. Macrophages are recruited to phagocytose debris.
- Regeneration: The proximal stump forms axonal sprouts. Schwann cells proliferate, forming Bands of Büngner that guide sprouts toward their distal target.

⭐ Axonal regeneration proceeds at ~1 mm/day or 1 inch/month. This slow rate is critical for predicting recovery time and preventing irreversible muscle atrophy after ~18-24 months.

Nerve Injury Classifications

Seddon Class	Sunderland Grade	Key Structure Damaged	Prognosis & Notes
Neuropraxia	I	Myelin (focal demyelination)	Excellent; full recovery in days/weeks.
Axonotmesis	II	Axon (endoneurium intact)	Good; requires axonal regeneration.
	III	+ Endoneurium	Fair; intrafascicular fibrosis can block.
	IV	+ Perineurium	Poor; neuroma-in-continuity common.
Neurotmesis	V	Full Transection (epineurium)	Very poor; requires surgical repair.

⚡ Clinical - Shocking Solutions & Fixes

Diagnosis:
- History & Physical: Note sensory (paresthesia, numbness) & motor (weakness, atrophy) deficits.
- Provocative Tests: Tinel's sign (tapping nerve → distal tingling), Phalen's test (wrist flexion → median nerve symptoms).
Electrodiagnostics (EDx):
- Nerve Conduction Study (NCS): Differentiates pathology.
  - ↓ Conduction Velocity → Demyelination (focal compression).
  - ↓ Amplitude → Axonal loss (severe injury).
- Electromyography (EMG): Needle in muscle assesses denervation.
  - Fibrillations & positive sharp waves indicate active denervation.

⭐ Wallerian degeneration (axonal breakdown distal to injury) begins 24-72 hours post-injury. However, EMG changes like fibrillations are not apparent for 2-3 weeks, making early EMG less useful for acute transection prognosis.

Management Flow:

⚡ Biggest Takeaways

Seddon classification guides management: Neurapraxia (conduction block, full recovery), Axonotmesis (axon loss, sheath intact), and Neurotmesis (transection, requires surgery).

Wallerian degeneration is the breakdown of the axon distal to the injury site in axonotmesis and neurotmesis.

Nerve regeneration proceeds at ~1 mm/day (1 inch/month); a positive Tinel's sign tracks progress.

EMG/NCS are essential to localize the lesion and assess severity and recovery.

Surgical options for neurotmesis include direct repair, nerve grafting (sural nerve donor), or nerve transfer.

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Peripheral nerve surgery basics