2.2 Peripheral Nerves & Plexuses

Peripheral Nervous System in the IONM Context

The peripheral nervous system (PNS) includes cranial nerves (covered in 2.3), spinal nerve roots, plexuses, and named peripheral nerves. For spine and peripheral cases, CNIM practice hinges on knowing where you stimulate, what you record, and how a change's pattern points to plexus stretch versus cord injury versus anesthetic drift.

Sensory SSEP stimulation activates large myelinated afferents under the cathode. Motor responses (MEP/EMG) depend on axons remaining excitable from cortex or root to muscle. A "bad signal" is not automatically a spinal cord alert—localization first.

Brachial Plexus: Anatomy That Explains Positioning Injuries

The brachial plexus (C5–T1 roots → trunks → divisions → cords → terminal nerves) supplies the upper limb. Terminal nerves most relevant to monitoring and injury patterns:

Upper trunk (C5–C6) injuries affect shoulder/elbow flexion patterns; lower trunk (C8–T1) injuries hit intrinsic hand muscles and ulnar-innervated SSEPs/EMG. In the OR, shoulder taping, Trendelenburg, sternal retractors (cardiac), and malpositioned arm boards produce plexopathy patterns that look alarming until you compare sides and peripheral potentials.

Lumbosacral Plexus and Lower-Limb Nerves

The lumbar plexus (L1–L4) and sacral plexus (L4–S4) form the lumbosacral plexus. Key nerves for CNIM:

• Femoral nerve (L2–L4) — quadriceps EMG; lithotomy / retractor risk in pelvic surgery
• Sciatic nerve (L4–S3) — divides into tibial and common peroneal; hip arthroplasty and stretch risk
• Tibial nerve — primary ankle SSEP stimulation site (medial malleolus / popliteal options)
• Common peroneal (fibular) nerve — alternative lower-extremity SSEP stim; highly vulnerable at the fibular head from stirrups or tight wraps
• Pudendal nerve — selected pelvic / sphincter monitoring contexts

Knowing root contributions helps when a surgeon asks whether an L5 root alert should affect tibial SSEP, EHL EMG, or both.

SSEP Stimulation Nerves: Practical Landmarks

Upper extremity

• Median nerve at the wrist — between palmaris longus and flexor carpi radialis tendons; cathode proximal for orthodromic sensory volley toward the cord. Generates robust cortical N20 in most patients.
• Ulnar nerve at the wrist or elbow — useful when median access is poor (trauma, lines) or when lower-trunk/ulnar neuropathy is the clinical concern. Elbow stimulation can bypass a distal lesion but may increase stimulus artifact.

Lower extremity

• Posterior tibial nerve at the ankle (posterior to medial malleolus) — workhorse for P37 cortical SSEP.
• Common peroneal nerve at the fibular head or ankle — alternate site; interpret cautiously if fibular compression is already present.

Always document stim site, intensity, and whether a peripheral/Erb's point or popliteal fossa potential was recorded. Those checkpoints are your peripheral vs central anchors.

Peripheral vs Central Change Localization

A structured comparison prevents false cord alerts:

1. Unilateral peripheral/plexus problem — ipsilateral peripheral potential drops or disappears; cervical/cortical peaks fall on that side only; contralateral limb baselines stable.
2. Root-level problem — may show EMG activity in myotome, dermatomal SSEP change, or selective MEP loss in one root's muscles without global cord pattern.
3. Cord-level problem — bilateral or level-appropriate changes in spinal and cortical SSEPs and/or MEPs above a preserved peripheral potential.
4. Brain / brainstem / anesthetic — cortical peaks degrade with preserved earlier potentials; often bilateral and linked to MAP, volatiles, or hematocrit.

Rule of thumb for CNIM reasoning: if the peripheral nerve action potential is gone, fix the arm/leg/plexus/technical setup before declaring a spinal cord injury. If the peripheral potential is intact and cervical/cortical responses collapse bilaterally after distraction, think cord.

Positioning Stretch and Compression Injuries

Non-surgical neuropathies are among the most common "signal change" explanations outside the sterile field:

• Ulnar nerve at the elbow — external compression on a hard arm board; early ulnar SSEP or hand EMG change
• Brachial plexus stretch — arm abduction >90°, contralateral neck tilt, sternotomy retraction
• Peroneal nerve at fibular head — candy-cane stirrups, tight wraps, lateral knee pressure
• Sciatic stretch — extreme hip flexion in lithotomy
• Femoral compression — self-retaining pelvic retractors

Prevention is part of monitoring professionalism: pad the ulnar groove, check arm position after bed turns, re-check impedances and peripheral potentials after repositioning, and communicate promptly when a unilateral peripheral pattern appears.

Quick localization checklist

• Same change in median and ulnar on one side → think plexus/root/cord, not a single distal nerve
• Only ulnar peripheral and cortical change → think elbow positioning first
• Bilateral tibial cortical loss with intact popliteal potentials after rod distraction → cord until proven otherwise
• Global cortical attenuation with stable subcortical peaks after raising sevoflurane → anesthetic effect

Mastering plexus and named-nerve anatomy turns you from a button-pusher into the person who can tell the surgeon where the nervous system is complaining.