7.1 Preganglionic versus Postganglionic Lesions

Why preganglionic versus postganglionic is an NCS question

Outline III.A.1.a asks you to localize lesions by their relationship to the dorsal root ganglion (DRG) and to state the electrodiagnostic effects. Independent OpenExamPrep study for the Registered Nerve Conduction Study Technologist (R.NCS.T.) candidate treats that geography as the reason sensory nerve action potentials (SNAPs) belong on trauma, radiculopathy-versus-plexopathy, and neuronopathy protocols. This chapter is independent teaching that follows published outline topics. It is not an official American Association of Electrodiagnostic Technologists (AAET) course and does not claim partnership with the certifying body.

Every axon you stimulate is kept alive by a cell body (soma). Separate the axon from that soma and Wallerian degeneration follows: the distal fragment becomes inexcitable over days (the clock is the next sections). Somatic motor somata live in the anterior (ventral) horn. Somatic sensory somata live in the DRG. Those two addresses are not interchangeable, so the same anatomic injury can kill motor axons and spare peripheral sensory axons — or kill both.

The sensory neuron is pseudounipolar. One process runs centrally through the dorsal root into the cord. The other runs peripherally through spinal nerve, plexus, and named nerve to receptors. The DRG nourishes both processes. A lesion of the central process — proximal to the DRG — does not automatically degenerate the peripheral sensory axon. A lesion of the peripheral process — distal to the DRG — does.

Do not confuse this sensory use of preganglionic with B-fiber preganglionic autonomic axons, whose ganglion is a sympathetic or parasympathetic autonomic ganglion. Here the ganglion is the DRG. Autonomic Horner-syndrome clues can still sit next to a T1 avulsion; they are neighboring anatomy, not a redefinition of SNAP geography.

Preganglionic lesions: proximal to the DRG

A preganglionic sensory lesion sits proximal to the DRG. The textbook traumatic example is root avulsion: dorsal rootlets torn from the cord, classically in high-energy brachial-plexus trauma. The DRG cell body and its peripheral sensory axon remain in continuity. Those peripheral sensory axons do not undergo Wallerian degeneration. A SNAP recorded from the corresponding cutaneous nerve can remain present even when the dermatome is completely anesthetic, because conscious sensation requires the central process and the cord. A recordable SNAP is not a certificate that the patient can feel.

Motor axons at the same segmental level tell the opposite story. Their cell bodies are in the anterior horn. Avulsion of the ventral root separates motor axons from those somata. Motor axons then degenerate. After Wallerian time, the compound muscle action potential (CMAP) in that myotome may fall. Physician needle electromyography (EMG) can show denervation, including paraspinal muscles supplied by the dorsal ramus. R.NCS.T. technologists do not perform needle examinations; they record the SNAPs and CMAPs that make the localization argument possible.

Handbook-style fact, stated as electrodiagnostic teaching: preganglionic injury proximal to the DRG results in Wallerian degeneration of motor axons only, not of peripheral sensory axons. That sentence is the SNAP-versus-CMAP contrast you apply on case stems. It is not a claim that every avulsion is electrically pure. Combined pre- and postganglionic injuries are common in the same plexus. An absent SNAP does not exclude avulsion; it means there is also postganglionic sensory axon loss, or that you recorded the wrong nerve, or that the study is too early for postganglionic axons to have degenerated.

Radiculopathy (compression or inflammation of a root, usually in the canal or foramen) is the everyday, non-avulsion cousin. For sensory fibers it typically spares the DRG cell body and the peripheral sensory axon. Standard teaching: SNAPs are spared in radiculopathy. The patient can still have radiating pain and dermatomal numbness because the central process or root is blocked or irritable, but the peripheral sensory cable remains connected to its soma. Motor axonal radiculopathy can still reduce CMAP amplitude if enough ventral-root axons are lost. Paraspinal denervation, when the physician needles those muscles, supports a lesion at or proximal to dorsal-ramus takeoff.

Exceptions belong on the same page. A very lateral disc or foraminal lesion can injure the DRG itself and drop SNAPs (postganglionic sensory at the ganglion). Multiple-root disease can mimic plexopathy. SNAPs in older feet may be technically absent. Use the model; do not treat one missing sural SNAP as proof of plexopathy.

Postganglionic lesions: plexus and peripheral nerve

A postganglionic lesion sits distal to the DRG: extraforaminal spinal nerve, plexus, or peripheral nerve. The peripheral sensory axon is cut off from its DRG soma and degenerates. SNAPs in that distribution are reduced or absent once Wallerian degeneration is complete. Motor axons in the same lesion are separated from anterior-horn somata, so CMAPs fall as well if enough axons are lost. Paraspinal muscles are typically spared in a pure plexopathy because the dorsal ramus has already branched.

This is why a low CMAP does not localize. Motor axonal loss occurs with avulsion, axonal radiculopathy, plexopathy, and named-nerve lesions. The SNAP tells you whether the peripheral sensory axon still has its DRG. If you only look at a small abductor pollicis brevis CMAP, you have not separated C8–T1 root disease from lower-trunk plexopathy from a median-nerve lesion.

A named peripheral-nerve lesion is also postganglionic. SNAP and CMAP drop follow one nerve's map, not a whole trunk or a whole myotome. That is how a fibular (peroneal) neuropathy at the fibular head differs from an L5 radiculopathy: the superficial fibular SNAP can fall in the nerve lesion and is typically spared in the root lesion, once degeneration time has passed and the recording is technically adequate.

Neuronopathy: the cell body itself is the target

Sensory neuronopathy (ganglionopathy) kills DRG neurons. Peripheral sensory axons degenerate. SNAPs are lost, often in a non-length-dependent pattern, while motor studies may be relatively spared. Teaching associations include toxic ganglionopathy (for example, high-dose pyridoxine) and immune sensory neuronopathy. The electrical signature is widespread SNAP loss that does not behave like a distal dying-back polyneuropathy.

Motor neuronopathy (anterior-horn cell disease) kills motor somata. CMAPs may fall; SNAPs remain because DRG cells are intact. That SNAP-present, motor-axonal pattern is the electrodiagnostic cousin of motor neuron disease, not of mixed axonal polyneuropathy.

Localization table

Why SNAPs are the technologist's localization tool

The CMAP is a poor sole localizer for the pre- versus postganglionic question. Motor axons degenerate with any disconnection distal to the anterior horn: ventral root, spinal nerve, plexus, or peripheral nerve. After Wallerian time, a small CMAP only says that enough motor axons, neuromuscular junctions, or muscle fibers are missing or blocked. It does not say where.

The SNAP reports sensory geography relative to the DRG:

1. Present SNAP in an anesthetic territory, after enough days that postganglionic axons would have degenerated, supports a preganglionic sensory lesion (avulsion or SNAP-sparing radiculopathy).
2. Absent SNAP in a plexus or named-nerve distribution supports postganglionic sensory axon loss (plexopathy, peripheral nerve, ganglionopathy, or combined injury).
3. SNAP map (which cutaneous nerves are out) distinguishes a trunk or cord from a single distal nerve, once you know plexus sensory territories from anatomy chapters.
4. Timing is part of the tool. SNAPs do not vanish the hour of injury. Very early studies can still show potentials that will later disappear if the lesion is postganglionic. Interpret SNAP presence on day 0 differently from SNAP presence on day 10.

Protocol design follows that logic. On a flail arm after trauma, recording lateral antebrachial cutaneous, median digital, ulnar digital, medial antebrachial cutaneous, and relevant CMAPs is not a random menu. Those SNAPs ask whether C6, C7, and C8–T1 peripheral sensory axons are still attached to their ganglia. A preserved SNAP in a numb thumb with a collapsed median motor response is localization, not a machine error. Lower-cervical and T1 preganglionic injury may be accompanied clinically by Horner syndrome because preganglionic sympathetics are neighboring anatomy — a bedside clue, not a SNAP substitute.

If you remember one sentence from III.A.1.a, remember this: the SNAP is the technologist's report of whether the peripheral sensory axon still lives with its DRG; the CMAP cannot carry that report alone. Independent OpenExamPrep study of lesion types starts there, then adds conduction block, demyelination, and Wallerian timing in the next sections.