5.3 Mononeuropathies & Radiculopathy Patterns

5.3 Mononeuropathies & Radiculopathy Patterns

Principles of Mononeuropathy Evaluation

Evaluating mononeuropathies requires nerve conduction studies across entrapment sites paired with needle EMG of distal and proximal muscles. Differentiating focal entrapment from radiculopathy or plexopathy is essential in PM&R practice.

Upper Extremity Entrapment Neuropathies

Carpal Tunnel Syndrome (CTS)

CTS results from median nerve compression beneath the transverse carpal ligament (flexor retinaculum).

• Clinical Features: Paresthesias in thumb, index, middle, and radial half of ring finger; thenar weakness/atrophy in severe cases.
• Sensory NCS Criteria: Most sensitive indicator. Prolonged median sensory distal latency (>3.5 ms over 14 cm wrist-to-digit 2, or >2.2 ms over 8 cm palm-to-wrist). Comparative studies: Median vs. Ulnar ring finger latency diff >0.4 ms.
• Motor NCS & Needle EMG: Prolonged distal motor latency (>4.2 ms) recording Abductor Pollicis Brevis (APB). EMG shows denervation in APB; proximal median muscles (Pronator Teres [PT], Flexor Carpi Radialis [FCR]) are normal.

Ulnar Neuropathy at the Elbow (UNE)

Compression of the ulnar nerve at the cubital tunnel or retroepicondylar groove.

• Clinical Features: Numbness in 5th and ulnar 4th digits; intrinsic hand muscle weakness, clawing, positive Froment sign, positive Wartenberg sign.
• Motor NCS Criteria: Across-elbow motor studies recording ADM or FDI: focal slowing (conduction velocity drop >10–11 m/s across elbow) or conduction block (>20–50% drop in CMAP amplitude).
• Needle EMG Pattern: Abnormalities in First Dorsal Interosseous [FDI], Abductor Digiti Minimi [ADM], Flexor Digitorum Profundus digits 4–5 [FDP 4–5], and Flexor Carpi Ulnaris [FCU].

Radial Neuropathy ("Saturday Night Palsy")

Compression of the radial nerve at the spiral groove of the mid-humerus.

• Clinical Features: Wrist drop, finger drop, sensory loss over dorsal first web space.
• Needle EMG Localization: Active denervation in radial muscles distal to spiral groove (Extensor Digitorum Communis [EDC], Extensor Indicis Proprius [EIP], Brachioradialis [BR]). The Triceps and Anconeus are spared (branches originate proximal to spiral groove).

             UPPER EXTREMITY ENTRAPMENT MONONEUROPATHIES
 +-------------------------------------------------------------------+
 | Carpal Tunnel Syndrome (Median)  --> Entrapment at Wrist (TCL)    |
 |   - Key Muscle Abnormal: APB     | Normal: PT, FCR, FPL           |
 | Ulnar Neuropathy at Elbow (UNE)  --> Entrapment at Cubital Tunnel |
 |   - Key Muscle Abnormal: FDI, ADM| Variable: FCU, FDP 4-5         |
 | Radial Neuropathy (Spiral Grv)  --> Entrapment at Mid-Humerus     |
 |   - Key Muscle Abnormal: EDC, EIP| Spared: Triceps, Anconeus      |
 +-------------------------------------------------------------------+

Lower Extremity Focal Neuropathies

Peroneal (Fibular) Neuropathy at Fibular Head

Compression of the common peroneal nerve winding around the neck of the fibula.

• Clinical Features: Foot drop, weak ankle dorsiflexion and foot eversion, sensory loss over lateral leg and foot dorsum.
• Motor NCS & Needle EMG: Conduction block or >10 m/s slowing across fibular head. EMG shows abnormalities in Tibialis Anterior [TA], Extensor Hallucis Longus [EHL], and Peroneus Longus/Brevis.
• Crucial Differentials: Tibialis Posterior [TP] (Tibial nerve, L5) and Short Head of Biceps Femoris (Sciatic nerve) are spared in peroneal neuropathy at fibular head, but abnormal in L5 radiculopathy or sciatic neuropathy.

Cervical & Lumbar Radiculopathy Patterns

Post-Ganglionic vs. Pre-Ganglionic Rule

Sensory Nerve Action Potentials (SNAPs) remain normal in pure radiculopathies. Radicular compression is proximal to the Dorsal Root Ganglion (DRG) (pre-ganglionic). Because the sensory cell body in the DRG and peripheral axon remain intact, Wallerian degeneration does not occur in the peripheral sensory nerve. Plexopathies and peripheral neuropathies occur distal to the DRG, resulting in abnormal/absent SNAPs.

       RADICULOPATHY VS PLEXOPATHY: DORSAL ROOT GANGLION (DRG)
 
 Spinal Cord ---[ Root Lesion ]---( DRG )==== Peripheral Nerve ====>
                  (Radiculopathy: PRE-ganglionic -> SNAP NORMAL)
 
 Spinal Cord ---------------------( DRG )==[ Plexus Lesion ]=======>
                  (Plexopathy: POST-ganglionic -> SNAP ABNORMAL)

Needle EMG Diagnostic Criteria

Radiculopathy requires active denervation or neurogenic MUAPs in:

1. At least two muscles innervated by the same spinal root, supplied by different peripheral nerves, AND
2. Cervical or lumbar paraspinal muscles (posterior primary rami).

High-Yield Myotome Maps

• C6 Radiculopathy: Abnormalities in Biceps Brachii (Musculocutaneous), Brachioradialis (Radial), Pronator Teres (Median), and C6 paraspinals. Lateral antebrachial cutaneous SNAP is normal.
• C7 Radiculopathy: Abnormalities in Triceps (Radial), Pronator Teres (Median), FCR (Median), EDC (Radial), and C7 paraspinals. Radial SNAP is normal.
• L5 Radiculopathy: Abnormalities in Tibialis Anterior (Deep Peroneal), Peroneus Longus (Superficial Peroneal), Tibialis Posterior (Tibial), Gluteus Medius (Superior Gluteal), and L5 paraspinals. Superficial Peroneal SNAP is normal.
• S1 Radiculopathy: Abnormalities in Gastrocnemius / Soleus (Tibial), Gluteus Maximus (Inferior Gluteal), and S1 paraspinals. Sural SNAP is normal. H-reflex is absent/prolonged.