EDX · EP 09 · ELECTRODIAGNOSTICS
Before You Listen
Before You Listen
- Prerequisites: lumbosacral plexus organization (lumbar plexus L1-L4 within psoas; sacral plexus L4-S3 on piriformis) from Episode 7; the dorsal root ganglion (DRG) principle that sensory nerve action potentials (SNAPs) are normal in radiculopathy and abnormal in plexus or peripheral nerve lesions; basics of nerve conduction studies (NCS) including compound muscle action potential (CMAP), sensory nerve action potential (SNAP), and the difference between conduction block (focal demyelination) and reduced distal CMAP amplitude (axonal loss with Wallerian degeneration); basics of needle electromyography (EMG) including fibrillation potentials, positive sharp waves, and the localization power of paraspinal muscle sampling; the upper-extremity entrapment patterns of Episode 8.
- Runtime: 1 hour 0 minutes 15 seconds.
- Topic in one line: the foot drop algorithm with the short head of the biceps femoris (normal in fibular at fibular head, abnormal in sciatic), the tibialis posterior (separates L5 radiculopathy from fibular neuropathy), and the superficial fibular SNAP (excludes radiculopathy by the DRG principle); fibular neuropathy at the fibular head with conduction block and the accessory deep peroneal nerve variant; sciatic neuropathy with preferential fibular-division involvement (most common iatrogenic cause: total hip arthroplasty); piriformis syndrome as a diagnosis of exclusion; tarsal tunnel syndrome and its technical limitations; meralgia paresthetica (pure sensory lateral femoral cutaneous entrapment); axonal vs demyelinating prognosis. Memorize them cold.
Vignette. A 62-year-old woman who underwent right total hip arthroplasty six weeks ago presents with right foot drop and numbness over the dorsum of her right foot. On examination, ankle dorsiflexion and great toe extension are 1/5, foot eversion is 2/5, and ankle inversion and plantarflexion are full strength. Sensation is reduced over the dorsum of the foot and the lateral lower leg in the superficial fibular distribution. The right hip and gluteal muscles appear normal in bulk and strength. Nerve conduction studies show a reduced right fibular compound muscle action potential amplitude recording from the extensor digitorum brevis, with no further amplitude drop between below-fibular-head and above-fibular-head stimulation. The right tibial motor study recording from the abductor hallucis is normal in amplitude and latency. The right superficial fibular sensory nerve action potential is absent. The right sural sensory nerve action potential is normal. Needle electromyography reveals fibrillation potentials and positive sharp waves in the right tibialis anterior, peroneus longus, extensor hallucis longus, and the right short head of the biceps femoris. The right tibialis posterior, medial gastrocnemius, gluteus medius, and lumbar paraspinal muscles are all normal.
What does the abnormal short head of the biceps femoris on needle electromyography tell you about the anatomic level of the lesion, why does the absent superficial fibular sensory nerve action potential exclude L5 radiculopathy as the cause, what is the clinical significance of the normal tibialis posterior in this scenario, and how does the timing six weeks after total hip arthroplasty fit the most common iatrogenic mechanism for this lesion?
(Answer at the end of this chapter)
Section 1: The Foot Drop Algorithm and the Short Head of the Biceps Femoris
Bottom line: the foot drop differential is the most heavily tested EDX scenario in the specialty; the short head of the biceps femoris is the only thigh muscle innervated by the fibular division of the sciatic nerve and is the single muscle that resolves the question (normal in fibular neuropathy at the fibular head, abnormal in sciatic neuropathy); the tibialis posterior separates L5 radiculopathy (abnormal) from fibular neuropathy (normal); the superficial fibular SNAP excludes L5 radiculopathy by the DRG principle (normal in radiculopathy, abnormal in fibular or sciatic neuropathy).
The differential diagnosis of foot drop is the single most frequently tested clinical scenario in electrodiagnostic medicine on the board examination. When a patient presents with ankle dorsiflexion weakness, the electromyographer must systematically distinguish three possibilities: common fibular neuropathy at the fibular head, sciatic neuropathy preferentially affecting the fibular division, and L5 radiculopathy. No single finding makes the diagnosis; the constellation points to the correct localization.
The short head of the biceps femoris is the single most important muscle and is tested relentlessly. It is the only muscle in the thigh innervated by the fibular division of the sciatic nerve (L5, S1). All other hamstring muscles (long head of biceps femoris, semimembranosus, semitendinosus) are innervated by the tibial division. In fibular neuropathy at the fibular head, the short head of the biceps femoris is normal because the fibular nerve is injured distal to where this muscle receives its innervation. In sciatic neuropathy with fibular predominance, the short head is abnormal because the fibular division is injured proximal to the branching point. In L5 radiculopathy, the short head may be abnormal but variably so. If you remember one muscle for the foot drop differential, it is the short head of the biceps femoris: normal = fibular at the knee; abnormal = sciatic or possibly radiculopathy.
The tibialis posterior is the most important muscle for distinguishing L5 radiculopathy from fibular neuropathy. It is innervated by the tibial nerve but receives its L5 root supply. In L5 radiculopathy the tibialis posterior is abnormal because it shares the L5 root with the fibular muscles even though supplied by a different peripheral nerve. In fibular neuropathy at the fibular head the tibialis posterior is normal because the tibial nerve is unaffected. Weak ankle inversion with tibialis posterior denervation places the lesion at the root or sciatic level.
Clinical Pearl — Three muscles, three answers
The foot drop algorithm collapses to three needle EMG questions. (1) Is the short head of the biceps femoris abnormal? If yes, the lesion is at or proximal to the sciatic nerve in the thigh; if no, the lesion is at the fibular head. (2) Is the tibialis posterior abnormal? If yes, the lesion is at the L5 root or sciatic level; if no, the tibial nerve and the L5 root are spared. (3) Is the superficial fibular SNAP abnormal? If yes, the lesion is postganglionic (fibular or sciatic); if no, the lesion is preganglionic (L5 radiculopathy by the DRG principle). Three muscles, three answers, complete localization.
Additional muscles confirm the localization. In L5 radiculopathy denervation extends beyond fibular muscles to other L5-innervated muscles supplied by different peripheral nerves. The gluteus medius and tensor fasciae latae are innervated by the superior gluteal nerve from L4-S1. Fibrillations in tibialis anterior, peroneus longus, tibialis posterior, and gluteus medius point strongly to an L5 root lesion because multiple peripheral nerves are involved but all share the L5 root. The medial gastrocnemius is primarily S1, so it is normal in L5 radiculopathy but may be abnormal in sciatic neuropathy involving the tibial division.
Paraspinal muscle examination provides additional localizing information. In L5 radiculopathy, fibrillations may be present in the lumbar paraspinals at the L5 level because the posterior primary ramus branches proximal to the plexus and peripheral nerve. Paraspinal fibrillations support a root-level lesion. In fibular and sciatic neuropathy, paraspinals are completely normal because both lesions are distal to the posterior primary ramus.
The superficial fibular SNAP is the final critical piece. In L5 radiculopathy it is normal because the lesion is preganglionic and the sensory axons remain intact distally (the DRG principle). In fibular and sciatic neuropathy the superficial fibular SNAP is reduced or absent because the lesion is postganglionic with Wallerian degeneration of the sensory axons.
The summary algorithm: L5 radiculopathy = weak ankle inversion (tibialis posterior abnormal), paraspinal fibrillations, abnormal gluteus medius, normal superficial fibular SNAP. Fibular neuropathy at the fibular head = normal inversion, normal paraspinals, normal short head of biceps femoris, conduction block or slowing across the fibular head. Sciatic neuropathy (fibular-predominant) = normal inversion, abnormal short head, normal paraspinals, normal gluteal muscles.
High Yield — The foot drop algorithm
- Three diagnoses to distinguish: fibular neuropathy at fibular head, sciatic neuropathy (fibular-predominant), L5 radiculopathy.
- Short head of the biceps femoris = only thigh muscle innervated by the fibular division of the sciatic nerve. Normal = fibular at fibular head; abnormal = sciatic or radiculopathy.
- Tibialis posterior (tibial nerve, L5 root) = abnormal in L5 radiculopathy and sciatic neuropathy; normal in fibular at fibular head.
- Superficial fibular SNAP = abnormal in fibular and sciatic neuropathy (postganglionic); normal in L5 radiculopathy (preganglionic, DRG principle).
- Paraspinal fibrillations = root-level lesion (L5); normal in fibular and sciatic neuropathy.
- Gluteus medius (superior gluteal, L4-S1) = abnormal in L5 radiculopathy; normal in sciatic neuropathy (gluteal nerves arise from plexus, not sciatic trunk) and in fibular neuropathy.
- Medial gastrocnemius = primarily S1, not L5; normal in L5 radiculopathy.