What does EHL (Extensor Hallucis Longus) tell you about a patient's neurological assessment?

The Role of Extensor Hallucis Longus (EHL) in Neurological Assessment

Testing the Extensor Hallucis Longus muscle provides critical information about L5 nerve root function and is an essential component of the neurological examination for detecting radiculopathy, peripheral neuropathy, and other neurological conditions affecting the lower extremities.

Anatomical and Functional Significance

• The Extensor Hallucis Longus (EHL) muscle is primarily innervated by the L5 nerve root via the deep peroneal nerve, making it a key indicator for L5 radiculopathy and deep peroneal nerve palsy 1, 2
• The EHL is responsible for extension of the great toe at both the metatarsophalangeal (MTP) and interphalangeal (IP) joints 3
• The muscle is located in the anterior compartment of the leg, with its midpoint approximately 12 cm above the bimalleolar line, or at about the distal 35% of the lower leg length 4

Clinical Examination Technique

• The most accurate assessment of EHL strength is performed by testing extension at the metatarsophalangeal (MTP) joint rather than the interphalangeal joint, with significantly higher reliability (κ = 1.00 for MTP testing) 3
• Manual muscle testing (MMT) of the EHL should be performed with the patient's ankle in a neutral position to isolate the EHL and prevent compensation by other muscles 3, 4
• Weakness of the EHL represents the vast majority of root involvement in discogenic lesions, particularly at the L5 level 2

Neurological Significance and Interpretation

• EHL weakness is a primary predictor of L5 nerve root radiculopathy, which is common in thoracolumbar fractures and disc herniation 5
• In thoracolumbar spine trauma, EHL function assessment contributes to the American Spinal Injury Association (ASIA) classification, which is a significant predictor of neurological recovery 5
• The EHL also plays a role in the Babinski response assessment - the Babinski sign is mediated by the EHL, not the extensor hallucis brevis 6
• Quantitative measurement of EHL strength can provide objective data for evaluating and following treatment of patients with discogenic lesions and can assist in surgical decision-making 2

Clinical Applications in Neurological Assessment

• Serial assessment of EHL strength can help monitor neurological recovery in patients with thoracolumbar injuries, as lumbar or conus injuries show the greatest neurologic recovery 5
• EHL testing is particularly valuable in evaluating patients with low back pain who may have nerve root compression, as it provides objective evidence of neurological involvement 2
• In thoracolumbar fracture patients, EHL assessment contributes to the overall neurological examination, which correlates with bladder function recovery and other important functional outcomes 5
• Quantitative EHL power measurement can help differentiate between true neurological deficits and non-organic causes of weakness, aiding in the evaluation of compensation cases 2

Limitations and Considerations

• EHL assessment should be considered alongside other neurological tests, as no single test provides complete neurological assessment 5
• Manual muscle testing of EHL may have inter-observer variability, particularly when testing at the IP joint in different positions 3
• For accurate electromyographic assessment of the EHL, needle placement at the distal two-thirds of the lower leg provides the most accurate access to the muscle belly 4