BACKGROUND: It has been reported that dermatomal somatosensory evoked potential (DSEP) can be used for diagnosing nerve root injury in patients with lumbar disc herniation (LDH), and that 83%-95% of patients suffer from the disease. Body height correction is not performed prior to determinations of latency and latency difference between the healthy and affected sides. However, latency noticeably correlates to body height. OBJECTIVE: This study aims to determine the lumbosacral nerve root injury in patients with LDH by DSEP, and to evaluate the sensitivity of the DSEP difference between the healthy and affected sides using a diagnostic index following body height correction.DESIGN: A case-control observation.SETTING: Department of Orthopedic Surgery, Hainan Provincial Peoples Hospital. PARTICIPANTS: Ninety-six patients, comprised of 67 males and 29 females, with an average age of 43 years and a mean body height of 1.65 m (range 1.48–1.81 m), were recruited for this study. These patients suffered from unilateral lower limb radiation pain and received treatment at the Department of Orthopedic Surgery, Hainan Provincial Peoples Hospital between January 2004 and December 2006. All patients were confirmed to suffer from LDH at the L3-4, L4-5, and/or L5–S1 by CT and/or MRI examinations. Central nervous system diseases were excluded. In order to obtain a normal reference value, DSEP was determined for a group of 50 subjects, who concurrently received health examinations in the same department. The subjects had no previous history of back leg pain or nervous system disease. The group of healthy controls included 26 males and 24 females, with an average age of 37 years and a mean body height of 1.63 m (range 1.50–1.80 m). Written informed consent was obtained from all subjects for laboratory samples. The protocol was approved by the Hospitals Ethics Committees. DSEP was determined with myoelectricity-evoked potential equipment (Keypoint, Batch No. 9020A0042591, Dantec Company, Denmark).METHODS: DSEP of patients with LDH was determined. Constant-voltage square pulse stimulation was used to determine DSEP, with the following parameters: a pulse wave width of 0.2 milliseconds; a saddle-like stimulating electrode; a stimulation intensity 3 times greater than the sensation threshold; a stimulation frequency of 1.5 Hz; mean superposition greater than 100 times; and inter-electrode impedance < 5 kΩ. The stimulation point was a skin key sensation point confirmed by the American Spinal Injury Association, I.e. L4 at medial malleolus, L5 at the third metatarsophalangeal joint on the dorsum of the foot, and S1 at the lateral heel. The recording electrode was a needle electrode, the recording point was Cz, and the reference electrode was Fz. DSEP latency of P40, and latency differences of P40, between the healthy side and the affected side, were determined. DSEP at L4, L5, and S1 nerve roots of the lower limbs of 50 healthy controls were bilaterally determined. The normal values of P40 latency and P40-N50 amplitude were statistically obtained.MAIN OUTCOME MEASURES: Determination of DSEP values.RESULTS: Ninety-six patients with LDH and fifty healthy controls participated in the final analysis. In the healthy controls, the amplitude of DSEP varied greatly, with a mean amplitude co-efficient of variation of 58% for L4, L5, and S1 dermatomes. P40 latency was stable, with a mean latency coefficient of variation of 4.7%. In patients with LDH, the P40 wave disappeared. P40 latency was 2.5 times prolonged compared to normal mean value. P40 latency difference between the healthy and the affected side was 2.5 times higher than the normal mean value of the healthy side. CONCLUSION: DSEP can reflect the functional status of lumbosacral nerve root. P40 latency difference between the healthy side and the affected side is the most sensitive diagnosis index for patients with LDH suffering from unilateral lower limb radiation pain.