Lumbosacral evoked potentials (LSEPs) and cortical somatosensory evoked potentials (SEPs) in patients with lesions of the conus medullaris and cauda equina.

Evoked potentials from unilateral stimulation of the posterior tibial nerve at the knee were recorded over the spinous processes S1, L4, L2, T12 and from the 'lower extremity' portion of the sensory cortex (Cz) in 29 patients who exhibited clinical and electromyographic signs of conus medullaris or cauda equina lesions. Simultaneous recording of the lumbosacral evoked potentials (LSEPs) and cortical somatosensory evoked potentials (SEPs) permitted evaluation of the relative effectiveness of the peripheral stimulus in eliciting responses in the lumbosacral segments of the spinal cord and in the cortex of the brain. In patients with cauda equina lesion, each major component of LSEP can be absent or the peak can have a reduced amplitude and a prolonged latency. The degree of impairment of the LSEP runs in parallel to the degree of severeness of the cauda equina lesion. The recording of LSEP responses with surface electrodes represents a reliable test for the detection of mild cauda equina abnormalities, but the surface recording technique is not sensitive enough to differentiate between severe incomplete and severe complete cauda equina lesions. On the other hand, concurrent recording of responses evoked at lumbosacral and cortical levels by the same stimuli did detect instances in which the first-order afferents were capable of delivering an adequate volley of impulses to evoke a sizeable cortical response without evidence of an associated postsynaptic response in the spinal cord. Such findings are good evidence of a problem localized in the gray matter of the spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)