![]() How are median nerve somatosensory evoked potentials (SEPs) recorded? What are somatosensory evoked potentials (SEPs)? SEPs are abnormal in diseases of the dorsal columns in which joint position sense and proprioception are impaired. SEPs can persist following lesions of the anterolateral spinal cord, however. Lesioning of the dorsal columns of the spinal cord rostral to the root levels where the afferent somatosensory activity enters the spinal cord abolishes the SEPs generated in the brain. The SEP components generated in the brainstem and in the cerebral cortex are mediated entirely by the dorsal columns (posterior columns) of the spinal cord, the fasciculus cuneatus for upper limb SEPs and the fasciculus gracilis for lower limb SEPs. However, an abnormal SEP result demonstrates that there is dysfunction within the somatosensory pathways subjects cannot volitionally make their SEPs abnormal. Since individuals have multiple parallel afferent somatosensory pathways (e.g., the anterior spinothalamic tract and the dorsal column tracts within the spinal cord), SEPs can be normal in patients with significant sensory deficits. Abnormal SEPs can result from dysfunction at the level of the peripheral nerve, plexus, spinal root, spinal cord, brain stem, thalamocortical projections, or primary somatosensory cortex. SEPs are used for clinical diagnosis in patients with neurologic diseases, to evaluate patients with sensory sympotoms that might be psychogenic, for prognostication in comatose patients, and for intraoperative monitoring during surgeries that place parts of the somatosensory pathways at risk. Recording electrodes are placed on the scalp, over the spine, and over peripheral nerves proximal to the stimulation site. Recordings of SEPs to stimulation of the ulnar nerves at the wrists are useful for intraoperative monitoring when the mid-cervical spinal cord or parts of the brachial plexus are at risk. SEPs can also be recorded following stimulation of branches of the trigeminal nerve, but this is technically challenging because of the proximity of the stimulating and recording electrodes and the short latencies of the responses the electrical stimulus artifact often overlaps with, and obscures, the trigeminal nerve SEPs. The stimulation sites typically used for clinical diagnostic SEP studies are the median nerve at the wrist, the common peroneal nerve at the knee, and the posterior tibial nerve at the ankle. While SEPs can be elicited by mechanical stimulation, clinical studies use electrical stimulation of peripheral nerves, which gives larger and more robust responses. Somatosensory evoked potentials (SEPs) consist of a series of waves that reflect sequential activation of neural structures along the somatosensory pathways. ![]() Auditory, visual, and somatosensory stimuli are used commonly for clinical evoked potential studies. Evoked potentials are the electrical signals generated by the nervous system in response to sensory stimuli.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |