Somatosensory evoked potentials

May use electrical stimulation of peripheral nerves (somatosensory or (SSEP)), auditory clicks through earphones (auditory or AEP, AKA BAER (brainstem auditory evoked response)) or flashing lights through goggles (visual EP or VEP).

Evoked potentials are most commonly used by neurosurgeons for intraoperative monitoring purposes. SSEP (especially from median nerve stimulation) also has prognostic significance in cervical spondylotic myelopathy, although use for this purpose is limited.

Evoked potential waveforms

SSEP monitors the dorsal column–medial lemniscus pathway, which mediates tactile discrimination, vibration, and proprioception ¹⁾.

Stimulation of sensory receptors in the skin initiates peripheral sensory nerves, which extend through the nerve root to the ipsilateral dorsal root ganglion at spinal levels. The projections from these first-order neurons form fasciculus gracilis and fasciculus cuneatus, which carry impulse from the lower and upper extremities, respectively. The first synapse occurs in the lower medulla, then the impulses cross over at the level of the brainstem and form medial lemniscus. The impulse then ascends to the contralateral thalamus and relays information to the primary somatosensory cortex in the parietal lobe. In the upper extremities, the median and ulnar nerve are monitored, whereas, in the lower extremities, the posterior tibial and peroneal nerve are monitored.

Useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex. SEP components include a series of positive and negative deflections that can be elicited by virtually any sensory stimuli. For example, SEPs can be obtained in response to a brief mechanical impact on the fingertip or to air puffs. However, SEPs are most commonly elicited by bipolar trancutaneous Electrostimulation applied on the skin over the trajectory of peripheral nerves of the upper limb (e.g., the median nerve) or lower limb (e.g., the posterior tibial nerve), and then recorded from the scalp.

In general, somatosensory stimuli evoke early cortical components (N25, P60, N80), generated in the contralateral primary somatosensory cortex (S1), related to the processing of the physical stimulus attributes. About 100 ms after stimulus application, additional cortical regions are activated, such as the secondary somatosensory cortex (S2), and the posterior parietal and frontal cortices, marked by a parietal P100 and bilateral frontal N140. SEPs are routinely used in neurology today to confirm and localize sensory abnormalities, to identify silent lesions and to monitor changes during surgical procedures.

Short-latency somatosensory evoked potentials (SEPs) were recorded in 45 freshly diagnosed cases of epilepsy before starting treatment. Follow-up recordings were made 6 weeks and 3 months after diphenylhydantoin, carbamazipine and phenobarbitone monotherapy were started. Serum drug levels were monitored. Both amplitude and latency of the initial component (N20) remained unchanged and were identical to a group of 30 age- and sex-matched normal individuals in whom SEPs were recorded during the period of study ²⁾.

Somatosensory evoked potentials indications.

Somatosensory evoked potentials (SSEP) were monitored in 152 cases, visual in 32, brainstem acoustic in 22, transcranial motor in 36; stimulation mapping of motor cortex was performed in 69 surgeries, and cranial nerves identification in 27. EEG was recorded in 7 patients, and 3 of them were woke up during the surgery for speech mapping.

The sensitivity of the SSEP in motor dysfunction detection was low (33%), while the specificity was relatively high (82%). These characteristics for visual and motor evoked potentials were close to 100% provided that the parameters of anesthesia met the corresponding requirements. The most effective methods in respect of prevention of postoperative dysfunctions were the stimulation mapping of functionally significant areas (motor and speech) and motor pathways mapping.

Intraoperative neuromonitoring reduces a number of neurological complications after neurosurgical operations. The SSEP method is not sensitive enough in surgeries that could affect motor centers and/or pathways, and multimodal monitoring combining SSEP and motor responses recording during transcranial and/or direct electrical brain stimulation. Successful monitoring requires highly coordinated actions between neurophysiologists, neurosurgeons and anesthesiologists ³⁾.