The SSEP on the ICU: current applications and pitfalls

Clinical neurological evaluation of patients in the intensive care unit (ICU) is often limited. Registration of the somatosensory evoked potential (SSEP) can assist in the neurological evaluation in these patients. In this paper, we discuss the principles, applications and limitations of the SSEPs in the ICU with a focus on prognostication in comatose patients. Registration of the SSEP is a very reliable and reproducible method, if it is performed and interpreted correctly. A bilateral absent cortical SSEP response is a reliable predictor for poor neurological outcome in patients with a post-anoxic coma, but not in patients with traumatic brain injury or subarachnoid haemorrhage. During SSEP recordings, great care should be taken in improving the signal to noise ratio. Since the interpreting clinician is often not present during the actual SSEP registration itself, the role of the lab technician is crucial in obtaining reliable SSEP results. If the noise level is too high, the peripheral responses are abnormal, or the response is not reproducible in a second set of stimuli, interpretation of the SSEP cannot be done reliably. Introduction Neurological evaluation of patients in the intensive care unit (ICU) is often limited, and clinical neurophysiology has provided several techniques to assist in the evaluation of the central and peripheral nervous system in these conditions. Techniques include the electroencephalogram (EEG), brainstem auditory evoked potentials (BAEP) and the somatosensory evoked potential (SSEP). In this paper, we discuss the principles, applications and limitations of the SSEP. SSEPs are used in a variety of clinical settings, including the evaluation of coma, neuromonitoring in the operating theatre, and the evaluation of traumatic spinal cord injury. Here, we focus on the use of the SSEP registration in the prognostication of comatose patients in the ICU. SSEP: Principles The somatosensory evoked potential is a small (< 10-50 μV) electrical signal, that can be recorded noninvasively from the skull, The SSEP on the ICU: current applications and pitfalls after giving a set of electrical stimuli to one of the peripheral nerves. Measurement of the SSEP evaluates the complete pathway from the peripheral sensory nervous system to the sensory cortex, which runs via the dorsal column lemniscal pathway via the spinal cord, brainstem and thalamus1,2. The dorsal column-lemniscal pathway consists of four neuronal populations. The cell bodies of the first-order neurons are situated in the dorsal root ganglia, the trigeminal ganglion, the midbrain trigeminal nucleus, and the vagal ganglion nodosum. The second-order neuron lies in the dorsal column nuclei (the cuneate nucleus and the gracile nucleus). Axons of these second neurons cross the midline and project to the ventroposterior nuclei of the thalamus (third-order neuron). From there the pathway projects into the network of somatosensory cortex areas (fourth-order neurons), which include primary and secondary somatosensory cortex, posterior parietal cortex, posterior and mid-insula, and mid-cingulate cortex1. Figure 1 shows the anatomical connections evaluated by the median nerve SSEP. SSEPs are usually evoked by bipolar transcutaneous electrical stimulation applied on the skin over the selected nerve, and registered with disk-electrodes along the tract. For example, in SSEP recordings of the median nerve registration electrodes can be placed at the elbow, Erb’s point, cervical, parietal and frontal cortex. The cortical response can only be interpreted reliably, when the peripheral responses are present. In the nomenclature of SSEP waveforms, N or P followed by an integer (e.g. N20) are used to indicate the polarity and the nominal post-stimulus latency (in ms) of the recorded wave in the healthy population1. The earliest cortical potential is the N20, which is generated in the primary somatosensory cortex, where thalamocortical cells make synaptic connections with the superficial and deep pyramidal cell layers3,4. In comparison to the later cortical responses, the N20 is the most robust, and is the latest waveform to disappear during increasing levels of encephalopathy. Furthermore, the N20 is relatively independent to the level of sedation1. As the M.C. Cloostermans1,2, J. Horn3, M.J.A.M. van Putten1,2 1clinical Neurophysiology, Mira – institute for Biomedical technology and technical Medicine, University of twente, enschede, the Netherlands 2Departments of clinical Neurophysiology and Neurology, Medisch Spectrum twente, enschede, the Netherlands 3Department of intensive care Medicine, academic Medical center, University of amsterdam, amsterdam, the Netherlands