Somatosensory evoked potential

The somatosensory evoked potentials (SEP or SSEP ) are records the electrical response of fast -conducting sensitive nerve fibers in the course of the loop path. It is derived by repeated electrical stimulation of peripheral nerves at various points in history, mostly. Equal to the entry into the spinal cord and the brain area in question

History

The SEP were first described by Dawson in 1947, then under difficult technical conditions, since as amplifier valves and tubes and recording oscillograph with documentation by photography were available. The use of computer technology to averaging and recording has led to significant reductions.

Principle

A possible close to the surface lying sensory nerve is repeatedly stimulated electrically. Thus, an action potential on the nerve spreads out, which ( rarely also needle electrodes) can be derived in the course by means of surface electrodes. Since the voltages are very small, the potential must be freed by averaging of noise.

The derivation can be done both in the course of the nerve ( median nerve at the Nth example above the Erb's point), although this is usually in the amount of entry into the spinal cord ( " spinal potential " - the median nerve, for example, over the spinous process of the seventh cervical vertebra ) and over the corresponding area of the sensory cortex ( " cortical potential "). The difference between the spinal and cortical potential is referred to as " central conduction time ." Due to the anatomy of the loop path includes all three switches ( Nc. cuneatus / gracilis, thalamus, dentate gyrus ) between the individual neurons in the central conduction time.

Standard values

In relevant books, there are standard values ​​for the individual components of the SSEP. This should serve as a guide. Ultimately, each laboratory should establish its own normal values ​​, mainly because the amplitudes, but also lower the latencies of the potentials of the technology used and the placement of the electrodes (not least the reference electrode ) depend.

Factors

Decisive influence on the result, the body length of the subjects. Therefore, all standard values ​​must be adapted to the body length. The body temperature also has an influence, especially cold extremities to the spinal latency. Lower influences also have age and body posture, gender liegen contradictory information.

Application

The SSEP allow an assessment of the function of sensitive nerves in the first proximal development, where they hardly can be examined due to the superposition of muscles, bones, etc. by neurography. Mostly however, they are used to test the central sensory path. When damage to the myelin sheaths ( " demyelination ", eg multiple sclerosis) lengthens the latency, reduction in the number of nerve fibers ( " Axonaler damage" ) decreases the amplitude of the potentials.

Further, the SSEP can be used to assess the prognosis of severe brain injury ( due to trauma or a lack of oxygen, for example ). The prognosis is very poor if they can successively derive from both sides of an answer to the spinal cord, but not the cortex. This finding suggests a bilateral brainstem injury that hardly allows an acceptable recovery. (. cf. Firsching et al, Dtsch Arztebl 2003, 100 (27 ): A- 1868)