Clinical Neurophysiology Electroneurography PDF

Clinical Neurophysiology Part II Electroneurography Karsten Schwerdtfeger Klinik für Neurochirurgie, Universitätsklinikum des Saarlandes. Studiengang: Neural Engineering Master WS 2024/2025  Intro Whats the similarity?...

Clinical Neurophysiology Part II Electroneurography Karsten Schwerdtfeger Klinik für Neurochirurgie, Universitätsklinikum des Saarlandes. Studiengang: Neural Engineering Master WS 2024/2025  Intro Whats the similarity?  Definitions Electrode: An electrode is a solid electric conductor that carries electric current into non-metallic solids, or liquids, or gases, or plasmas, or vacuums. Electrodes are typically good electric conductors, but they need not be metals. Electrolyte: A substance that breaks up into ions (particles with electrical charges) when it is dissolved in water or body fluids. Some examples of ions are sodium(Na+), potassium(K+), calcium(Ca2+), chloride(Cl-), and phosphate(P3-).  Electrode-electrolyte reaction Example Example Electrode- IHP - Inner Helmholtz plane Electrolyte: Material: OHP - Outer Helmholtz plane KCl AgCl dispersed in Water (Saline) Model describing the electrode-skin interface Circuit Model Electrode Electrolyte interface Circuit Model Electrolyte Skin interface Electroneurography - Mixed nerve Most of our nerves are mixed nerves: – The efferent motor fibers are the axons of the montoneurons in the brain stem or the spinal cord extending to the voluntary muscles of the body. – The afferent sensory fibers carry the information from free nerve endings or specialized receptors to the CNS. – The somata of afferent somatosensory nerves are located in the spinal ganglions just outside the spinal column or within the foramen intervertebrale. Formally, only the part proximal to the ganglion is an axon and the distal part is a dendrite which however generates APs and directly pass into the axon so bypassing the soma (pseudounipolar ganglion cells) – Proximal to the spinal ganglion the fibers form the spinal roots with efferents in the ventral root and afferents in the dorsal root. – Distal to the spinal ganglion the fibers form the spinal nerve – Mixed nerves also contain vegetative fibers which in clinical practice, however, are not assessed electrophysiologically. Electroneurography - Muscle innervation Muscle innervation is a bit more complex: – Two different efferent fibers Aα-fibers from α-motoneurons to the main contractile muscle cells Aγ-fibers from γ-motoneurons to the contractile elements of the muscle spindles, a special receptor within muscles measuring the length of the muscle – Two afferent sensory fibers from the muscle spindle Ia – fibers carrying information of the velocity of length change II – fibers carrying information about the absolute length – From receptors in the tendons Ib – fibers carry information about the tension in the tendons. – The afferent fibers establish synaptic contacts either directly or via interneurons with the α-motoneurons Afferents from the spindle form the circuit for the deep tendon reflexes tested during a neurological examination. Neurology Exam : Reflexes (youtube.com) Afferents from the tendons inhibit the motoneurons thus protecting the muscle for damage like rupture. Electroneurography - Muscle innervation Muscle spindles – The contractile elements within the spindles (intrafusal muscle cells) are located at both ends of the cells. The center of the cells is elastic – The contraction of the ends may compensate length changes due to contraction of the outer (extrafusal) muscle cells and thus preserves the sensitivity of the receptor – Or may be used to modify the reflex circuit as part of a particular voluntary initiated movement. – A way to demonstrate the modification via the spinal Eigenapparat is the Jendrassik maneuver ( https://www.youtube.com/watch?v= MyQ23ZlL8Iw ) Electroneurography - Muscle innervation Neuromuscular junction – Each motoric nerve fiber has a terminal branching thus innervating several muscle cells. – As the diameter of the branches decreases AP propagation velocity decreases correspondingly – Signal transmission occurs via a synapse the motor end plate. Synaptic transmission is causing a delay of 0.5 ms Motor unit – A motoneuron and the entirety of muscle fibers innervated by one nerve fiber is called motor unit. – In the big tonic muscles for example the gastrocnemial muscle up to 2,000 muscle cells were innervated by one motoneuron. – In muscles showing well-graded force development the motor units are much smaller (a few hundred muscle cells) – Motor pool is the entirety of all motor units of one muscle. ENG - Stimulation and recording Direct stimulating of a nerve – Usually nerves are stimulated with bipolar electrodes and a square-wave current creating an anode and a cathode. – Depolarization occurs under the cathode so it should be placed to the direction you want to analyze nerve conduction. – The anodal block is a fiction. An increased latency of the recorded responses is possible when changing the polarity. Direct recording from a nerve – When recording from a nerve with many fibers, the response differs from the biphasic or triphasic shape. – There are several peaks which disperse with increasing distance to the stimulation site – They correspond to fiber groups of different diameters and myelinisation and therefore different AP propagation velocities which can be associated to distinct functions as we have already seen with the muscle innervation  ENG – fiber classifiction 100 m/s 50 m/s Erlanger/ 20 m/s Gasser 15 m/s 7 m/s 1 m/s 75 m/s 75 m/s Lloyd/Hunt 55 m/s 11 m/s 1 m/s ENG – Motor nerve conduction Stimulation of the nerve under study. Recording from a (distal) muscle innervated by the nerve under study. Usually no surgical exposure of the nerve occurs. Stimulation and recording is done with surface electrodes Motor fibers are identified by recording the response from a muscle innervated by the nerve under study The evoked response is called compound muscle action potential CMAP Due to the terminal slowing and synaptic delay at the end-plate the calculation of the nerve conduction velocity between stimulation and recording site is not reasonable ENG – Motor nerve conduction Instead the latency between stimulation and the onset of the response, the distal motor latency is noticed. Stimulation at different sites along a nerve allows to calculate conduction velocities by dividing the distance between two stimulation sites through the latency difference. Ensure that the cathode always shows in the same direction. Stimulation should occur with supramaximal stimuli activating the whole motor pool – Increase stimulus strength until no further increase in response can be recorded – Add 2 mA to the intensity As latency is measured to the onset of the muscle response, only the velocity of the fastest fibers is assessed. ENG – Motor nerve conduction Pitfalls – Difficulties with stimulation due to thick subcutaneous tissue or deep, intramuscular course of the nerve – Irradiation of the stimulus to neighboring nerves, especially at the wrist – Different or rather low temperature of the extremities ENG – Motor nerve conduction - assessment distal motor latency – Side differences – Absolute values Amplitude – Side differences – Differences between stimulation sites – Absolute – difficult due to the variability Velocity – Side differences – Differences between stimulation sites ENG – Motor nerve conduction - Age  ENG – peripheral nerve lesion types Acute (injury) – Neurapraxia – segmental degeneration of the myelin sheath Regeneration with shorter distances of Ranvier nodes – NCV decreases – Axonotmesis – disruption of the axons with intact connective tissue sheaths Partial block – amplitude decreases initially Wallerian degeneration Regeneration (1mm/day) with shorter internodal distances – NCV decreases – Neurotmesis – complete disruption of the entire nerve Total block Neuroma/retrograde degeneration with neuron loss Indication for surgical reconstruction Chronic (nerve entrapment syndromes, nerve compression syndromes) – Repetitive degeneration/regeneration of the myelin sheath – Axonal damage due to reduced blood perfusion ENG – Peripheral nerval lesion types Left: normal findings Middle: chronic damage due to an ulnar entrapment syndrome at the elbow Right: acute injury to the elbow with partial nerve block ENG – Sensory nerve conduction Usually not successful with skin stimulation Tests of nerves which are pure sensory either at stimulation or at recording site. Recording of the nerve response – Antidrome or orthodrome – Small amplitudes (up to 20 uV) – Large stimulus artifact Suited – Digital nerves – Suralis nerve https://www.youtube.com/watch?v=E6DSR Kdrqxw Clinical applications – carpal tunnel syndrome Most frequent peripheral nerve compression syndrome Incidence: 345/100.000/year main symptoms are pain, numbness and tingling in the thumb, index finger, middle finger and the thumb side of the ring finger. Symptoms typically start during the night and at rest. They are improved by “shaking” of the hand. In advanced cases permanent numbness and muscle atrophy of the thenar occur. Clinical applications – carpal tunnel syndrome Distal chronic compression syndrome with complaints on the right side Clinical applications – carpal tunnel syndrome Distal chronic compression syndrome with complaints on the right side Motor nerve conduction: the distal motor latency is increased on the right side. Clinical applications – carpal tunnel syndrome Distal chronic compression syndrome with complaints on the right side Motor nerve conduction: the distal motor latency is increased on the right side. Sensory nerve conduction: – As the fourth finger is innervated on the radial side by the median nerve and on the ulnar side by the ulnar nerve which is not affected in CTS the nerve conduction velocities should be compared Clinical applications – carpal tunnel syndrome Distal chronic compression syndrome with complaints on the right side Motor nerve conduction: the distal motor latency is increased on the right side. Sensory nerve conduction: – As the fourth finger is innervated on the radial side by the median nerve and on the ulnar side by the ulnar nerve which is not affected in CTS the nerve conduction velocities should be compared – There is a marked difference with slowing of the median sensory nerve conduction especially on the right side Clinical applications – carpal tunnel syndrome Especially in the beginning nerve conduction may appear normal. Pathological nerve conduction values without symptoms are irrelevant. In dubious cases a sonography or a MRI-scan of the wrist should occur Therapy: – With minor symptoms – splinting during the night – With major symptoms or refractory to splinting …. Clinical applications – carpal tunnel syndrome Surgical decompression – Open – Endoscopically Monoportal Biportal (Esmarch ischaemia - Tourniquet) Clinical applications – Cubital tunnel syndrome The second most frequent nerve compression syndrome Weakness of the hand Numbness D4 (ulnar), D5, over the hypothenar Muscle atrophy Clinical applications – Cubital tunnel syndrome Endoscopically assisted decompression. Clinical applications – Cubital tunnel syndrome ENG – Loge de Guyon - Syndrome Symptoms similar to the cubital tunnel syndrome Nerve conduction tests – Prolonging of the distal motor latency – Slowing of the distal sensory nerve conduction – Normal values at the elbow. ENG – Meralgia paresthetica Compression of the lateral femoral cutaneous nerve under the inguinal ligament Pain, Numbness on the lateral aspect of the thigh Unfortunately no reliable sensory nerve conduction test could be established Assessment with somatosensory evoked potentials may be helpful  ENG – Polyneuropathy Polyneuropathy is damage or Inherited causes are disease affecting peripheral hereditary motor neuropathies, nerves in roughly the same areas Charcot-Marie-Tooth disease, on both sides of the body, and hereditary neuropathy with featuring weakness, numbness, and burning pain. liability to pressure palsy. It usually begins in the hands and Acquired causes are diabetes feet and may progress to the arms mellitus, vascular neuropathy, and legs and sometimes to other alcohol abuse, and vitamin parts of the body where it may affect the autonomic nervous B12 deficiency. system. CIDP is regarded as an It may be acute or chronic. autoimmune disease  ENG – Polyneuropathy Classification – Distal axonopathy is the result of interrupted function of the peripheral nerves. It is the most common response of neurons to metabolic or toxic disturbances. – Myelinopathy, is due to a loss of myelin. – Neuronopathy is the result of issues in the peripheral nervous system neurons cell bodies. ENG – F-waves and H-reflex – F-waves are evoked by strong electrical stimuli (supramaximal) applied to the skin surface above the distal portion of a nerve. – The impulse travels in an orthodromic (towards the muscle) as well as antidromic fashion (towards the cell body in the spinal cord) – As the orthodromic impulse reaches innervated muscle fibers, a strong direct motor response (M) is evoked. – As the antidromic impulse reaches the cell bodies within the spinal cord, a part of the alpha motor neurons, (roughly 5-10%) generate a second AP after the refractory period due to persistend depolarization of the dendrites ('backfire‘). – This 'backfiring' elicits an orthodromic impulse, towards innervated muscle fibers (F-response) ENG – F-waves and H-reflex – The H-reflex (or Hoffman’s reflex) is a reflecting the monosynaptic reflex circuit of spinal stretch reflexes. – The H-reflex test is performed using an electric stimulator, which gives usually a square-wave current of short duration and small amplitude. – The response is usually a clear wave, called H-wave, 28-35 ms after the stimulus when applied to the tibial nerve at the poplitea. – The gastrocnemial muscle is the best-suited muscle to elicit a H-reflex. – With increasing stimulation strength the H- wave decreases, and at supramaximal stimulus, the H-wave will disappear. – This is the main difference to the F-wave, as the F-wave slightly increase with strengthening the stimulus. ENG – F-waves and H-reflex – A prolonged or absent F-wave/H- reflex may be caused anywhere in the depicted pathway. – Together with normal distal conduction tests a pathological F- wave/H-reflex points to a proximal lesion, either of the nerve roots or the brachial/lumbosacral plexus – Further elucidation can be done by clinical investigation and motor evoked potentials. To be continued