Nerve Conduction Study and Electroencephalogram

Nerve Conduction Study

  • A nerve conduction study (NCS) is a medical diagnostic test. It is used to estimate the function and the capability of electrical conduction, of the motor nerves and sensory nerves of the human body. 
  • Nerve conduction velocity (NCV) is frequently measured during this test. NCS laterally with electromyography measure nerve and muscle function. Diagnosis of defective spinal nerve compression, or any other neurologic disorder or injuries are undertaken for study by NCS process. 
  • Evaluation of numbness of limbs, weakness of the legs and arms as well tingling or burning sensation in certain areas of the body are the central area of diagnosis.. Nerve conduction study mainly comprise of the following studies.

Motor Nerve Conduction Study

  • It is performed by electrical stimulation of a peripheral nerve and recording from a muscle to which these nerve supplies. Latency is defined as the time taken for the electrical impulse to travel from the stimulation to that muscle and is usually measured in milliseconds. 
  • The target muscle generates a response whose size is called the amplitude. Motor amplitudes are measured in millivolts. Determination of NCV across different segments of the nerve is a primary goal of the Motor NCS study.  
  • It is done by the stimulation of two or more different locations along the same nerve With the help of the difference in latencies from the two points of stimulation as well as the distance between the different stimulating electrodes, one can calculate the NCVs across different segments.

Sensory Nerve Conduction Study

  • It is similar to Motor NCS and is performed by electrical stimulation of a peripheral nerve and but here the recording is done from a truly sensory portion of the nerve. Sensory latencies are measured milliseconds but sensory amplitudes are much smaller than the motor amplitudes (microvolt). Sensory NCV is calculated in the same way as motor NCVs.

F - wave study

  • The motor and sensory segments are concerned about nerve conduction velocities in sections/segments of limb whereas the F-wave latency is used to derive the conduction velocity of nerve between the limb and spine. In a typical F wave study, a strong electrical stimulus is applied above the distal portion of a nerve. The resulting the impulse travels both in both directions: one towards the muscle fibre and the other back to the motor neurons of the spinal cord. These directions are referred to as orthodromic and antidromic, respectively. 
  • The orthodromic stimulus on reaching the muscle fibre elicits a strong M-response indicative of muscle contraction. Meanwhile the antidromic stimulus on reaching the motor neuron cell bodies excites a small portion of the motor neurons causing them to backfire resulting in orthodromic wave which travels back down the nerve towards the muscle. This reflected stimulus evokes a smaller response of the muscle fibres resulting in a second CMAP called the F wave. The limb length, D (in milli metres)is taken into account for calculations of Conduction velocity.

H - reflex study

  • This study is similar to F-wave study and evaluates conduction between the limb and the spinal cord. Although a subtle difference exists in that here the impulses going toward the spinal cord are in sensory nerves while the impulses coming from the spinal cord are in motor nerves. The interpretation of nerve conduction studies is a complex affair and expert medical practitioners such as neurologists, physiatrists or clinical neurophysiologists are routinely involved. 
  • NCS have proven to be very helpful in diagnosis of many diseases related to the nerves. The process is non-invasive, albeit sometimes it can be painful due to minor electrical shocks. Although the low amount of electrical current is considered safe, patients with a harboring electrical devices such as permanent pacemaker are advised to avoid this kind of test or tell the examiner prior to the test.

Electro-Encephalograph (EEG)

  • In 1929, Hens Berger is credited to have found some electrical activity when he connect a galvanometer to human scalp. It gave birth to electro-encephalography. EEG is an electrically operated instrument having array of 16-30 electrode, which when attached to the scalp for short time gives electric wave signals. It operates by detecting the brain’s electrical charge which is maintained by billions of neurons. The Neurons are electrically charged due to continuous pumping of ions by membrane transport proteins across their membrane. 
  • Neurons constantly exchange ions with the extracellular fluid, e.g. to maintain resting membrane potential. When many ions having similar charge are pushed out of several neurons at the same time, they can push their neighbors, who further apply force to their neighbors, and so on such that a wave forms. When the wave of ions reaches the electrodes attached to the scalp, they can give or take electrons on or from the metal of the electrodes. Since metal can conducts these electrons easily, voltages difference between any two electrodes can be measured by a voltmeter. Recording these voltages for a specific time gives us the EEG.
Nerve Conduction Study

Image Source: medlineplus.gov

Wave Patterns Form EEG Recording

  1. Delta waves: The frequency of Delta wave is below 4 Hz. It is the highest in amplitude and the slowest waves. It originate normally in adults during sleep. It is also seen normally in babies. It can also be observed in patients during coma.
  2. Theta waves: The frequency of theta wave range from 4 Hz to 7 Hz. It is seen normally in young children. It may be seen in older children and adults under stress or during meditation. Excess theta for age represents abnormal activity.
  3. Alpha waves: The frequency of alpha wave range from 7 Hz to 14 Hz. An awake but resting person produces alpha wave. Hans Berger termed “alpha wave” when he saw the first rhythmic EEG activity. This was the “posterior basic rhythm” seen in the occipital regions of the brain. It arises with closing of the eyes and with relaxation, and weakens with eye opening or mental labor. In addition to the posterior basic rhythm, there are other normal alpha rhythms such as the “mu rhythm” which arises when the hands and arms are indolent.
  4. Beta waves: The frequency of beta wave range from 15 Hz to about 30 Hz. During extreme mental activity, beta wave initiates from frontal and parietal regions. Beta activity is closely linked to motor behavior and is generally weakened during active movements. An alert wide awake person shows unsynchronized beta wave.
  5. Gamma waves: The frequency of gamma wave is nearly 30–100 Hz. Gamma rhythms represent binding of different populations of neurons together into a network for the purpose of carrying a certain motor function.
  6. Mu waves: The frequency of mu wave is 8–13 Hz. It partly overlaps with other frequencies. It denotes the synchronous firing of motor neurons in rest state. Deviations from normal pattern indicate brain disorder and change in physiological state of brain. EEG can diagnose epilepsy, brain tumour, abscess, sleep disorders, metabolic and drug effects on brain.

Further Readings