During an exam, the neurologist reviews the patient’s health history with special attention to the current condition. Typically, the exam tests mental status, function of the cranial nerves, vision, strength, coord- ination, reflexes and sensation. This helps the doctor determine if the problem exists in the nervous system. Below is a glossary of the most common tests.

What is Electromyography (EMG)?

 

Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles... (click to read more)

EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells are electrically or neurologically activated. This is accomplished by inserting a needle electrode through the skin into the muscle tissue. The physician observes the electrical activity while inserting the electrode. The insertional activity provides valuable information about the state of the muscle and its innervating nerve. Normal muscles at rest make certain, normal electrical signals when the needle is inserted into them. Then the electrical activity when the muscle is at rest is studied. Abnormal spontaneous activity might indicate some nerve and/or muscle damage. Then the patient is asked to contract the muscle smoothly. The shape, size, and frequency of the resulting motor unit potentials are judged. Each electrode insertion gives a samply of the activity of the whole muscle. Because skeletal muscles differ in the inner structure, the electrode may need to be placed at various locations to obtain an accurate study.
 
A motor unit is defined as one motor neuron and all of the muscle fibers it innervates. When a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. The area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate. After the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibers of that particular motor unit. The sum of all this electrical activity is known as a motor unit action potential (MUAP). This electrophysiologic activity from multiple motor units is the signal typically evaluated during an EMG. The composition of the motor unit, the number of muscle fibers per motor unit, the type of muscle fibres and many other factors affect the shape of the motor unit potentials in the myogram.
 
Some patients can find the procedure somewhat painful, whereas others experience only a small amount of discomfort when the needle is inserted. The muscle or muscles being tested may be slightly sore for a few hours after the procedure.

What is Nerve Conduction Study (NCS)?

 

A nerve conduction study (NCS) is used to evaluate the electrical conduction of the motor and sensory nerves of the human body... (click to read more)


A nerve conduction study (NCS) is used to evaluate the electrical conduction of the motor and sensory nerves of the human body.
 
Nerve conduction velocity (NCV) is a common measurement made during this test. There are various other parameters analyzed including conduction latency, amplitude, and morphology.
 
Nerve conduction studies are very helpful to diagnose certain diseases of the nerves of the body. The test is not invasive, but can be a little painful due to the electrical shocks. The shocks are associated with a low amount of electrical current so they are not dangerous to anyone. Patients with a permanent pacemaker or other such implanted stimulators such as deep brain stimulators or spinal cord stimulators must tell the examiner prior to the study. This does not prevent the study, but special precautions are taken.

What is Electroencephalography (EEG)?

 

Electroencephalography (EEG) is the recording of electrical activity along the scalp produced by the firing of neurons within the brain. .. (click to read more)


In clinical contexts, EEG refers to the recording of the brain’s spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from multiple electrodes placed on the scalp. In neurology, the main diagnostic application of EEG is in the case of epilepsy, as epileptic activity can create abnormalities on an EEG study. A secondary clinical use of EEG is in the diagnosis of coma, encephalopathies, and brain death. EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.
 
Derivatives of the EEG technique include evoked potentials (EP), which involves averaging the EEG activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory).
 
In conventional scalp EEG, the recording is obtained by placing electrodes on the scalp with a conductive gel or paste, usually after preparing the scalp area by light abrasion to reduce impedance due to dead skin cells. Many systems typically use electrodes, each of which is attached to an individual wire. Electrode locations and names are specified by the International 10–20 system for most clinical and research applications. This system ensures that the naming of electrodes is consistent across laboratories. In most clinical applications, 19 recording electrodes (plus ground and system reference) are used. (A smaller number of electrodes are typically used when recording an EEG on infants.) Additional electrodes can be added to the standard set-up when a clinical or research application demands increased spatial resolution for a particular area of the brain.
 
During the EEG, a series of activation procedures may be used. These procedures may induce normal or abnormal EEG activity that might not otherwise be seen. These procedures include hyperventilation, photic stimulation (with a strobe light), eye closure, mental activity, sleep and sleep deprivation. During (inpatient) epilepsy monitoring, a patient’s typical seizure medications may be withdrawn.

What is Evoked Potentials?

 

An evoked potential or evoked response is an electrical potential recorded from the nervous system following presentation of a stimulus, as distinct from spontaneous potentials detected by electroencephalography (EEG)... (click to read more)

.Evoked potential amplitudes tend to be low, ranging from less than a microvolt to several microvolts, compared to tens of microvolts for EEG, millivolts for EMG, and often close to a volt for ECG. To resolve these low-amplitude potentials against the background of ongoing EEG, ECG, EMG and other biological signals and ambient noise, signal averaging is usually required. The signal is time-locked to the stimulus and most of the noise occurs randomly, allowing the noise to be averaged out with averaging of repeated responses.
 
Signals can be recorded from cerebral cortex, brain stem, spinal cord and peripheral nerves. Usually the term “evoked potential” is reserved for responses involving either recording from, or stimulation of, central nervous system structures.

What is Somatosensory Evoked Potentials (SSEPs)?

 

Somatosensory Evoked Potentials (SSEPs) are used in neuromonitoring to assess the function of a patient's spinal cord during surgery...(click to read more)

They are recorded by stimulating peripheral nerves, most commonly the tibial nerve, median nerve or ulnar nerve, typically with an electrical stimulus. The response is then recorded from the patient’s scalp.
 
Because of the low amplitude of the signal once it reaches the patient’s scalp and the relatively high amount of electrical noise caused by background EEG, scalp muscle EMG or electrical devices in the room, the signal must be averaged. The use of averaging improves the signal-to-noise ratio. Typically, in the operating room, over 100 and up to 1,000 averages must be used to adequately resolve the evoked potential.
 
The two most looked at aspects of an SSEP are the amplitude and latency of the peaks. The most predominant peaks have been studied and named in labs. Each peak is given a letter and a number in its name. For example, N20 refers to a negative peak (N) at 20ms. This peak is recorded from the cortex when the median nerve is stimulated. It most likely corresponds to the signal reaching the somatosensory cortex. When used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient’s post-intubation baseline is a crucial piece of information. Dramatic increases in latency or decreases in amplitude are indicators of neurological dysfunction.
 
During surgery, the large amounts of anesthetic gases used can affect the amplitude and latencies of SSEPs. Any of the halogenated agents or nitrous oxide will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected. For this reason, an anesthetic utilizing less halogenated agent and more intravenous hypnotic and narcotic is typically used.

What is Polysomnography (PSG)?

 

Polysomnography (PSG), also known as a sleep study, is a multi-parametric test used in the study of sleep and as a diagnostic tool in sleep medicine...(click to read more)

The test result is called a polysomnogram, also abbreviated PSG. The name is derived from Greek and Latin roots: the Greek πολύς (polus for “many, much”, indicating many channels), the Latin somnus (“sleep”), and the Greek γράφειν (graphein, “to write”).
 
Polysomnography is a comprehensive recording of the biophysiological changes that occur during sleep. It is usually performed at night, when most people sleep, though some labs can accommodate shift workers and people with circadian rhythm sleep disorders and do the test at other times of day. The PSG monitors many body functions including the brain (EEG), eye movements (EOG), muscle activity (EMG), heart rhythm (EKG), respiratory airflow, respiratory effort indicators, and peripheral pulse oximetry.
 
Polysomnography is used to diagnose, or rule out, many types of sleep disorders including narcolepsy, periodic limb movement disorder (PLMD), REM behavior disorder, parasomnias, and most commonly sleep apnea. It is often ordered for patients with complaints of daytime fatigue or sleepiness that may be caused by interrupted sleep. Although it is not directly useful in diagnosing circadian rhythm sleep disorders, it may be used to rule out other sleep disorders.
 
For the standard test the patient comes to a sleep lab in the early evening, and is introduced to the setting and “wired up” so that multiple channels of data can be recorded when he/she falls asleep. The sleep lab may be in a hospital, a free-standing facility, or in a hotel. Our local sleep labs are free-standing facilities. A sleep technician should always be in attendance and is responsible for attaching the electrodes to the patient and monitoring the patient during the study.
 
During the study, the technician observes sleep activity by looking at the video monitor and the computer screen that displays all the data second by second. In most labs the test is completed and the patient is discharged home by 7 a.m. unless a Multiple Sleep Latency Test (MSLT) is to be done during the day to test for narcolepsy.

What is Intraoperative monitoring?

 

The purpose of IONM is to reduce the risk to the patient of iatrogenic damage to the nervous system, and/or to provide functional guidance to the surgeon and anesthesiologist...(click to read more)

Intraoperative neurophysiological monitoring (IONM) or intraoperative neuromonitoring (IOM) is the use of electrophysiological methods such as electroencephalography (EEG), electromyography (EMG), and evoked potentials to monitor the functional integrity of certain neural structures (e.g., parts of the brain, spinal cord and peripheral nerves) during surgery.
 
Neuromonitoring employs various electrophysiologic modalities, such as SSEP, transcranial motor evoked potentials (TcMEP), EEG, EMG, and auditory brainstem response (ABR). For a given surgery, the set of modalities used depends on which neural structures are at risk. IONM techniques have significantly reduced the rates of morbidity and mortality without introducing additional risks. By doing so, ONM techniques reduce health care costs.
 
To accomplish these objectives, a member of the surgical team with special training in neurophysiology obtains and co-interprets triggered and spontaneous electrophysiologic signals from the patient periodically or continuously throughout the course of the operation. Patients who benefit from neuromonitoring are those undergoing operations involving the nervous system or which pose risk to its anatomic or physiologic integrity. In general, a trained neurophysiologist attaches a computer system to the patient using stimulating and recording electrodes. Interactive software running on the system carries out two tasks:
 
Selective activation of stimulating electrodes with appropriate timing
Processing and displaying of the electrophysiologic signals as they are picked up by the recording electrodes.
 
The neurophysiologist can thus observe and document the electrophysiologic signals in realtime during the surgery. The signals change according to a various factors, including anesthesia, arterial pressure, tissue temperature, surgical stage, and tissue stresses. Differentiating the signal changes along these lines – with particular attention paid to stresses – is the joint task of the surgical triad: surgeon, anesthesiologist, and neurophysiologist.
 
Patients benefit from neuromonitoring during certain surgical procedures, namely any surgery where there is risk to the CNS or to a peripheral nerve. Most neuromonitoring is utilized by spine surgeons but vascular surgeons, otolarygologists and urologists have all utilized neuromonitoring as well. The most common applications are in spinal surgery, selected brain surgeries, carotid endarterectomy, ENT procedures, thyroid surgeries, and peripheral nerve surgery. Motor evoked potentials have also been used in surgery for TAAA (thoracic-abdominal aortic aneurysms). Intraoperative monitoring is used to localize neural structures, for example to locate cranial nerves during skull base surgery; to test function of these structures; and for early detection of intraoperative injury, allowing for immediate corrective measures. For example, during any surgery on the thoracic or cervical spinal column, there is some risk to the spinal cord. Since the 1970s, SSEP (somatosensory evoked potentials) have been used to monitor spinal cord function by stimulating a nerve distal to the surgery, and recording from the cerebral cortex or other locations rostral to the surgery. A baseline is obtained, and if there are no significant changes, the assumption is that the spinal cord has not been injured. If there is a significant change, corrective measures can be taken; for example, the hardware can be removed. More recently TcMEP have also been used for spinal cord monitoring. This is the reverse of SSEP; the motor cortex is stimulated transcranially, and recordings made from muscles in the limbs, or from spinal cord caudal to the surgery. This allows direct monitoring of motor tracts in the spinal cord. Electroencephalography is used for monitoring of cerebral function in neurovascular cases (cerebral aneurysms, carotid endarectomy) and for defining tumor margins in epilepsy surgery and some cerebral tumors. EMG is used for cranial nerve monitoring in skull base cases and for nerve root monitoring and testing in spinal surgery. ABR (aka BSEP, BAER, BAEP, etc.) is used for monitoring of the acoustic nerve during acoustic neuroma and brainstem tumor resections.