Information for the patient

In the event that you are a patient, you may need Electroneurodiagnostic testing. These are mostly non-invasive exams conducted at a hospital or a private clinic. They may be done for many reasons such as diagnosing seizures to certain types of neuropathy. It is only a professional practice that the physician ordering the EEG explain to the patient certain things like why they are ordering the test and approximately how long the exam will take. Unfortunately many patients are not informed of anything while arriving and are often disgruntled buy the lack of disclosure. The patient should always be prudent ing in asking questions since they are dealing with their health.

Here are some of the Exams


An EEG, Electroencephalogram is a test performed by an END or EEG technician at a hospital or clinic. It Involves the placement of approximately 22 electrodes on the scalp, spaced in 10 to 20 percent intervals in accordance to measurements of the head. These electrodes will record electrical activity that is generated mostly beneath the scalp, mainly the brain. A Clinical EEG is a non invasive exam and does not involve any radiation or electricity administered to the patient. The placement of these electrodes involves the initial cleaning and abrasion of the scalp to improve the conductivity between the scalp and the electrode. This is usually done by rubbing a cotton swab dabbed in a exfoliating compound, at the site on the scalp where the electrode will be placed. The electrode is a small metal cup about 2-3 mm in diameter attached to a wire that is connected to an amplifier. The electrode cup is filled with a conductive sticky paste that adheres the electrode to the scalp. It is important for the patient to have clean hair and scalp for the exam; hence it is recommended that the Patient wash their hair and dry it before the EEG exam. It is also important that the Patient does not apply and oil, conditioner or gel after washing their hair before the Exam. The application process takes about 20 minutes. During the application the patient must remain still so the technician may properly place the electrodes and keep them in place. It is also important that the patient remain still during the recording process of a routine EEG. Any movement of the head is picked up by the EEG machine and overrides and interferes with the signals that are important. Poor cooperation form the patient may make information acquired from the EEG exam inaccurate or incomplete. This makes it difficult to acquire good data form certain patients such as pediatrics and the mentally challenged. Once the the electrodes are applied it is critical that the patient remain still for the recording. The recording time of a routine EEG can be anywhere form 20 minutes to 1 hour.

EEG Head 23237636045_3acf35b2dc_z


Sensory evoked potentials (SEP) are recorded from the central nervous system following stimulation of sense organs (for example, visual evoked potentials elicited by a flashing light or changing pattern on a monitor;[2] auditory evoked potentials by a click or tone stimulus presented through earphones) or by tactile or somatosensory evoked potential (SSEP) elicited by tactile or electrical stimulation of a sensory or mixed nerve in the periphery.

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

Dr K Ng)
(Image courtesy of

Visual evoked potential

Visual evoked potentials (VEPs) are described by O'Shea et al. (2009).[2] They are caused by sensory stimulation of a subject's visual field and are observed using electroencephalography. Commonly used visual stimuli are flashing lights, or checkerboards on a video screen that flicker between black on white to white on black (invert contrast). The resulting waveform includes the C1 and P1 followed by the visual N1.
Visual evoked potentials are very useful in detecting blindness in patients that cannot communicate, such as babies or animals. If repeated stimulation of the visual field causes no changes in EEG potentials, then the subject's brain is probably not receiving any signals from his/her eyes. Other applications include the diagnosis of optic neuritis, which causes the signal to be delayed. Such a delay is also a classic finding in Multiple Sclerosis. Visual evoked potentials are furthermore used in the investigation of basic functions of visual perception. VEPs are also sometimes used to determine if someone is fraudulently alleging blindness.
The term "visual evoked potential" is used interchangeably with "visually evoked potential". It usually refers to responses recorded from the occipital cortex. Sometimes, the term "visual evoked cortical potential" (VECP) is used to distinguish the VEP from retinal or subcortical potentials.

Auditory evoked potential

Auditory evoked potential can be used to trace the signal generated by a sound through the ascending auditory pathway. The evoked potential is generated in the cochlea, goes through the cochlear nerve, through the cochlear nucleus, superior olivary complex, lateral lemniscus, to the inferior colliculus in the midbrain, on to the medial geniculate body, and finally to the cortex.[21]
Auditory evoked potentials (AEPs) are a subclass of event-related potentials (ERP)s. ERPs are brain responses that are time-locked to some “event”, such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. For AEPs, the “event” is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc.


Testing as and inpatient or outpatient.

Electroneurodiagnostic exams such as EEG's can be performed bedside while you are staying in the hospital or as an outpatient at the hospital or private clinic. Many times this is not a choice for the patient, but if it is, conducting an exam as an outpatient such a s a controlled and isolated lab is much better. In an EEG exam, the patient must be as relaxed as possible, free of extraneous noise and distractions. Any movement of the pt head or body can be translated into false test results. It is also encouraged that the patient be allowed to be drowsy and or fall asleep during an EEG. EEGs are done on all types of patients, including some who are in altered mental status, such as confused and unable to follow commands. Recordings in a controlled lab/clinic are usually more accurate, revealing and conclusive than those tests ordered bedside in a patients hospital room.