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What They Are
1. Electroencephalogram (EEG)
What It Is : An EEG records the electrical activity of the central switchboard – the brain. In other words, this test measures your brainwave patterns. Several external electrodes are placed on your scalp. These record the electrical impulses emanating from different centres or areas of your brain that are picked up by the electrodes and transmitted to a polygraph. This is a machine that produces separate graphs on moving paper or on a computer screen.

Why It’s Done : An EEG is performed on individuals with epilepsy or those who exhibit seizures of any type. Epilepsy is a brain disorder where clusters or groups of nerve cells fire abnormally. This is like a mini electrical storm in a part of the brain. A seizure may cause strange sensations, emotions and behaviour or convulsions, muscle spasms and, at times, even loss of consciousness.

During a seizure, the neurons in the brain may fire as many as 500 signals per second. Some individuals may experience this only occasionally while others may experience it more than once a day.

2. Video EEG
What It Is : As the name suggests, Video EEG is an EEG performed while the process is videotaped. A Video EEG typically takes several hours and sessions may continue for days or weeks. It may be performed on an out-patient basis or in severe cases on in-patients.

Why It’s Done : A Video EEG allows the doctor / neurologist to observe an individual during a seizure. Just like the electrical signals, the individual’s presentation too offers clues to the disorder, its exact nature and severity. Together, this helps make a precise and accurate diagnosis. A Video picks up seizure activity when the patient is clinical experiencing a seizure.

3. Electromyogram / Nerve Conduction Study (EMG NCS)
An EMG or Electromyogram and NCS or Nerve Conduction Study are two separate but complementary tests.

Electromyogram (EMG)
What It Is : An EMG essentially measures the electrical activity in the muscles. Micro needles are placed in the muscles and the electrical activity is measured. The presence, shape, size and pattern of electrical activity emanating from the muscle are displayed on an oscilloscope, which is a type of monitor.

The test is painless and takes about an hour. There may be minor discomfort while the needles are inserted but it lasts only for a fraction of a second.

Why It’s Done : An EMG evaluation is usually sought to detect muscle diseases such as myopathies, muscular dystrophy and alcoholic myopathies; nervous system disorders such as radiculopathy and peripheral nerve injury; and neuromuscular disorders such as myasthenia gravis. Sometimes, an EMG is performed to detect polio.

Nerve Conduction Study (NCS)
What It is : An NCS is a test used to evaluate the function or electrical conduction of the motor (nerves that go into muscles) and sensory (nerves that provide sensation) nerves in the human body. The test is performed by stimulating a peripheral nerve and recording its reaction displayed on a monitor.

This test is usually performed by stimulating nerves through electrodes by administering a mild electrical current. It may therefore cause mild discomfort. Patients with a pacemaker or implanted stimulators such as deep brain stimulators must inform the examiner before the test begins so that appropriate precautions may be taken.

Why It’s Done : An NCS is performed on patients who report unexplained muscle weakness, swelling, numbness (paresthesias), twitching, tingling or a burning sensation in the limbs. The test detects neuropathies, radiculopathies, Guillian Barre Syndrome, diabetic neuropathy and myasthenia gravis.

4. Somato-Sensory Evoked Potential (SSEP)
What It Is : This test assesses the relay of body sensations to your brain via the spinal cord and to see how this central switchboard receives these sensations.

The examiner places surface electrodes on your limbs and a mild electrical current is switched on that stimulates your senses. The response to these electrical signals is recorded by electrodes placed on your scalp and spine. The resultant pattern of signals is used as a diagnostic tool.

Why It’s Done : An SSEP test is usually performed on individuals who experience unexplained numbness in any area of the body. It is also one of many tests used to diagnose multiple sclerosis.

5. Visual Evoked Response (VER)
What It Is : This diagnostic test is used to detect disorders in vision by stimulating the patient’s visual field. A visual response is ‘evoked’ or stimulated with the use of flashing or flickering lights or a checkerboard pattern on a monitor in a darkened room.

Since the optic nerve, the nerve that transmits signals from the eyes to the brain, is stimulated, electrodes are placed on the scalp over the area that corresponds to the occipital lobe in the brain. Thus an EMG machine is used to measure these signals.

Why It’s Done : A VER test is performed to detect blindness in individuals who cannot communicate or articulate themselves such as babies. It is also used to confirm optical neuritis, multiple sclerosis, stroke, lesions, or partial blindness caused by the inflammation of the optic nerve. Additionally, a VER test is one of many tests used to detect multiple sclerosis as a classic symptom of this disease is a delay in the transmission of signals from the optic nerve to the brain.

6. Brainstem Auditory Evoked Response (BAER / AER)
What It Is : A BAER test is basically used to study an individual’s auditory (hearing) response. It is often used in newborns and children.

Also called Auditory Brainstem Response Audiometry or Brainstem Evoked Response Audiometry (BERA), the test detects the functions of the ears, cranial nerves and functioning of the ‘lower’ part of the auditory system or the brainstem (as opposed to the cerebral cortex).

The individual is asked to listen to sounds such as clicks or tones which stimulate the ear. In response to these sounds, signals from the ear are transmitted to the brainstem and are recorded by electrodes placed at the vertex of the scalp and the ear lobes.

Why It’s Done : The BAER test is used to detect deafness and other types of hearing loss, especially in cases of multiple sclerosis or stroke or lesions. It is also used to detect and measure vertigo as this disorder involves the inner ear and hence the auditory apparatus.