Various medical conditions can result in seizures. Before diagnosing epilepsy, a doctor will consider other possible causes of the seizures.
A high fever or a metabolic disturbance could cause seizures to occur. For example, an electrolyte disturbance as a result of severe diarrhea or vomiting, or a glucose imbalance caused by low blood sugar, may result in seizures.
Laboratory tests such as blood tests are sometimes used to find medical conditions other than epilepsy that may be causing seizures.
In addition to a thorough physical examination, the procedures used to establish a diagnosis of epilepsy often include a medical history and diagnostic tests.
Medical history is important in a doctor’s assessment. Typically, it involves a family health history and a detailed description of the characteristics, onset, and frequency of the seizures. Determining the type of seizure an individual is having is valuable in both the diagnosis and the prescribing of the appropriate treatment.
Diagnostic tests usually include an electroencephalogram (EEG). An EEG is used to record the brain’s electrical activity and it is an important tool in the diagnosis of epilepsy.
Neuroimaging tests are often used to provide pictures of the brain. Computed tomography (CT) and magnetic resonance imaging (MRI) scans provide pictures of the brain structures.
Other neuroimaging tests such as magnetic resonance spectroscopy (MRS) and positron emission tomography (PET) show how the brain functions and are used to evaluate the possibilities for epilepsy surgery.
It is important to note that sometimes a diagnostic test does not detect abnormalities. For example, a person with epilepsy may have a normal EEG because abnormal activity is not present during the recording or the activity is too deeply located in the brain to be recorded.
Diagnostic tests used in the diagnosis of epilepsy may include:
An EEG is a painless, non-invasive test that is used to measure a person’s brain wave pattern. The electrical impulses of the brain are recorded by small metal discs placed on the person’s scalp, connected through wires, with the EEG machine. The EEG recordings can detect abnormalities in the brain’s electrical activity.
Although an abnormal EEG can confirm a diagnosis of epilepsy, a normal EEG does not rule out the presence of epilepsy. The EEG records the activity in the brain at the time of the recording. Usually the recording session lasts for less than an hour.
Hyperventilation (over-breathing) and photic stimulation (flashing lights) are routinely used to reveal abnormal changes in the brain activity. Sleep deprivation may also be used.
Ambulatory EEG units are sometimes used to monitor a person for longer periods of time. The individual wears a portable EEG unit that records brain activity during normal activities at home, at work, or during sleep.
EEG video telemetry, a technique that combines EEG recording with videotaping, may also be used over longer periods of time to record a clinical seizure on tape. Behaviour during a seizure can then be studied in combination with EEG recordings.
CT SCAN (Computed Tomography)
A CT (or CAT) scan is used to detect physical conditions in the brain that may be causing seizures such as tumors or scar tissue. The CT machine takes a series of x-rays to show the brain’s structures. Typically, the person lies on a CT scan table while the surrounding scanner takes the x-rays. An intravenous injection with a so-called contrast medium is sometimes used to make abnormalities on the scan more visible.
MRI (Magnetic Resonance Imaging)
An MRI is used to provide structural information such as the presence in the brain of tumors, scar tissue, or abnormal blood vessels. Magnetic fields instead of x-rays are used to produce precise two- or three-dimensional images of the brain. The MRI shows a more detailed picture of the brain than the CT scan. Sometimes both studies are needed. During the procedure, the individual usually lies on a scanning table in the tunnel-like magnetic chamber.
MRS (Magnetic Resonance Spectroscopy)
Essentially an MRI with a different computer program, the MRS provides information about chemical activity in the brain. This information can be used to detect metabolic abnormalities in the brain during, after, and in between seizures.
PET (Positron Emission Tomography)
PET scanning produces three-dimensional computer images of the brain processes at work. An intravenous with a very low dose of a radioactive glucose substance is given to the person. The scanning images show how much glucose is being used by different parts of the brain. These images provide information on the chemistry, blood flow, and glucose consumption of the brain that is useful in locating the origin of the seizures. Usually the individual lies on an examination table that is slowly moved into the machine so that the head is inside the circular opening of the scanner.
SPECT (Single Photon Emission Computed Tomography)
This test helps to locate the site where the seizure begins. A compound with a small amount of radioactive substance is injected into a vein and then three-dimensional images are taken to view blood flow or metabolism. There are two separate injections. One is given during a seizure and the other is given in between seizures. The SPECT scans are taken an hour or two after the injections. The scans are then compared to identify the changes in blood flow. The individual lies very still on a bed while a large camera takes pictures.
MSI or MEG (Magnetic Source Imaging or Magnetoencephalography)
This test is used to assess the function of brain tissue. It is similar to the EEG, but magnetic rather than electrical brain waves are recorded in a three-dimensional fashion through sensors located in a machine placed near the person’s head.
Reprinted in part from Living with Epilepsy (Epilepsy Education Series, Edmonton Epilepsy Association