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How are EEGs used to Detect Epilepsy?

March 6, 2018

Most of us have probably heard of the term “EEG scan”. Got a persisting headache? Go for an EEG scan. Trying to confirm a diagnosis for seizures or epilepsy? An EEG scan is what you need.

 

But… what is this odd-sounding name, and how does it help us?

 

Put simply, here’s what an EEG (Electroencephalogram) does in a sentence: It measures electrical activity in your brain by placing electrodes on the human scalp.

 

Here is a small breakdown so you can better understand its function:

-    Your brain contains about 100 billion neurons.

-    These neurons send ‘messages’ to each other in the form of nerve impulses, which give off electricity.

-    This electrical activity can be measured by electrodes. These are small flat metal discs that pick up electrical activity and send the electrical signals to a computer.

-    This electrical activity recorded is also known as brain waves.

 

Simple, right?

 

Now, why do we need these devices? Essentially, EEGs measure brain activity, which is crucial to the diagnosis of several neurological conditions, including:

 

i. Encephalitis (inflammation of the brain)

ii. Stroke (reduced blood supply to the brain)

iii. Epilepsy (recurrent convulsions and loss of consciousness)

 

… and several more. These scans provide crucial information, and, if you follow my guide for the rest of the article, you can get a much better insight into what the lines on the EEG scans actually represent.

 

To answer this question , we must first look at how the EEG scan looks like.

 

Although  it may look difficult, it is certainly not impossible to get the basics of reading EEG scans correctly.  And by following this recipe I’m going to make it seem as easy as… well, a piece of cake.

 

Here are the steps:

 

STEP 1: THE INGREDIENTS (Letters, Numbers)

 

(1)  LETTERS

 

As mentioned earlier, an EEG is taken by placing 25-30 electrodes on different locations of the human scalp. The letters on the side of the scan indicate the parts of the brain on which noteworthy electrical activity occurs. This helps to determine the origin of a seizure, or explain the reason behind a certain symptom.

 

Each electrode has a unique letter. These letters describe the parts of the brain they record. Below is a quick key of the letters and what they stand for:

  • F = frontal

  • Fp = frontopolar

  • T = temporal

  • C = central

  • P = parietal

  • O = occipital

  • A = auricular (ear electrode)

  • Z = electrodes on the midline of the head.

(2)  NUMBERS

 

Now that you are able to identify the letter, what function it serves, and which part of the brain it corresponds to, you will need to know what the numbers on the side mean. This step is extremely simple. Even numbers (2, 4, 6…) identify electrode positions on the right side of the head, while odd numbers (1, 3, 5… ) correspond to the left side of the brain.

 

 

STEP 2: DETERMINE THE FREQUENCY

 

Now that we know what the letters and numbers mean, we have to look at the brain waves. Notice how some are very close together while others are more spread out.

The type of wave can be defined by it’s frequency, which essentially means the number of ‘waves’ per second. Correspondingly, the frequency values for each wave are as follows:

 

Delta waves = less than 4 Hz

Theta bands = 4-8 Hz

Alpha bands = 8-12 Hz

Beta bands = 14-30 Hz

Gamma bands = 30-80 Hz

 

However … what does this signify? Well, values of about 8 Hz are normal findings of EEG in an awake adult, while 7 Hz is normal in children or sleeping adults. Anything less or more than these values indicates an anomaly, possibly describing a brain tumour, infection, injury, stroke, or epilepsy.

 

STEP 3: PINPOINT THE ABNORMALITIES

 

All the information garnered so far can be used to broadly categorize the anomalies into different diagnoses. Although it is impossible to form an accurate diagnosis without patient data, it is possible to broadly categorize it to a certain extent.

 

Some of the possible categorizations is as follows:

 

Partial seizures – This occurs when brain abnormalities occur on only one part of the brain. These are indicated by ‘sharps’, which are waves lasting from 0.8 to 2 seconds in length.

 

Generalized seizures: Abnormal activity is picked up by all electrodes from all parts of the brain.

 

Absence seizures – These occur primarily in children, and lasts only for a few seconds - so short that it is almost mistaken for daydreaming. These are generally identified by ‘spikes’ (very fast waves, under 0.8 seconds long) on the EEG. Repeated spikes form a ‘spike-wave’, which is also an indicator for this seizure.

 

Idiopathic seizures – This type of seizure occurs as a result of one of a group of epileptic disorders (known as Idiopathic General Epilepsy, or IGE), that are genetically determined. It is often defined by poly-spikes, which are a series of spikes that occur quickly.

 

Although the EEG is one of the most applicable and efficient forms of brain monitoring in the world today, it is by no means a perfect test. It has many limitations: poor detection of electrical signals, lack of interpretation of the signal messages, and lack of specific patterns to identify certain seizures. However, even having a little bit of information into how this system works can be extremely insightful.

 

 

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