Photic Stimulation - An Overview

Introduction:

Epilepsy is one of the most common neurological disorders. Worldwide, fifty million people are affected by this central nervous system disorder. It affects 4 out of 1000 people globally. Loss of sensations, awareness, and seizure episodes characterize epilepsy. Among different types of epilepsy, photosensitive epilepsy is one of the most common, affecting almost five percent of patients. Epileptic or seizure events are greatly influenced by the photic or pattern-induced stimulus.

Proper diagnosis and identification are needed to determine the type of epilepsy. Photic or intermittent photic stimulations are routinely used along with electroencephalograms to determine the epilepsy type. It helps to determine medications for epileptic patients and guides the counseling process. Apart from this, photic stimulation can be used for visual fixation and correcting visual reflection reflexes.

Where Is It Used?

Intermittent photic stimulation (IPS) activates electroencephalography (EEG) recordings to elicit paroxysmal photo responses. Mainly in epileptic patients, it is done for the following reasons.

• To understand whether the patient is photosensitive or not. In photosensitive patients, paroxysmal photo responses are positive. These responses consist of spikes, spike–waves, or intermittent slow waves, which are only confined to occipital regions.

• Paroxysmal photosensitive responses and seizures can occur in epileptic patients. Photic stimulation tests are necessary to identify the exact type of epilepsy.

• Quantifying the response to intermittent photic stimulation and tailored studies can indicate the use of antiepileptic drugs.

• This method can treat strabismic amblyopia in older children and young adults.

• Intermittent photic stimulation can be used in the treatment of nystagmus also.

What Is the Method of Testing?

Electroencephalograms do not always show epileptic changes in the records. Different activation methods are used to elicit epileptiform abnormalities in the EEG. Photic stimulation is one of them.

• For the test purpose, a photo stimulator is used with a range of 1 to 60 Hz, which means up to 60 flashes per second.

• A photo stimulator is placed around 30 cm away from the subject in a darkened room with att EEG leads attached to the body,

• Intermittent photic stimulation is initiated at 18 flashes or seconds with eyes open for three seconds, then on eye closed and for a further three seconds with eyes shut.

• Flashes are delivered in different frequencies of 10 seconds (eyes open and with one eye closed, and three seconds with eyes shut).

• Then the process is reversed, starting from one flash per second to up to 50 flashes per second.

• The procedure was repeated for five minutes but terminated earlier if two or more photo paroxysmal responses occurred.

• Intermittent photic stimulation should not be performed during or less than three minutes after hyperventilation.

• The test should be stopped immediately if an epileptiform event occurs.

• A second sequence should start at 60 Hz and decrease as follows: 60, 50, 40, 30, 25, and 20.

• The stimulator must be turned off immediately if a generalized response is seen and the sequence stops.

In most patients, photosensitivity is maximal, with 14 to 16 flashes. Children suspected of progressive myoclonic epilepsies undergo ≤5Hz flash frequencies.

Mallett intermittent photic stimulation is the protocol used to treat nystagmus and amblyopia. In this method, the better eye is occluded, and the flashing frequency is four times per second. Clement Clark and Curpax synoptophore and slides with a red background are being used. Each eye must be stimulated for 15 to 20 minutes, with 30 to 40 minutes per season. The concept behind this protocol is that red light helps the eye to align properly each time the flash comes on. This is also believed to stimulate the eye to line up centrally. The detail on the slides is designed to stimulate different types of cells that run from the eye to the brain, and concentrating on the targets helps train the brain to use the lazy eye.

What Is the EEG Response?

• Focal and generalized spikes.

• Sharp waves.

• Spike-and-slow wave complexes.

• Polyspikes - These spikes are known as interictal spikes. The properties of these spikes are as followers.

• Duration less than 200 ms: 70 to 200 ms for a sharp wave and 20 to70 ms for a spike.

• Asymmetric contour with steep upslope and with adownstroke

• An abrupt change in polarity occurs over several milliseconds.

EEG Response to Photic Stimulation -

While interpreting intermittent photic stimulation, a clear distinction in epileptiform responses should be made. It should be determined whether the response is confined to the occipital area (OSW), starting occipitally and spreading to frontal regions (OGSW), or whether generalized from the start (GSW), other responses (OR), including generalized spike, should be made.

1. GSW: Generalized regular, spike-and-waves, or poly spike-and-waves.

2. OGSW: Temporo parieto occipital, in the beginning, generalized regular, spike-and-waves, or polyspike-and-waves.

3. OSW: Temporo parieto occipital regular, spike and-waves, or polyspike-and-waves or other responses.

How Does It Work?

The response of photo stimulus over the brain is not fully understood. Different scientists postulated different views regarding this. According to some scientists, photosensitive stimuli cause excitement in pyramidal cells. This excitement can be inhibited by γ - aminobutyric acid (GABA) ergic inhibition. But in patients with marked deficiency of γ - aminobutyric acid, it causes the spread of excitation, and a sudden sensory stimulus can engage sensory regions of the brain.

According to other scientists, the visual cortex is located in the occipital lobe, triggered by the visual stimulus, leading to a synchronized response of many brain cells and high cerebral blood oxygenation. Following this, other brain areas like the occipital cortex, Brodmann’s areas 17, 18, 19, hypothalamus, caudate, hippocampus, and scattered other regions. For persons sensitive to photo stimuli, the amount of activation of the visual cortex is much more.

Where Is It Contraindicated?

In certain cases, the photic stimulation test is contraindicated; there are

• Intracranial hemorrhage.

• Increased intracranial pressure.

• Pregnancy.

• Ongoing status epilepticus.

• History of epileptic seizures induced by photic stimulations.

• Inability to cooperate.

Conclusion:

The mechanism of photosensitivity and its effect on cerebral tissues is not fully understood. People found to be positive for photosensitivity do not have a history of seizures. That is why it is essential to establish patients' responses under photic stimulations. This will not only help in the diagnosis but also help to determine the treatment plan.