Pattern-Sensitive Epilepsy

Demographic Data Onset. The onset of pattern sensitivity is most likely at puberty or a few years before, and it can remain throughout life. Gender. Girls are nearly twice as likely to be affected as boys. Prevalence. Photosensitivity occurs in about 4% of adults with epilepsy, but the prevalence is greater in the young. Pattern sensitivity is present in most photosensitive patients, but with clinical significance only in about 30–50% (Harding and Jeavons 1994).


Clinical Manifestations
Seizures are more likely if the patterns are striped , subtend a large visual angle at the eye ( > Fig. 209-1b), if they are brightly lit and strongly contrasted ( > Fig. 209-1c), and if the periodic elements within the pattern are regularly spaced (> Fig. 209-1d) and have a spatial frequency close to 3 cycles per degree ( > Fig. 209-1e).Epileptogenic patterns include gratings, the metal stair tread of escalators, and striped clothing.The seizures can be of any type, ranging from fleeting absence to major convulsion (Wilkins 1995).

Etiology
Etiology is most often idiopathic, but can be secondary.Diseases that render the visual cortex hyper excitable can potentially give pattern-sensitive epilepsy.

Pathophysiology
The seizures arise when normal cortical excitation involves a region of the visual cortex of sufficient size (> Fig. 209-2), stimulating complex cells within a limited number of orientation columns (> Box 1 and > Fig. 209-3).The discharge begins within one cerebral hemisphere and can generalize, or remain confined within that hemisphere, in which case it is associated with an ipsilateral posterior epileptiform EEG ( > Box 1 and > Fig. 209-4).For seizures to occur, the excitation needs to be (1) strong and (2) synchronized.Synchronization occurs when the pattern is stationary, and its retinal image is moving by virtue of the normal instability of the eye during fixation.The motion stimulates neurons selective to one direction of motion then another, synchronizing the activity.The epileptogenic potential of the pattern is greatly increased if it alternates in phase at a frequency of about 10-20 Hz, or if it vibrates at similar frequency in a direction orthogonal to the stripes ( > Fig. 209-5).If the patterns drift continuously in one direction (at a similar rate), they are not epileptogenic (Wilkins 1995).

Diagnostic Procedures
Most patients with pattern-sensitive epilepsy are sensitive not only to patterns but also to flickering light, and conventional diagnostic procedures using intermittent photic stimulation may be expected to give rise to a photo-paroxysmal EEG response (PPR).However, there are exceptional patients who show a photoparoxysmal response only to patterns.The most epileptogenic patterns are strongly illuminated (mean luminance >100 cd m À2 ), subtend at least 20 of arc at the eye, and consist of stripes subtending about 15 min of arc.Several such patterns at a variety of orientations should be available for routine testing during the EEG examination (Wilkins 1995).

Differential Diagnosis
The demonstration of pattern sensitivity in a photosensitive patient is clinically important because it shows the additional range of visual stimulation to which the patient is susceptible.Text can provide a sufficient pattern stimulus in some patients (horizontal bars, > Fig. 209-1).Patients with migraine show aversion and perceptual distortion (dotted curves, > Fig. 209-1) when viewing epileptogenic patterns, but this is because the patterns induce a strong neurological response; not because the response is synchronized -drifting patterns are not epileptogenic, but are aversive for individuals with migraine (Wilkins 1995).

Prognosis
The prognosis is as for photosensitive epilepsy; 75% of patients retain their susceptibility to patterns for life, but some lose their sensitivity after their 20s (Harding and Jeavons 1994).

Management
The management is as for photosensitive epilepsy.Appropriately tinted glasses can be an effective treatment.Blue glasses have been shown to reduce seizures in some cases.There are Pattern-Sensitive Epilepsy also initial indications that an individually chosen colour can offer a more acceptable and effective treatment.Where there is an awareness of symptoms, an appropriate color can be selected subjectively using the Intuitive Colorimeter.If physical means of protection are insufficient, the drugs of choice are sodium valproate or lamotrigine.

Declaration of Interest
The British Medical Research Council owns the rights to the Intuitive Colorimeter.The author receives from the Council a proportion of the royalties on sales in the form of an Award to Inventors.

Figure 209 - 1 .
Figure 209-1.Probability of epileptiform EEG activity in patients with pattern-sensitive epilepsy shown as a function of several spatial characteristics of the pattern (solid curves).The dotted curves show the number of illusions of color, shape or motion reported by normal observers, illusions to which those with migraine are particularly susceptible.The horizontal bars show the characteristics of text, when considered as a striped pattern.Icons beneath each graph represent variation in the relevant spatial characteristic (Wilkins 1995)

Figure 209
Figure 209-2.Proportion of patients showing epileptiform EEG activity in response to a pattern of stripes, shown as a function of the proportion of the visual cortex to which the pattern projects.The data are taken from several studies that included the patterns shown in > Fig. 209-3a-d (Wilkins 1995)