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- Dynamic, continuous, tiny dots across the entire visual field1, persisting for >3 months
- Additional visual symptoms of at least two of the following four types:
- enhanced entoptic phenomena3
- impaired night vision (nyctalopia)
- Symptoms are not consistent with typical migraine visual aura4
- Symptoms are not better accounted for by another disorder5
- Patients compare visual snow to television static (“television snow”). The dots are usually black or grey on a white background and grey or white on a black background, but also reported are transparent dots, white flashing dots and coloured dots.
- Palinopsia may be visual after-images and/or trailing of moving objects. Visual after-images are different from retinal after-images, which occur only after staring at a high-contrast image and are in complementary colour.
- These phenomena, arising from the structure of the visual system itself, include excessive floaters in both eyes, excessive blue field entoptic phenomenon (uncountable little grey/white/black dots or rings shooting over the visual field of both eyes when looking at homogeneous bright surfaces such as the blue sky), self-lighting of the eye (coloured waves or clouds perceived when closing the eyes in the dark) and spontaneous photopsia (bright flashes of light).
- As described under 1.2.1 Migraine with typical aura.
- Normal ophthalmology tests (corrected visual acuity, dilated-pupil fundoscopy, visual field examination and electroretinography) and no intake of psychotropic drugs.
A1.4.6 Visual snow is newly included in the Appendix of ICHD-3. It may not per se be part of the migraine spectrum but appears to be epidemiologically associated with 1.2 Migraine with aura. Further research is needed into whether these disorders share pathophysiological mechanisms causing visual symptoms but, meanwhile, it is hypothesized that cortical hyperexcitability plays a role in both. Patients with 1. Migraine have an increased prevalence of palinopsia and heightened visual sensitivity outside attacks: A1.4.6 Visual snow features both palinopsia and photophobia. Patients with A1.4.6 Visual snow and comorbid 1. Migraine more often have palinopsia, spontaneous photopsia, photophobia, nyctalopia and tinnitus than those without comorbid migraine.
Two other reasons support inclusion of A1.4.6 Visual snow in ICHD-3. First, it creates awareness of this condition, and aids physicians in recognizing it. Patients complaining of visual snow as a symptom often have (a history of) 1. Migraine; physicians unaware of A1.4.6 Visual snow may misinterpret its symptoms as persistent visual aura. Second, in a similar argument applied to research, future studies on persistent visual symptoms need homogenous study groups; inclusion of criteria for A1.4.6 Visual snow makes it clear to researchers how this disorder is currently defined.