Visual snow syndrome or “what's in a name?”

I read with interest the recent article by Weatherall (1) and agree it is unlikely that the visual snow syndrome (VSS) has arisen de novo in recent decades despite the first case series appearing as recently as 2014 (2,3). A clinical description did appear in 1983 by Abraham as the Hallucinogen Persisting Perception Disorder (HPPD) (4), including mention of prolonged after-images, a common feature of VSS. I would, however, take issue with Weatherall (and others ² ) in using the term “palinopsia” as synonymous with the generation of physiological after-images (PA-I).

The term (as “paliopsia”) was first coined by Macdonald Critchley (5) to convey “seeing again”. He characterised a disorder in which an image reappears after the original image is no longer visible: not persistence of an image. Although a subgroup of palinopsia may behave in a similar manner (known as visual persistence), later studies (6) demonstrated fundamental differences between palinopsia and PA-IG.

The persistent images in VSS are neither complementary in colour nor negative in achromatic contrast and Abraham correctly classified these as “positive” after images, which is a class of PA-I. In PA-I, images persist following removal of the initial image with no time delay and remain retinotopically localised. The image appears to move with voluntary (but not passive) movements of the eye. This is not the case with palinopsia where the image reappears after a noticeable time delay, is localised in a different location in the visual field (sometimes in external space) and is often distorted in some way (commonly increased in size – Critchley's “visual spread”). The perceived size of a PA-I depends on the viewing distance of the surface being viewed and will become progressively smaller if a vergence effort is made in the dark. Following monocular viewing of a provoking stimulus a PA-I will remain monocular whereas a palinoptic image will transfer binocularly.

Only positive PA-I of this type can give rise to another symptom common in VSS and HPPD: the appearance of a streak or trail following a moving image. This cannot occur in palinopsia but is compatible with PA-I. This can be experienced by any of us when dark adapted: the longer retinal processing time gives rise to a universal experience – remembered by most as the trail of light that follows a “sparkler” seen at a firework party.

The term “entoptic” refers to any visual experience which arises in the eye itself. This includes phosphenes such as those generated by mechanical deformation of the retina or by cosmic rays in astronauts. Another common error (of which Weatherall is guilty) is the misuse of the term “entopic”, which is the converse of “ectopic”. An entopic organ is one that is in its proper location – not a description often heard! In VSS, patients report the entoptic experiences generated by intra-ocular structures positioned between the external light source and the photoreceptor layer of the retina. Other symptoms are a result of limitations of the optics of the eye, such as “star bursts” around streetlights.

Floaters (or mosche volanti as they are still called in Italy) can only be observed when there is light entering the eye because they are the shadows of imperfections in the vitreous humour falling on the retina. The images appear very large and patients are often reassured when it is explained that the imperfections are microscopically small, appearing much magnified as the shadows are projected directly on the retina. Indeed, we can “see” our own leukocytes in the “blue-field entoptic phenomenon”. The reference to “floaters” in English and “flying flies” in Latin and Italian conveys the apparent motion of the shadows that do not follow the motion of the retinal image during eye- and ego-motion. What is significant in relation to VSS is that many individuals go through their life without ever noticing these entoptic phenomena, which are universal, whereas others can be troubled by a persistent awareness of them to an extent that they become intrusive. The illustrations by Wharton Jones and Brewster in Weatherall's article are undoubtedly vitreous floaters. I can agree with Weatherall that many of these early descriptions of patients with troubling visual experiences overlap with the panoply of features in contemporary descriptions of VSS.

Why does any of this matter? The most parsimonious explanation of VSS is that the phenomena described are universally present but can be “filtered out” in the processes underlying visual perception. A failure of this filtering can lead to them becoming intrusive. Indeed, many patients with VSS report that the experience for them has been lifelong. Recent research has shown that a striking feature of visual perception is that it operates on both a Bayesian and a “need to know” basis in interpreting and filtering information contained in the retinal image. Examples are the “moon illusion”, “change blindness”, “inattentional blindness” and many more. “Visual snow” itself may represent a level of spatio-temporal “noise” in the visual system that does not normally reach perception. This is supported by the high prevalence of tinnitus in the published cohorts. However, for this to be a universal explanation, we need to be sure that the persistence of images in VSS is due to persistence of PA-I and is not related to the group of pathological phenomena known as palinopsia.

The age of analogue television, when TV noise or “snow” was a common experience, came in the 20th Century and has disappeared in the 21st. It is interesting to note that, in many language groups, it was the black dots that dominated and it was referred to as the “war of the ants”. Had that been universal we might now have a “visual formication” syndrome rather than one involving snow. Indeed, patients now often refer to their vision as “pixellated” reflecting the digital age. I mention this because Weatherall has reminded us that in seeking historical evidence of a disorder we need to be mindful of changes in the use of language over time and to take care to be accurate in our own usage.