Hallucinogen Persisting Perception Disorder
Hallucinogen Persisting Perception Disorder or HPPD is a form of visual disorder characterized by visual snow, light sensitivity, halos, and other visual disturbances akin to those demonstrated by hallucinogens. Fun as this sounds, it does get annoying when you try for the fifth time to read War and Peace only to be foiled by the words curving and changing colour. It's an interesting, but often difficult condition. It also sometimes comes alongside depersonalization and derealization, which only exacerbate things further.
Typical symptoms include heightened awareness of floaters and tracers, halos of light around bright objects, curving effects at the edge of the peripheral vision, visual snow, sensitivity to light, changes in the hue of objects, and deficiencies in depth perception. In extreme cases, depersonalization and derealization have also been reported.
 Causes of HPPD
Little is currently understood about the biological mechanism underlying the disorder. One theory is that the disorder is a form of nervous hypersensitivity related to vision, but at the moment there's no experimental evidence to support this. In many reported cases HPPD is triggered by frequent and relatively heavy use of hallucinogens. However in some cases HPPD-like symptoms have been reported even after a single, relatively modest trip. It is possible that genetic and environmental factors may make certain individuals more susceptible to develop the disorder, but until more is understood about the nature of the condition there are no firm predictive criteria.
 Prevalence of HPPD among hallucinogen users
Anecdotal evidence would suggest that HPPD occurs at a significant, albeit low rate amongst hallucinogen users. Estimates range anywhere from 0.66% up to 4.1% of hallucinogen users will develop HPPD, however no substantial medical study has been carried out.
 Diagnostic criteria for Hallucinogen Persisting Perception Disorder
As the name suggests, the use of a hallucinogen is a prerequisite for diagnosis. It is however possible that HPPD belongs to a wider spectrum of perceptual disorders that can also occur in individuals with no history of hallucinogen use.
Diagnostic criteria from DSM-IV:
A. The re-experiencing, following cessation of use of a hallucinogen, of one or more of the perceptual symptoms that were experienced while intoxicated with the hallucinogen (e.g., geometric hallucinations, false perceptions of movement in the peripheral visual fields, flashes of colors, intensified colors, trails of images of moving objects, positive afterimages, halos around objects, macropsia, and micropsia.
B. The symptoms in Criterion A cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
C. The symptoms are not due to a general medical condition (e.g., anatomical lesions and infections of the brain, visual epilepsies) and are not better accounted for another mental disorder (e.g., delirium, dementia, Schizophrenia) or hypnopompic hallucinations.
If you do not meet all three criteria, you do not have HPPD. Typically the symptoms must persist even after several months of abstinence from hallucinogen use for a physician to definitively diagnose it as a persisting disorder. It is also worth noting that many general practitioners are unlikely to have encountered HPPD and may not even know what it is. It is important to be honest about your history of drug use; few physicians will be able to identify HPPD unless you indicate to them that the symptoms developed following use of a hallucinogen. Without this piece of information they may believe the symptoms to be related to one of the arguably more serious conditions listed under criterion C above.
There is currently no known effective treatments to manage or cure HPPD. Many sufferers find the negative symptoms weaken and may even disappear with abstinence from hallucinogen use. It is unclear in such cases whether the condition is truly cured or simply lying dormant (i.e. may return and even worsen if the sufferer later resumes the use of hallucinogens).
There is debate over the existence, severity, and likelihood of HPPD developing. A large amount of the existing information about it is anecdotal, yet it is almost always cited as a potential negative side-effect of hallucinogen use. HPPD certainly has been documented, but more research into the phenomenon is necessary.