I can be big or small. Only you can see me. If you have me you are not crazy. What am I?

The answer to the riddle describes Alice in Wonderland (Review) Syndrome (AIWS), a rare neurological condition that profoundly distorts an individual's perception of themselves and the world around them. The clue "I can be big or small" directly refers to the primary symptoms of AIWS, where objects, or even parts of one's own body, may appear significantly larger (macropsia) or smaller (micropsia) than they actually are. This unsettling experience can also make things seem closer or farther away. The phrase "Only you can see me" highlights the subjective nature of these distortions; they are not visual impairments but rather a disruption in how the brain processes sensory input, making the altered reality unique to the person experiencing it.

Despite the bizarre nature of these perceptions, the statement "If you have me you are not crazy" is crucial. Alice in Wonderland Syndrome is a recognized neurological disorder, not a sign of psychosis or mental instability. It was named in 1955 by British psychiatrist John Todd, who observed patients experiencing symptoms similar to Alice's shrinking (Review) and growing adventures in Lewis Carroll's famous stories. In fact, some speculate that Carroll himself may have experienced such perceptual distortions, possibly due to migraines, which often feature as an underlying cause of AIWS.

Episodes of Alice in Wonderland Syndrome are typically temporary, lasting from a few minutes to an hour, and can be quite unsettling for those who experience them. While the exact cause is not fully understood, AIWS is frequently associated with migraines, particularly in adults, and with viral infections like the Epstein-Barr virus, especially in children. It can also be linked to other conditions such as epilepsy, head trauma, and certain medications. Treatment usually focuses on managing the underlying cause to alleviate the perceptual disturbances.