What is Itch?

Itch is defined as an uncomfortable sensation on the skin that evokes a desire to scratch. The sensation of itch is a universal experience that everyone has encountered at some point in their lives. Whether it’s a mosquito bite, an allergic reaction, or a chronic skin condition, itching can range from mild annoyance to an insufferable nuisance. Here, we will delve into the precise features of itch.

While traditionally considered a mild form of pain, itch is now appreciated to be a distinct sensation. When an itch stimulus is activated in the skin, sensory nerves send an electrical impulse to the spinal cord. From the spinal cord, the signals are relayed to the brain where itch is ultimately perceived. These perceived uncomfortable feelings are manifested through  desires and/or uncontrollable urges to scratch to alleviate itch.

However, itch, in addition to triggering a conscious desire, can also activate an unconscious reflex to scratch. A reflex is an immediate motor response to a sensation. For example, pulling your arm away upon touching a hot stove is a reflex. Similarly, scratching can be evoked independently of one’s brain, whereby an itch elicits an unconscious reflex to scratch. This is why many people scratch unknowingly in their sleep.

Transient scratching is likely a protective response by which an allergen or a parasite might be removed from the skin surface. However, excessive scratching can actually exacerbate itch, leading to a vicious cycle of itching and scratching. This itch-scratch reflex indicates why it is essentially impossible for someone who itches to stop scratching.

In healthy individuals, itching can occur spontaneously because of day-to-day stimuli on the skin including stray hairs, occasional dry weather, or encounter with an irritant in the environment. However, itching can be more pathologic too in cases of being bitten by mosquitos, exposed to poison ivy, or developing a hive reaction to a medicine. Further, in more extreme situations, itching can become a disease state where there no external trigger can be blamed for someone chronically itching. Taken together, itch is a distinct sensation that can be elicited by a host of external stimuli that, in its chronic form, becomes highly pathologic.