PRURITUS
#Pruritus (Itching) is a bizarre sensation. Unlike most other senses, its sole purpose seems to be to compel a person to scratch the affected part of the skin.
We all know that itching, also known as pruritus, is #irritating, but it is unique as a sensation in that the affected part of the body stops itching and a pleasant sensation is felt once it is scratch.
It is similar to pain, since both are unpleasant and causes a behavioural response - in pain, we withdraw our affected part of the body away, whereas with itching, we scratch.
There is so much irritation that our #skin receives everyday, from our clothes, hair, sweat, dust, body lice and our pets.
Over 1,000 species of #bacteria live on our skin. Our skin can become flaky or dry or scarred or damaged. We can have as much as 500 million bacterial cells per square inch. And let’s not get started on fungi! Athlete’s foot or yeast infections plague our skins. It’s a wonder that we’re not constantly itching.
Itching can be a symptom of many, many conditions including an #allergic reaction, body lice and other #parasites, #herpes, #insect bites, swimmer’s itch, #chickenpox, #hives, #dandruff, #psoriasis, #eczema, #sunburn, athlete’s foot, etc. But strangely, it can also come about after shaving or whilst a scab is healing.
Dermal or Pruritoceptive Itch
There are different types of itching sensations, depending on where the sensation comes from. Most of the time it is an actual sensation coming from the skin, and this is known as a dermal or pruritoceptive itch - resulting from sensory organs in the skin. In fact, the sense of itch travels to the brain from the skin along the same type of nerve fibres for pain.
Unlike most senses, where the sensory input is just from one or two stimuli, the input for this sensation can be due to movement, chemical, thermal and electrical stimulation. With this kind of itch, the sensation usually ceases when scratching.
But why does it stop when the skin is scratched?
The Scratch Reflex
With pain, it is easy to see why there is a withdrawal #reflex. #Pain is caused by something harmful, for example, a very hot cup of tea. But the scratch reflex associated with itching has probably evolved as a result of lice and parasites on the skin, and any light skin sensations caused by these parasites would have caused the person to scratch at the area to remove the irritant.
Another theory is that itching evolved as a result of histamine production caused by allergens; the body scratches in a bid to remove the allergens.
The scratch reflex is still very enigmatic as it uses different parts of the brain and does not seem to be localised. Researchers are still to discover why the scratch reflex is stopped upon scratching - is it as a result of the sensory neurons being deactivated or inhibited, or is it to do with the reward centre in the brain that decides that it is pleasurable to scratch, or both? No one is really sure.
One thing is for sure, however, is that the neurones that provide the stimulus for the itching sensation is only located in the skin, specifically concentrated around the basal cell layer (the part of the epidermis closest to the dermis). As a result, itching is never felt inside the body - your liver can’t itch as it does not have the “machinery” to provide an itch #signals to the brain!
Although most of the time, itches are as a result of sensory input from the skin, itches can actually also arise for @neuropathic reasons.
#NEUROPATHIC ITCH
Neuropathy is the damage of #nerves, and the damaged nerve can fire, which tells the brain that an itch is being detected, even though there is no itch sensation. The problem with this is that it can cause an intense itch that does not go away when scratched, as a result of the damaged nerve.
For example, in brachioradial pruritus, nerves near the top part of the spine (cervical spine) that innervate or connect to the sensory nerves found in the upper arms are damaged (this is known as cervical radiculopathy).
This tricks the brain into thinking that the sensory nerves are firing and as a result, an itching sensation is felt. Brachioradial pruritus can be so severe that it causes the patients to scratch at their own skin so much that their skin bleeds. Anti-pruritic (anti-itch) creams or medication to relieve histamines or itching do not work because it is not the sensory organ that is receiving an itch stimulus, but it is the damage to the nerves that needs to be fixed. An ice pack, however, relieves itching for some patients, and this may be to do with inhibition of the nerves in that area.
This condition is also enigmatic and further research needs to be done as there seems to be no international #scientific consensus that brachioradial pruritus is purely cervicogenic (solely arising from the damage in the cervical spine). Some scientists believe that the condition results from exposure to the sun (photodermatosis), but critics challenge this hypothesis as the condition can occur in both summer and winter months. Similarly, proponents of the photodermatotic hypothesis challenge the cervicogenic hypothesis as brachioradial pruritus is not constant - it is episodic - and cervical spinal damage is not present in all patients.
#NEUROGENIC ITCH
An itch can also arise from the central nervous system. An example of a neurogenic itch is pruritus caused by cholestasis. #Cholestasis occurs when bile cannot flow from the liver into the intestine - for which there are many reasons, mainly hepatobiliary disorders. As a result, bile salts build up in tissues and in the blood, which can act on nerves in the brain. It is thought that these salts mimic the effects of opioids like morphine (and indeed, one of the side effects of morphine is itching) and receptors on nerve cells in the brain detect the salts, which then produces the sensation of itching.
Like neuropathic itch, #neurogenic itches are “unscratchable” and produces neither relief nor pleasure upon scratching, which can result in skin mutilation by the patient.
#PSYCHOGENIC ITCH
Another type of itch is a #psychogenic itch, where the sensation of itching comes as a result of a psychiatric disorder. An example of this is delusional parasitosis, where the patient becomes delusional and irrationally believes that they are infested with parasites (of course, there are no real parasites - only ones in their mind!). This can cause them to itch or develop a lesser form of itching known as formication (not to be confused with fornication!), which is the feeling that there are bugs under your skin.
Another psychogenic itch is an Obsessive Compulsive Disorder for scratching where neurotic patients cannot help but scratch themselves to no avail or relief, leading to skin bleeding.