Why do you suddenly feel itchy? Why does scratching feel good? We answer your itch-related questions

How does our brain register an itch? We find out from a National Skin Centre expert why medical conditions such as chronic kidney disease and liver disease can make you itchy, and why medications don't always work.

What purpose does an itch serve, other than to irritate the hell out of you? Well, it’s your body’s alarm system alerting you that, hey, you have something in your skin and you need to get it out, stat.

“The scratching that is triggered prevents external elements like insects, toxins or irritants from getting into the deeper part of the skin and the bloodstream,” said Associate Professor Tey Hong Liang, a senior consultant and consultant-in-charge at National Skin Centre’s Itch Clinic.

Although itch and pain are distinctly different sensations, they are closely related and actually share some neural pathways, he said. So how does your brain decide to make you scratch or wince in pain? And why does scratching a mosquito bite – or even better, making an “X” indentation with your fingernail in your skin – feel so good?

What about those non-specific, unreachable back itches that start with “higher, lower… no, go higher but to the left” – but ultimately, no one seems to be able to hit the spot? We scratch the itch to find out.

HOW DOES OUR BRAIN REGISTER AN ITCH?

“Itch is triggered when certain chemicals, such as histamines, interact with receptors on specialised nerve fibres in the skin known as pruriceptors,” explained Assoc Prof Tey. “The signals from pruriceptors then travel to the spinal cord, where they interact with neurons that can transmit both pain and itch signals to the brain.”

Interestingly, the pain-processing areas in your brain – the somatosensory cortex, anterior cingulate cortex and insular cortex – are triggered when itch signals are received, said Assoc Prof Tey.

“However, different patterns of activation help distinguish itch from pain,” he explained. “In short, itch and pain are processed as separate sensations due to the differences in nerve fibre activation, chemical signalling and brain processing pathways.

“This ensures that itch is experienced as a distinct sensation, prompting scratching to remove irritants on the skin surface rather than pain, which prompts the withdrawal of the body part as the deeper part of the skin is perceived to have been damaged.”

WHY DOES SCRATCHING FEEL SO GOOD, INCLUDING MAKING AN “X” INDENTATION IN OUR SKIN?

Scratching (yes, along with making the “X” mark) creates a mild pain stimulus to temporarily override the itch signal. This provides momentary relief from the itch, said Assoc Prof Tey.

You’ll also feel good because scratching activates certain areas of the brain, such as the striatum, that are associated with reward and pleasure, he said. “This stimulation leads to the release of dopamine, a neurotransmitter linked to pleasure and satisfaction, making the act of scratching feel gratifying.”

But too much of a good thing always end badly. Scratching actually damages the skin, which causes your body to release more histamines and inflammatory chemicals. “These chemicals activate more pruriceptors and intensify the itch,” said Assoc Prof Tey.

Repeated scratching also sensitises pruriceptors and “as a result, even a light touch or stimulus can trigger an intense itch sensation”, he said, trapping you in an itch-scratch cycle.

“Once you scratch, the itch may briefly subside but the inflammation and sensitisation increase, leading to a return of the itch, often with greater intensity,” said Assoc Prof Tey.

WHY ARE SOME ITCHES SO UNSPECIFIC, WE CAN’T SEEM TO PINPOINT THEIR LOCATIONS?

C-fibres, which are nerve fibres or pruriceptors for detecting itch, are mostly involved when you feel itchy. They have broad receptive fields, said Assoc Prof Tey, which “can be triggered by stimuli over a relatively large area of the skin”.

“When you experience an itch, it may be difficult to pinpoint the exact location because multiple nerve fibres might be activated, creating a somewhat diffuse sensation.”

The areas in the brain that process itch signals are also “less precise in mapping out the exact location, making it hard to target the exact spot”, he said. Together, these mechanisms explain why you often can’t pinpoint the itchy location.

WHY DO WE SOMETIMES FEEL ITCHY WHEN WE’RE JUST READING OR SCROLLING THROUGH SOCIAL MEDIA?

It could be your brain becoming more attuned to subtle sensations when you’re at rest, said Assoc Prof Tey.

“When you're not engaged in an activity, your brain has more bandwidth to notice sensations that you might otherwise ignore,” he said. “This allows itch signals, which might normally be suppressed or overshadowed by other sensations, to become more prominent.”

You’ll also notice feeling itchy when you’re anxious, stressed, or simply bored or resting. Not being actively engaged in something can heighten your awareness of bodily sensations, making you more likely to notice itches, explained Assoc Prof Tey.

CAN AN ITCH FEEL DIFFERENTLY IN DIFFERENT PARTS OF THE BODY?

Yes, it can. “This variation is due to differences in skin structure, nerve distribution, sensitivity, and the presence of specialised receptors,” said Assoc Prof Tey. Here are some examples:

Face: The face has a high density of sensory receptors and a delicate, thin layer of skin, making it highly sensitive. Facial itches are often experienced as a light, tickling sensation.

Scalp: The scalp is densely packed with hair follicles, sebaceous glands and nerve fibres, so scalp itches are often more intense and feel deep.

Back: The back has a larger surface area but fewer nerve endings than other body parts. So, an itch here often feels diffused, which can lead to the "roaming" itchy-back sensation that seems to feel like it's always just out of reach.

Genital area: The skin here is delicate and has a high density of sensory nerve fibres, making it extremely sensitive. Itching in this area can feel intense, uncomfortable or even painful.

Arms and hands: The arms have thick skin and varying densities of nerve fibres; they are also more exposed to environmental irritants. The palms have thick, hairless skin with many sensory nerve endings but fewer itch-specific nerve fibres. An itch on the palm often feels more ticklish or tingly.

Legs and feet: The shins are typically drier and tend to itch more than the other parts of the body. These itches tend to feel broad and generalised.

The soles of the feet may have a thick upper layer of skin with few itch-sensitive nerve fibres, but they are also prone to sweating and friction. As a result, itchy feet, especially the soles, often feel intense and deep, making it more challenging to relieve.

HOW DOES THE ITCH FROM A MOSQUITO BITE DIFFER FROM ECZEMA OR A MEDICAL CONDITION SUCH AS CHRONIC KIDNEY DISEASE?

The itch from a mosquito is concentrated in the bite site. “The itch may last a few hours to a couple of days, depending on individual sensitivity,” said Assoc Prof Tey. “Scratching provides temporary relief but can quickly make the itch more intense and cause swelling.”

Meanwhile, the itch from eczema or dry skin can be widespread and persistent, often intensifying at night, he said. “It tends to come and go, with periods of flare-ups and remission. It ranges from mild to severe, with eczema often feeling deeper and more intense.”

If you have eczema or dry skin, you’ll know that scratching worsens the skin inflammation and contributes to the "itch-scratch cycle".

Medical conditions such as chronic kidney disease (CKD), liver disease and shingles can also make you scratch your skin raw.

In CKD, the widespread and diffused itch is due to dry skin, which is caused by a buildup of toxins in the blood as a result of reduced kidney function. “It affects large areas, such as the back, chest or arms, and may be worse at night,” said Assoc Prof Tey. “The itch ranges from mild to severe, and is often described as a crawling or prickling sensation.”

The itch from liver disease is often generalised but can be more intense on the palms, soles or back. The sensation is caused by an accumulation of bile salts or toxins in the skin due to impaired bile flow activating the nerve fibres. 

“It can be relentless and worsen at night or in response to heat,” said Assoc Prof Tey. “The itching is felt deeply, often with a burning sensation.”

Shingle sufferers experience itching in a very specific area along a nerve pathway on one side of the body. “It often precedes or accompanies a painful rash, and can feel like a combination of itching and burning,” said Assoc Prof Tey. “The itch lasts for weeks, coinciding with the rash phase but may persist.

HOW DO MEDICATIONS WORK ON ITCHES? WHY DON’T THEY ALWAYS WORK?

There are a few ways they work: By targeting different aspects of the itch pathway, such as reducing inflammation, blocking histamine release, or interfering with nerve signalling, explained Assoc Prof Tey.

“However, their effectiveness varies, depending on the underlying cause of the itch and individual patient factors.” Here’s a look at why they might not always be effective, he said:

• Oral and topical corticosteroids

The topical version reduces inflammation by inhibiting the body’s production of inflammatory agents such as prostaglandins and cytokines. Oral corticosteroids (such as prednisolone) suppress the immune response and reduce widespread inflammation, which can help with severe, generalised itching.

Limitations: The topical version’s effect is delayed. Plus, its long-term use on non-inflamed skin can lead to skin thinning. The prolonged or repeated use of oral corticosteroids can cause significant side effects, such as weight gain, osteoporosis and diabetes.

• Oral and topical antihistamines

Topical antihistamines (such as diphenhydramine creams) block histamine receptors on the skin, and prevent histamine from binding and triggering itch signals. The oral versions (such as cetirizine and fexofenadine) block histamine receptors systemically to prevent histamine-induced itch from spreading.

Limitations: Topical antihistamines have limited penetration of the skin and can sometimes cause skin allergy. The oral version may work on urticarial and allergic-type itch but it isn’t sufficient for more advanced stages of inflammatory diseases such as eczema.

• Topical anaesthetics

Products containing pramoxine or lidocaine numb the skin by blocking nerve signals that transmit itch sensations to the brain.

Limitations: You get temporary relief but it may not penetrate the skin enough to address the itch.

• Moisturisers

They hydrate the skin to repair the skin barrier and reduce the recurrence of itch.

Limitations: They don’t relieve itch immediately and are less effective for itches unrelated to dryness or skin barrier dysfunction. 

Incidentally, NSC has formulated non-steroidal, itch-relieving moisturisers that contain menthol and ceramide to reduce itch within minutes. They can also repair skin surface to reduce the recurrence of itch in the longer term. The range covers various specific body parts such as the body, scalp and face as well as formulations for different age groups. 

However, for more severe itch, topical therapy is not sufficient and oral medications are required.

• Immune modulators or immunosuppressants

They inhibit the inflammatory pathways and can effectively inhibit itch.

Limitations: Dupilumab, an immune modulator, is effective and safe but it requires injection and is very expensive. The immunosuppressant, JAK inhibitor, are new and their long-term safety is still uncertain.