Itch

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Itch, known as pruritus or automatic tickle


A sensation that causes the desire or reflex to scratch.


An unpleasant sensory experience, creating a a scratch reflex.


Globally, 4% of the population, have difficulty with itchiness.


Unmyelinated nerve fibers for itch and pain both originate in the skin.


Information for pain and itch fibers ate conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract.


Itch generates the  feeling of a foreign object underneath or upon the skin and also the urge to remove it. 


Hedonic aspects to scratching, is finding  noxious scratching to be highly pleasurable for some.


Motivational aspects of scratching include the possibility of frontal brain areas of reward and decision making, 

contributing  to the compulsive nature of itch and scratching.


The presence of a contagious itch are common, and even just a discussion on the topic can precipitate the desire to scratch.


The process is likely to be more than a localized phenomenon where one scratches: itching and scratching can be induced purely by visual stimuli.


A human mirror neuron system exists that allows imitation of  certain motor actions when we view others performing the same action: contagious yawning.


Itch can be reduced/inhibited by pain, 

heat, rubbing/scratching, noxious chemicals, and electric shock.


Causes:


Scabies


Swimmer’s itch


Athlete’s foot 


Body louse


Cutaneous larva migrans


Head lice


Herpes


Insect bites


Pubic lice


Shaving


Varicella


Environmental and allergic- specific chemicals, such as urushiol, derived from poison ivy or poison oak, or Balsam of Peru, found in many foods and fragrances.


Foreign objects on the skin- the most common cause of non-pathological itching.


Photodermatitis –chemicals in the skin reacting to sunlight, lead to the formation of irritant metabolites.


Urticaria 


Dandruff


Punctate palmoplantar keratoderma


Formication, a sensation that of small insects crawling on or under the skin.


Pruritus ani, irritation of skin at the exit of the rectum, causing the desire to scratch.


Skin conditions-psoriasis, eczema, seborrhoeic dermatitis, sunburn, athlete’s foot, and hidradenitis suppurativa.


Most causes of itching are inflammatory in nature.


Xerosis, or dry skin, is the most common cause of itching.


Diabetes mellitus


Hyperparathyroidism


Iron deficiency anemia


Jaundice and cholestasis 


Malignancy or internal cancer


Polycythemia


Psychiatric disease


Thyroid illness


Uremia


Drugs that activate histamine (H1) receptors or trigger histamine release: 


Chloroquine


Bile acid congeners.


Gestational pemphigoid


Intrahepatic cholestasis of pregnancy.


Pruritic urticarial papules and plaques of pregnancy 


Menopause, or changes in hormonal balances associated with aging.


It can originate in the peripheral  nervous system or in the central nervous system.


Itching without an identifiable cause is known as essential pruritus.


Itch originating in the skin can be induced by a variety of stimuli: mechanical, chemical, thermal, and electrical stimulation. 


Itch originating in the skin is pruritoceptive.


Afferent neurons responsible for histamine-induced itch are unmyelinated C-fibres.


Two major classes of human C-fiber nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. 


Mechano-insensitive receptors respond mostly to itch induced by histamine. 


Iitch receptors are found only on the epidermis and the epidermal/dermal transition layers.


 The maximal sensitivity to itch found at the basal cell layer of the epidermis. 


The sensation of Itch is never felt in muscle or joints.


Itch is abolished in skin areas treated with nociceptor excitotoxin capsaicin.


Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. 


Itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin.


Neuropathic itch can originate along the afferent pathway as a result of damage to the nervous system: central nervous system or peripheral nervous system.


Neuropathic itch: notalgia paresthetica, brachioradial pruritus, brain tumors, multiple sclerosis, peripheral neuropathy, and nerve irritation.


Neurogenic itch is itch induced centrally, and is mostly associated with increased accumulation of opioids.


It is  associated with some psychiatric disorders such as tactile hallucinations, delusions of parasitosis, or obsessive-compulsive disorders.


Inflammatory mediators, bradykinin, serotonin, and prostaglandins, are released during inflammatory conditions and can activate pruriceptors and cause acute sensitization of the nociceptors. 


Neuro growth factors (NGF) can cause structural changes in nociceptors.


NGF is known to up-regulate neuropeptides, especially substance P, and increased NGF is also found in atopic dermatitis, a hereditary and non-contagious skin disease with chronic inflammation.


Substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells.


Itch treatment:


OTC and prescription anti-itch drugs are available. 


Non-chemical efforts include cooling, warming, soft stimulation.


Topical antipruritics in the form of creams and sprays are 

 available.


 Oral anti-itch drugs:


Antihistamines


Corticosteroids, hydrocortisone topical cream; 


Counterirritants, such as mint oil, menthol, or camphor


Crotamiton 


Local anesthetics


Phototherapy


Scratching may relieve isolated itch, but

can intensify itching and even cause further damage to the skin:the itch-scratch-itch cycle.


Therapy for dry skin is to maintain adequate skin moisture and use of topical emollients.





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