Itch can be defined as an unpleasant cutaneous sensation associated with an urge to scratch. It is more or less voluntary, yet can very often be a subconscious motor activity [1].

The sensation of itch may have evolved in order to protect the skin from agents (e.g. parasites, plant toxins) that would potentially breach its barrier to harm the organism. In this way, the itch response has similarities to the pain response, and indeed itch (pruriceptive) and pain (nociceptive) pathways seem to have evolved in tandem.

However, the two have differences. Whereas the sensation of pain causes reflex withdrawal away from the source of pain in an attempt to avoid the agent, itching causes an opposite response, that being to scratch, perhaps in an attempt to rid the skin of an agent that has already breached the skin’s defences.

Pruriceptive and nociceptive pathways have evolved in order to provide us with potential survival benefits. As with other body systems that have evolved over generations, the pruriceptive system can become maladaptive in a number of individuals. This is usually in response to one or more causative agents, manifesting as miserable, intractable itching [1].

Itch is generated by specialised, itch-dedicated, cutaneous unmyelinated C fibres that have dense sensory nerve endings. These are distinct from the polymodal nociceptor mechanoreceptors involved in pain signalling as they do not respond to heat, mechanical or chemical stimuli. They show a sustained response to histamine which is a potent pruritogen and express the cell membrane receptor transient receptor potential cation channel subfamily V member 1 (TrpV1), also known as the capsaicin receptor [2].

Pruriceptive fibres ascend the spinothalamic tract into the thalamus which in turn has projections into the cerebral cortex. Spinal and higher cortical projections interact with ascending fibres, and it is hypothesised that these projections attenuate the itch signal, much like the gating mechanism that exists for pain perception. Ikoma et al. have written an excellent and comprehensive review on the neurobiology of itch [3].

On a behavioural level, patients undergo a vicious itch-scratch-itch cycle. A pruritogen causes the skin to itch. The itch drives a scratch reflex. Scratching traumatises the skin and induces pain. Pain in turn suppresses itch for a length of time but the skin damage stimulates the release of pruritogenic inflammatory mediators leading to further itching and scratching and so on ad infinitum. The desire to scratch can be denied as there is a degree of higher cortical control over motor function; however, a stronger and stronger desire to satisfy the urge to scratch develops until the sufferer must relent. What follows is a vigorous and traumatising episode of scratching that produces a feeling that can be described as a combination of guilt, pain and exquisite pleasure. These symptoms are not confined to daylight hours. Patients often wake from sleep to find their fingernails bleeding, having traumatised their skin by scratching vigorously during sleep.

Pruritus may be a reflection of an underlying systemic disease, a primary dermatological illness, a psychiatric or behavioural problem or a condition affecting the anorectum [4]. These causes and their various management options are discussed in detail elsewhere in this book but are summarised in Box 21.1. If investigation reveals no specific condition to be causing their symptoms, the patients are diagnosed as having idiopathic pruritus ani, a notoriously difficult and depressing condition to contend with, both as a patient and as that patient’s physician. This chapter will deal specifically with idiopathic pruritus ani (IPA).

The true incidence of IPA is difficult to establish but in general is considered to be common albeit with a wide spectrum of severity. Men are afflicted more than women in a ratio of 4:1 [5].

A number of theories have been postulated in an attempt to describe an aetiological cause for pruritic symptoms.

Specific dietary factors have been reported as important as a causative factor that once removed sees resolution of symptoms [6]. There is, however, little robust evidence for this and reports are largely anecdotal. Caffeine in particular has been reported as an irritant as well as being reported to cause transient weakness in the anal sphincter after its ingestion [7, 8]. In this, there may be an explanation for pruritic symptoms in the setting of subclinical incontinence (see next section).

Poor perianal hygiene has been implicated as a cause of pruritus ani [7]. In an elegant experiment, Caplan applied autologous faeces to the perianal and underarm skin of a group of patients with (n = 12) and a group of patients without (n = 15) pruritus ani. A further group (n = 10) had topical faecal application simulated to act as control [9].

Twelve of 27 of these subjects complained of perianal itching with an onset between 1 and 6 h of faecal application to the perianal skin. Four of these subjects had a history of pruritus ani, eight had no prior history. None of the control group suffered symptoms. Pruritus was instantly relieved with cleansing. A single subject developed pruritus on application to the arm. The conclusion of the study was that faeces acted as an irritant rather than an allergen.

Farouk studied rectal and internal anal sphincter pressures in a group of pruritus ani patients [10]. Those with pruritus had higher rectal pressures with lower internal anal sphincter pressures and prolonged internal sphincter relaxation than the control group. Pruritus was reported within an hour of the abnormal internal sphincter relaxation. The authors’ conclusion was that occult faecal leakage was a cause of pruritus secondary to abnormal internal sphincter relaxation.

Given the above findings, chronic leakage of irritant faeces causing itch with subsequent mechanical skin trauma makes an interesting hypothesis. If indeed this is the case, symptoms may be amenable to treatment with a stool thickener such as loperamide. No trial data have been presented to support the hypothesis although anecdotally this approach can be successful.

The perianal region is subject to the same skin commensals as the remainder of the body. Due to its anatomy within the warm, moist gluteal folds and at the outlet from the gastrointestinal tract, additional flora may exist and thrive. Bacterial, viral, fungal and parasitic organisms all have been implicated as an aetiology; therefore, thorough investigation with swab, scraping and Wood’s light examination is essential. Sexually transmitted infection is common and therefore appropriate questioning on history taking is essential.

Dasan reports an interesting series of consecutive patients presenting to a combined dermatological and coloproctological clinic [11]. Out of 40 patients, 2 were identified as suffering from an anorectal condition that required surgical intervention. Thirty-four out of 40 patients were suffering from an underlying dermatosis, treatment of which improved or resolved their symptoms. Patch testing was undertaken in 32 out of 40. Eighteen of these patients showed hypersensitivity to allergens which are commonly found in remedies for pruritus ani. A patient was found to be sensitive to an ingredient of his wife’s shampoo. Symptoms resolved on cessation of her practice of washing her hair in their shared bath water.

Patch testing as a useful instrument in the investigation of chronic pruritus ani is supported by Harrington who tested 80 patients with PA [12]. Fifty-five of these patients were patch test ‘positive’, 38 of them for a medication commonly used as a remedy for pruritus. As well as topical preparations, the advice to use ‘wet wipes’ is commonly given to patients in the clinic. Ingredients of wet wipes are occasionally allergenic on patch testing [13] and should be avoided.

Daniel et al. report in their series of 109 patients with pruritus ani that 75 % of these had an underlying coloproctological disorder: 20 % had haemorrhoids and 12 % had anal fissures; however, 19 % had an underlying coloproctological malignancy (11 % rectal cancer, 6 % anal cancer, 2 % colonic cancer) [7]. It is interesting to note this group’s definition of chronic pruritus ani as being a condition with symptoms lasting over 6 weeks. Mentes’ group had a median symptom length of 24 months [14]. Underlying anorectal conditions, whilst clearly important to exclude and treat if appropriate, do not seem to be as prevalent in other groups investigating idiopathic pruritus ani with a longer time course [11, 14–16].