Chapter 122

### Introduction

It is easy to stand a pain, but difficult to stand an itch.

Chang Ch’ao 1676

Pruritus (the Latin word for itch) is defined simply as the desire to scratch.

It is one of the most important dermatological symptoms and is usually a symptom of primary skin disease with a visible rash. However, it is a subjective symptom and diagnostic difficulties arise when pruritus is the presenting symptom of a systemic disease with or without a rash—‘pruritus sine materia’. An associated rash may also be a manifestation of the underlying disease.

The broad differential diagnoses of pruritus are:

• skin disease

• systemic disease

• psychological and emotional disorders

### Physiology1

Itch arises from the same nerve pathway as pain, but pain and itch are distinct sensations. The difference is in the intensity of the stimulus. Unrelieved chronic itch, like unrelieved pain, can be intolerable and cause suicide. There are many similarities: both are abolished by analgesia and anaesthesia; subdued by counter-irritation, cold, heat and vibration; and referred itch occurs just like referred pain. Antihistamines that act on the H1 receptor are often ineffective, suggesting that histamine is not the only mediator of itch.1

### Localised pruritus

Pruritus may be either localised or generalised. Localised itching is generally caused by common skin conditions such as atopic dermatitis (see Table 122.1). Scratch marks are generally presented. Pruritus is a feature of dry skin. An intense, localised itch is suggestive of scabies, also known as ‘the itch’.

Table 122.1

Primary skin disorders causing significant pruritus

Itching of the scalp, anal or vulval areas is a common presentation in general practice.

A careful examination is necessary to exclude primary skin disease; a detailed history and examination should be undertaken to determine if one of the various systemic diseases is responsible.

Notalgia paraesthetica is a common localised itch and/or paraesthesia (possibly also pain) in the interscapular area. It is considered to be due to pressure on spinal nerves from spinal dysfunction. It is usually relieved by physical therapy to the thoracic spine (see CHAPTER 24).2 Pruritic rashes in children commonly include atopic dermatitis, varicella, urticaria (hives), insect bites and scabies.

### Generalised pruritus3

Pruritus may be a manifestation of systemic disease. It can accompany pregnancy, especially towards the end of the third trimester (beware of cholestasis), and disappear after childbirth. These women are then prone to pruritus if they take the contraceptive pill.4

Systemic causes ...

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