Vết gãi do ngứa (Pruritus)
Mô tả: Scratch marks represent a sign related to an underlying symptom (pruritus) that is, in simple terms, the feeling of being itchy. The absence of scratch marks in hard-to- reach places (e.g. between the shoulder blades) when they are present on the rest of the body may be an indication of severity of itch.
Nguyên nhân: Pruritus is associated with numerous skin systemic diseases that cause pruritus include, but are not limited to, those given in Table 6.6. • 25–86% in uraemic patients with chronic renal failure93,99 • 20–25% in patients with jaundice; prevalent in 100% of primary biliary cirrhosis and a presenting symptom in 50%100 • 25–75% in polycythaemia vera97 • 4–11% in patients with thyrotoxicosis.101 Pruritus may precede the onset of disease by 5 years in Hodgkin’s lymphoma.102
Cơ chế: The skin has many unmyelinated C-fibres that synapse with itch-specific secondary neurons. It is the irritation of the unmyelinated C-fibres by chemical mediators or ‘pruritogens’ that causes the sensation of pruritus or itch.93 The main pruritogen is histamine. However, there are numerous others and more are being found each year. Potential mediators of pruritus are list in Table 6.7. These factors stimulate pruritus by: 1 directly acting on epidermal nerve endings (e.g. histamine) 2 liberating histamine from mast cells (e.g. neuropeptides) 3 potentiating histamine (e.g. PGE2, endogenous opioids). Chronic renal failure Many factors are thought to contribute to pruritus in chronic renal failure. The accumulation of pruritogenic factors due to the kidney’s inability to excrete them is thought to be the primary issue. Features seen in chronic renal failure that contribute to pruritus include:93,94 • xerosis (dry skin) • abnormal cutaneous mast cell proliferation • secondary hyperparathyroidism • increased pruritogenic cytokines • increased vitamin A levels • increased endogenous opioids • impaired sweating • peripheral neuropathy • increased levels of magnesium, stimulating release of histamine • increased levels of phosphate (cutaneous calcifications stimulating itch receptors). Hepatobiliary Like pruritus of chronic renal failure, the disorders is thought to be multi-factorial. Traditional teaching has been that increased bile salts accumulate in blood and tissues inducing pruritus. However, the latest research suggests that, although bile salts may directly or indirectly play a role in pruritus, the evidence for a key role of bile salts in the induction of pruritus in cholestasis is weak.95 Steroids, steroid metabolites, histamine, serotonin, GABA and cannabinoids are just a few of the pruritogens thought to play a role in the development of itch in cholestasis. One recent study96 found that lysophosphatidic acid may cause a rise in intracellular calcium that, in turn, activates itch-inducing nerve fibres in patients with cholestasis. Diseasecondition Histamine Substance P Dopamine Prostaglandins Serotonin VIP Cytokines Bradykinins Directly stimulate C-fibres Cause histamine release Potentiate histamine Pruritus Pruritic scratch marks Increased ‘pruritogen’
Ý nghĩa lâm sàng: Little research has been directed towards the value of pruritus as a symptom or sign. Given the wide variety of causes, its specificity is low. Pruritic scratch markspruritus 486 Its prevalence in some of the above
