Antihistamines

Antihistamines

Antihistamines

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Antihistamines

  • OTC medicines used to treat:
    • allergies
    • reactions to insect bites or stings
    • motion sickness
    • nausea related to migraine
    • short term treatment for sleeping difficulties
  • Divided into two main groups:
    • 1st generation (drowsy - cross blood-brain barrier) - chlorphenamine (Piriton), Diphenhydramine (Nytol), Promethazine (Phenargan), Buclizine (found in combination in Migraleve)
    • 2nd generation (non-drowsy - do not cross blood-brain barrier) - Loratidine (Clarityn), Cetirizine, Acrivastine (Benadryl)
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How do antihistamines work?

  • Histamine
    • Basic amine present in most tissues. Increased concentration in lungs, skin and particularly in GI tract
    • At a cellular level found mainly in mast cells and basophils
    • Four types of receptors (G-protein coupled) H1, H2, H3 and H4
  • Main action in humans:
    • Stimulation of gastric acid secretion (H1)
    • Contraction of smooth muscle, except blood vessels (H1)
    • Cardiac stimulation (H2)
    • Increased vascular permeability (H1)
    • Stimulation of arousal in the circadian
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How do antihistamines work?

  • Main pathophysiological role of histamines:
    • Stimulation of gastric acid secretion (treated with H2 receptor antagonists)
    • Mediatior of type 1 hypersensitivity reactions
  • Allergic reaction overview
    • Initial contact with allergen
    • Plasma cell releases antibodies
    • Antihistamines bind to receptors on mast cells
    • Subsequent contact with allergen
    • Histamine and other chemicals released - allergic reaction
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Seasonal Allergic Rhinitis

  • Immune system caused and mistakenly identifies normally harmless substance as an intruder
  • This substance is called an allergen - pollen
  • Immune system responds by releasing histamine and chemical modulators that cause the typical hayfever symptoms
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Hay fever symptoms

  • Rhinorrhoea
    • Runny nose with a discharge that is thin, clear and watery
  • Nasal congestion
    • Inflammatory response to allergen that causes vasodilation and congestion
  • Nasal itching
  • Eye symptoms
    • Eyes may be itchy and watery - result of tear duct congestion and pollen grain caught in the eye
  • Sneezing
  • Classic response is sneezing then rhinorrhoea then congestion
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Allergen Avoidance Advice

  • Wearing wraparound sunglasses to stop pollen getting in eyes when outdoors
  • Apply a small amount of petroleum gel to nasal openings to trap pollen grains
  • Avoid cutting grass, playing or walking in grassy areas
  • Taking a shower and changing clothes after being outdoors
  • Do not keep fresh flowers in the house
  • Keep car windows closed - pollen filter can be bought for air vents in cars
  • Staying in doors when pollen count is high 
  • Vacuum regular - high-efficency particle arresting (HEPA) filter
  • Damp dust
  • Keep windows closed - particularly morning and evening
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When to Refer

  • Wheezing
    • Difficulty with breathing, possibly with a cough suggests an asthma attack
  • Earache and facial pain
    • Allergic rhinitis can be complicated by a secondary bacterial infection
  • Purulent conjunctivitis
    • More painful and discharge changes from watery to coloured and sticky
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Hayfever Treatment

  • Antihistamines - Cetirizine, Loratadine, Chlorphenamine, Acrivastine
  • Steroid nasal sprays - Beconase
  • Sodium cromoglicate
  • Decongestants
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Motion Sickness Symptoms

  • Nausea and/or vomiting
  • Dizziness
  • Pallor
  • Cold sweats
  • Hypersalivation
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Motion Sickness Treatment and Referral

  • Treatment with hyoscine hydrobromide
    • minimum age 4 years, anti-muscarinic adverse effects, interacting medicines, short-acting (4 hours), take 30 minutes before travel - drug of choice
  • Treatment with antihistamines - contraindications and adverse effects same as hyoscine hydrobromide
    • Cinnarizine - minimum age 5 years, lasts up to 8 hours, take 2 hours before travel
    • Promethazine - minimum age 2 years, lasts longer than 8 hours, take night before travel
  • Referral
    • OTC failure - continuing symptoms
    • Age - under 2
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Insomnia

  • Difficulty getting to sleep or staying asleep long enough to feel refreshed in the morning
  • 1 in 3 people in the UK are affected - particularly common in the elderly
  • May be short or long term
  • Persistent insomnia can have a significant impact on quality of life potentially limiting productivity, affects mood and relationships with friends, family and colleagues 
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Insomnia Causes

  • Underlying medical conditions
  • Biological, e.g. aging, pregnancy
  • Behavioural, e.g. children
  • Poor sleep hygeine
  • Mental health, e.g. depression
  • Psychological, e.g. stress
  • Altered patterns, e.g. shifts
  • Environmental noise
  • Medicines
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Medications that Cause Insomnia

  • Selective Serotonin Reuptake Inhibitors (SSRIs)
  • Dopamine agonists
  • Amphetamines
  • Anticonvulsants
  • Cold medicines and decongestants
  • Steroids
  • Beta agonists
  • Theophylline
  • Diuretics
  • Caffeine
  • Alcohol
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Sleep Hygiene

  • Maintain a routine with regular bedtime and waking time 
  • Alcoholic and caffeine containing drinks should be avoided
  • Avoid sleeping in very warm rooms
  • Take daytime and not evening exercise
  • No daytime naps
  • No sleeping in to catch up
  • No strenuous mental activity at bed time
  • Associate bed with sleep, not TV or phones
  • If unable to sleep - get up and do something, return to bed when tired
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Treating Insomnia and When to Refer

  • Pharmacologicsl interventions
    • Sedating antihistamnes - promethazine, diphenhydramine
  • Referral
    • Under 12
    • Duration >3 weeks
    • No known cause
    • Previous undiagnosed medical conditions
    • Symptoms suggestive of anxiety, stress or depression
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