Antihistamines

Question 1

What is the MOA for antihistamines?

Answer 1

Block action of histamine at receptor sites

Question 2

Histamine is released from ______ and causes _______

Answer 2

Mast cells

Vasodilation, Increased capillary permeability, and increased GI and pulmonary secretions

Question 3

What are the possible routes of administration for antihistamines?

Answer 3

Oral
Nasal spray
Eye drops
Topical
IM
IV

Question 4

Clinical indications for antihistamines

Answer 4

Allergic conditions (rhinitis, dermatitis, conjunctivitis)

Hypersensitivity reactions to drugs

Anaphylaxis

Urticaria (acute or chronic)

Motion sickness

Vertigo

Insomnia

Question 5

1st gen antihistamines block histamine at the ________

Answer 5

H1 receptor

Question 6

Why do 1st gen antihistamines have more CNS side effects?

Answer 6

Because they readily cross the BBB

Question 7

Name the 1st gen oral antihistamines

Answer 7

Diphenhydramine (Benadryl)

Chlorpheniramine (Chlor-trimeton)

Dimenhydrinate (Dramamine)

Promethazine (Phenergan)

Meclizine (Antivert)

Hydroxyzine (Atarax, Vistaril)

Question 8

Which H1 blocker is best for vertigo?

Answer 8

Meclizine (Antivert)

Question 9

Which H1 blocker causes the most extreme sedation?

Answer 9

Hydroxyzine (Atarax)

Question 10

2nd gen H1 blockers are known to be …

Answer 10

Less sedating than 1st gen (b/c don’t cross the BBB as much)

Question 11

What is the one 2nd gen H1 blocker that can be sedating

Answer 11

Cetirizine (Zyrtec)

Question 12

What are the three 2nd gen H1 blockers?

Answer 12

Loratadine (Claritin)

Cetirizine (Zyrtec)

Fexofenadine (Allegra)

Question 13

Name the two H2 blockers we discussed

Answer 13

Ranitidine (Zantac)

Famotidine (Pepcid)

Question 14

What are some examples of nasal spray antihistamines?

Answer 14

Azelastine (Astelin, Astepro)

Azelastine + Fluticasone (Dymista)

Question 15

What are some examples of antihistamine eye drops?

Answer 15

Azelastine (Optivar)

Ketotifen (Zatidor) AH + MCS

Naphazoline (Naphcon A)

Olopatadine (Patanol) AH + MCS

Question 16

Can you use 2nd gen H1 blockers for motion sickness or insomnia?

Answer 16

NOPE

You need the anticholinergic properties of 1st gens, that cross the BBB

Question 17

What are the side effects of 1st gen H1 blockers?

Answer 17

ANTICHOLINERGIC

Red as a Beet
Dry as a Bone
Blind as a Bat
Mad as a Hatter
Hot as a Hare

Question 18

Which antihistamines are approved by use by pilots after an adequate trial without SE

Answer 18

Loratadine (Claritin)
Fexofenadine (Allegra)

Others (including Zyrtec) are not allowed

Question 19

What does the FAA require for use of any non-approved antihistamine?

Answer 19

Pilot must ground themselves for at least 5x the drug’s half-life (for Benadryl, that would be 60 hours)

Question 20

Other occupations that should avoid first gen H1 blockers other than pilots

Answer 20

Heavy machinery operator
Construction workers or electricians working at heights
Mass transit (ie bus driver)
Healthcare providers working 24 hour shifts or night shifts
PA students who cannot fall asleep during class

Question 21

What would be your recommendations for an elderly patient with BPH who’s hay fever is not responsive to his current regimen of Benadryl and Sudafed?

Answer 21

Stop both drugs - you want to avoid decongestants and 1st gen h1 blockers as they may trigger urinary retention

Use nasal steroid first line instead

Question 22

What drugs are contraindicated in patients with glaucoma?

Answer 22

Decongestants, antihistamines, and steroids (including nasal steroids) - all may increase IOP

Question 23

How do you approach treating hay fever in a patient with glaucoma?

Answer 23

Consult patient’s ophthalmologist to determine plan

May require closer monitoring of IOP if requiring long term allergy tx

Consider immunotherapy (allergy shots)

Question 24

For patients with glaucoma or BPH, avoid using…

Answer 24

Phenergen for gastroenteritis (use zofran instead)

Antivert for labyrinthitis

H1 blockers for motion sickness or insomnia

Question 25

Scabies is infestation of the skin by…

Answer 25

The mite Sarcoptes scabiei

—> intense nighttime itching and burrow lesions

Question 26

DOC for scabies

Answer 26

Permethrin cream 5% from neck to soles of feet for 8-12 hours

Consider Atarax (hydroxyzine) or Benadryl for sedation

Question 27

Chronic urticaria is sometimes linked to…

Answer 27

Autoimmune causes

Question 28

Chronic urticaria will typically respond well to …

Answer 28

Non-sedating antihistamines (ie Claritin, Allegra)

Question 29

How do you treat poison ivy?

Answer 29

Steroid cream (ie Triamcinolone or Kenalog) - remember, no high potency over face

Non-sedating oral antihistamine (ie Zyrtec)

Might need to add oral steroid taper if not improving

Warn patient to watch for signs of infection

Question 30

Your dumbass patient with a PCN allergy took his buddy’s augmentin for some unknown reason. How do you treat the rash he developed?

Answer 30

Stop the augmentin

Give h1 blocker (Zyrtec ok)

Consider IM steroids and/or IM antihistamine - don’t let them drive themselves home after this

Consider tapering course of oral steroids based on severity

Question 31

What drug is not part of the first line treatment for urticaria but can be a useful adjunct if patient not responding to treatment?

Answer 31

H2 blockers (Ranitidine or Famotidine)

Question 32

How do you treat anaphylaxis?

Answer 32

EPINEPHRINE

Benadryl

IM/IV glucocorticoid (ie Dexamethasone)

H2 blocker (Famotidine, Ramitidine)

Question 33

Severe potentially life-threatening allergic reaction to food, meds, latex, insect stings etc

Answer 33

Anaphylaxis

Can affect >1 body part at the same time
• Skin and mucosa - 90%
• Respiratory - 70%
• GI - 45%
• Cardiac (hypotension, syncope, tachycardia) - 45%