Questions & explanations
1. Compare atopy patch testing with a blood test for food-specific IgG antibodies.
Atopy patch testing and IgG blood tests both look for delayed reactions, but they measure different things. Patch testing involves placing food on the skin and observing a local immune response, while IgG blood tests measure antibodies in the blood that may indicate exposure, not necessarily allergy. IgG tests are controversial because many healthy people have IgG to foods they eat regularly, so results can be misleading. Patch testing is more specific for delayed hypersensitivity mediated by T-cells, whereas IgG tests are not recommended by allergy guidelines. Doctors prefer patch testing only in specific situations, while IgG tests are often considered unproven.
2. What is SJS/TEN and how are they related?
Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are severe, life-threatening skin reactions usually caused by drugs. They are considered the same disease spectrum, differing only by the extent of skin detachment. SJS involves less than 10% of body surface area detachment, TEN involves more than 30%, and SJS/TEN overlap is 10-30%. The reaction typically starts with fever and flu-like symptoms, followed by a painful rash that blisters and peels. Mucous membranes (mouth, eyes, genitals) are often involved. Common culprit drugs include allopurinol, anticonvulsants, sulfonamides, and NSAIDs. Treatment requires stopping the drug and intensive care.
3. What is AGEP and what does it stand for?
AGEP stands for Acute Generalized Exanthematous Pustulosis. It is a rare skin reaction, usually caused by drugs, characterized by sudden onset of numerous small, sterile pustules (pus-filled bumps) on red skin. It often starts on the face and flexural areas, then spreads. Fever and elevated white blood cell count (neutrophils) are common. The reaction typically occurs within 24-48 hours after starting the drug, which is faster than other drug reactions. Common culprit drugs include antibiotics (especially aminopenicillins and macrolides), calcium channel blockers, and antimalarials. AGEP usually resolves quickly after stopping the drug, with skin peeling.
4. What is DRESS syndrome and what does the acronym stand for?
DRESS stands for Drug Reaction with Eosinophilia and Systemic Symptoms. It is a severe, life-threatening drug reaction that involves skin rash, fever, swollen lymph nodes, and internal organ involvement, especially the liver, kidneys, and lungs. It often occurs 2 to 8 weeks after starting a drug. Common culprit drugs include anticonvulsants (like carbamazepine, phenytoin), allopurinol, and sulfonamides. The reaction is characterized by high eosinophil count (a type of white blood cell) and other immune abnormalities. Diagnosis is based on clinical criteria and exclusion of other causes. Treatment involves stopping the drug and supportive care.
5. How does AGEP differ from pustular psoriasis?
AGEP is drug-induced and has a sudden onset, usually within 48 hours of drug intake. Pustular psoriasis (generalized pustular psoriasis) is a chronic inflammatory condition that can be triggered by drugs but often has a history of psoriasis. In AGEP, pustules are small, superficial, and resolve quickly after drug withdrawal. Histology in AGEP shows subcorneal pustules with spongiosis, while pustular psoriasis shows intraepidermal pustules with acanthosis. Fever and neutrophilia are common in both, but AGEP lacks the systemic inflammation of psoriasis. The key distinguishing feature is the temporal relationship with a drug and rapid resolution.
6. A patient on carbamazepine for 4 weeks develops fever, rash, and elevated liver enzymes. Eosinophil count is 2,000/µL. How would you apply the RegiSCAR criteria?
This patient likely meets RegiSCAR criteria for DRESS. The features include fever (≥38.5°C), rash (>50% body surface), eosinophilia (≥700/µL), and liver involvement (elevated enzymes). The latency of 4 weeks is typical. The patient should be hospitalized if not already. Other causes like infection should be ruled out. According to RegiSCAR, the presence of at least three of the major criteria (fever, rash, eosinophilia, lymphadenopathy, organ involvement) plus drug causality suggests definite DRESS. The score would be high. Immediate discontinuation of carbamazepine is essential. Treatment includes corticosteroids and supportive care.
7. What are cross-reactivity patterns in fixed drug eruption?
Cross-reactivity means that a drug similar in structure to the culprit drug can also cause the same reaction. In fixed drug eruption, cross-reactivity is common among drugs within the same class. For example, if a patient reacts to one NSAID (like ibuprofen), they may also react to other NSAIDs (like naproxen). Similarly, cross-reactivity among tetracyclines (doxycycline, minocycline) and among sulfonamides (sulfamethoxazole, sulfadiazine) is seen. However, cross-reactivity is not always predictable. Therefore, patients should avoid all drugs in the same class unless proven safe. Patch testing may help identify safe alternatives.
8. Compare the treatment of ACE inhibitor-induced angioedema with that of hereditary angioedema.
Both conditions are bradykinin-mediated, so treatment is similar. For acute attacks, icatibant (bradykinin blocker) or ecallantide (kallikrein inhibitor) are used. C1 inhibitor concentrate can also help. In ACE inhibitor-induced angioedema, the most important step is stopping the ACE inhibitor. In hereditary angioedema, the drug is not the cause, so patients need long-term prevention. For ACE inhibitor angioedema, antihistamines and steroids do not work. Both conditions require airway monitoring. If the throat swells, intubation or tracheostomy may be needed. After recovery, ACE inhibitor patients should avoid all ACE inhibitors.
9. What is a fixed drug eruption and how does it present?
A fixed drug eruption (FDE) is a type of drug reaction that appears as one or more round, red patches on the skin or mucous membranes. It is called 'fixed' because the same spots reappear in the same locations each time the drug is taken. The lesions often become hyperpigmented (dark) after healing. Common sites include the lips, genitals, hands, and feet. The reaction usually occurs within hours to a day after drug intake. Symptoms include itching or burning. Common culprit drugs are NSAIDs, antibiotics (tetracyclines, sulfonamides), and paracetamol (acetaminophen). Diagnosis is based on history and recurrence with rechallenge.
10. How is celiac disease different from FPIES?
Celiac disease is an autoimmune reaction to gluten, a protein in wheat, barley, and rye, while FPIES is a non-IgE-mediated food allergy to various foods. In celiac disease, the immune system attacks the small intestine, causing damage over time, leading to symptoms like diarrhea, weight loss, and fatigue. FPIES causes acute vomiting and diarrhea hours after eating, but does not cause long-term intestinal damage. Celiac disease requires lifelong gluten avoidance, while FPIES is often outgrown. Diagnosis of celiac disease involves blood tests for specific antibodies and intestinal biopsy, whereas FPIES is diagnosed by history.
11. How does FPIES differ from allergic proctocolitis?
FPIES and allergic proctocolitis are both non-IgE-mediated food allergies, but they affect different parts of the gut and have different symptoms. FPIES involves the small intestine and colon, causing severe vomiting and diarrhea hours after eating. Allergic proctocolitis affects the rectum and colon, causing bloody stools in infants, usually within days of ingesting the trigger. FPIES can cause dehydration and shock, while allergic proctocolitis is milder and babies grow well. FPIES triggers include milk, soy, and grains; allergic proctocolitis is usually triggered by cow's milk protein in breastfed or formula-fed infants.
12. What are the differences between fixed drug eruption and other drug rashes like urticaria?
Fixed drug eruption (FDE) appears as well-defined, round patches that recur in the same spots, often leaving dark marks. Urticaria (hives) is raised, itchy welts that come and go in different places and do not leave marks. FDE is usually not itchy or mildly itchy, while urticaria is very itchy. FDE is caused by drugs like NSAIDs and antibiotics, while urticaria can be caused by drugs, food, or infections. FDE lesions last for days and heal with hyperpigmentation; urticaria lesions last less than 24 hours. Diagnosis of FDE is confirmed by recurrence with drug challenge, while urticaria is diagnosed by history and skin tests.