Antimicrobials · Macrolides
The facts most likely to be tested
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Note this is off-label and limited to short courses (1-4 weeks) because of tachyphylaxis.
The most significant adverse effect of erythromycin in neonates is an increased risk of pyloric stenosis.
Erythromycin is a potent CYP450 inhibitor, which frequently leads to drug-drug interactions with medications like theophylline or warfarin.
Systemic erythromycin use is associated with QT interval prolongation, increasing the risk of torsades de pointes.
Erythromycin is the preferred treatment for chlamydial conjunctivitis in newborns, though it carries a risk of hypertrophic pyloric stenosis.
The primary mechanism of action for erythromycin is the inhibition of bacterial protein synthesis by binding to the 50S ribosomal subunit.
Erythromycin is a common cause of cholestatic hepatitis, which typically presents with jaundice and elevated alkaline phosphatase.
Vignette unlocked
A 3-week-old male infant is brought to the clinic by his mother due to persistent non-bilious, projectile vomiting after feedings. The infant was treated with oral erythromycin two weeks ago for suspected chlamydial conjunctivitis. On physical examination, the infant appears hungry but dehydrated, and a palpable olive-shaped mass is noted in the epigastrium. Laboratory studies reveal a hypochloremic, hypokalemic metabolic alkalosis.
What is the most likely diagnosis and the underlying cause related to the infant's medication history?
Infantile hypertrophic pyloric stenosis secondary to erythromycin exposure.
This vignette tests the high-yield association between systemic macrolide use in neonates and the development of pyloric stenosis, a classic board examination topic.
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High yield triage
Classification
Macrolide antibiotic; bacteriostatic protein synthesis inhibitor.
Indications
Neonatal chlamydial conjunctivitis/pneumonia, Pertussis (alternative), Gastroparesis (short-term prokinetic, off-label).
Mechanism of Action
Binds 50S ribosomal subunit to inhibit protein synthesis.
Side Effects
GI upset (motilin agonist), QT prolongation, ototoxicity.
Contraindications / Monitoring
QT-prolonging CYP3A4 substrates (pimozide, ergotamine, lovastatin, simvastatin); monitor QTc interval and LFTs.
Full handout
Mechanism of Action
Erythromycin binds reversibly to the 50S ribosomal subunit, blocking the translocation of peptidyl-tRNA. This action inhibits bacterial protein synthesis, effectively halting bacterial growth. It also acts as a potent agonist of motilin receptors in the GI tract, stimulating smooth muscle contraction.
Unique Properties
Unlike other macrolides, erythromycin is a potent prokinetic agent used to treat diabetic gastroparesis. It is significantly more prone to causing GI distress due to its direct stimulation of motilin receptors.
Indications
The CDC-recommended agent for neonatal chlamydial conjunctivitis and infant chlamydial pneumonia (erythromycin base/ethylsuccinate 50 mg/kg/day PO divided QID x14 days). It is an alternative for Bordetella pertussis. Erythromycin is NO LONGER recommended for chlamydia in pregnancy (CDC 2021) - azithromycin 1 g is recommended, amoxicillin is the alternative. It is also utilized for erythrasma and as an alternative for streptococcal pharyngitis in penicillin-allergic patients.
Pharmacokinetics
Metabolized extensively by the hepatic CYP3A4 enzyme system, leading to numerous drug-drug interactions. It is primarily excreted in the bile, requiring caution in patients with hepatic impairment.
Side Effects & Adverse Events
Common effects include abdominal cramping, nausea, and diarrhea. QT prolongation is a major concern, potentially leading to Torsades de Pointes. Ototoxicity may occur, particularly at high doses or in patients with renal failure.
Contraindications
Absolute contraindication with concurrent use of CYP3A4 substrates that prolong the QT interval (e.g., cisapride, pimozide). Avoid in patients with a history of cholestatic jaundice associated with prior macrolide use.
Monitoring
Perform baseline and periodic ECG to monitor the QTc interval in high-risk patients. Monitor liver function tests if therapy is prolonged, as it can cause cholestatic hepatitis.
Clinical Pearls
Always remember the motilin agonist effect for board questions regarding drug-induced diarrhea or gastroparesis. If a question describes a pregnant patient with Chlamydia, azithromycin (single 1 g dose) is first-line, with amoxicillin as the alternative; doxycycline is contraindicated in the second and third trimesters, and erythromycin is no longer recommended in pregnancy at all (CDC 2021). Its remaining chlamydia niche is the neonate.