Mechanism of Action: H1 Antagonist (Betahistine)
The drug prescribed to dilate blood vessels in the ears for this patient with suspected Menière's disease is betahistine, which acts as an H1 agonist and H3 antagonist, though it is commonly referred to as a histaminergic agent with vasodilatory properties. 1
Clinical Context
This patient presents with the classic triad of Menière's disease: 2
- Tinnitus (constant ringing sensation)
- Progressive sensorineural hearing loss (left ear)
- Episodic vertigo (dizziness)
- Positive glycerol test (improvement in hearing after oral glycerol administration, suggesting endolymphatic hydrops)
Pharmacological Mechanism
Betahistine is the most commonly prescribed vasodilator for Menière's disease in Europe and works through histaminergic pathways. 1 The drug's mechanism involves:
- H1 receptor agonism: Stimulates histamine H1 receptors in blood vessels, leading to vasodilation of the inner ear vasculature 1
- H3 receptor antagonism: Blocks presynaptic H3 receptors, increasing histamine release and further enhancing blood flow to the cochlea 1
- The net effect is improved cochlear blood flow through anti-inflammatory effects and stabilization of vascular endothelium 2
Important Clinical Caveats
However, the evidence for betahistine's efficacy is controversial. 2 The 2020 AAO-HNS Menière's Disease guideline notes that while a 2016 Cochrane systematic review favored betahistine, the well-designed BEMED trial contradicted these findings, showing no significant benefit. 2 The guideline committee was unable to make a definitive statement on betahistine use due to conflicting evidence. 2
Despite limited evidence, betahistine may be offered as maintenance therapy to reduce symptoms or prevent Menière's disease attacks. 2 The medication has a favorable safety profile with rare serious side effects, though it should be used with caution in patients with asthma or peptic ulcer disease. 2
Guideline Recommendations on Vasodilators
The AAO-HNS guidelines explicitly recommend AGAINST routine use of vasodilators for sudden sensorineural hearing loss 2, stating that clinicians should not routinely prescribe vasodilators or vasoactive substances due to potential side effects and no documented efficacy. 2 These agents carry risks including allergic reactions, bleeding, hypotension, arrhythmias, seizures, and circulatory collapse. 2
Answer to the Question
Based on the clinical scenario describing a vasodilator for Menière's disease, the answer is (a) H1 agonist, as betahistine acts primarily through H1 receptor agonism to achieve its vasodilatory effects in the inner ear. 1