💊 SNRIs
Drug Class & Overview
Serotonin-Noradrenaline Reuptake Inhibitors (SNRIs) are a class of antidepressant medications that inhibit the reuptake of both serotonin and noradrenaline.
Mechanism of Action
SNRIs block the reuptake of both serotonin (5-HT) and noradrenaline (NA) into the presynaptic neuron. This dual action leads to increased concentrations of both neurotransmitters in the synaptic cleft, enhancing their effects.
Key Indications
Major Depressive Disorder is a primary indication. They are also used for Generalised Anxiety Disorder, Social Anxiety Disorder, and Panic Disorder. Some SNRIs are specifically indicated for neuropathic pain (e.g., duloxetine) and fibromyalgia.
Contraindications
Absolute contraindications include concomitant use with MAO inhibitors due to the risk of serotonin syndrome. Caution is required in patients with uncontrolled hypertension, narrow-angle glaucoma, or a history of seizures. They should be used with caution in patients with significant cardiac disease.
Adverse Effects
Common side effects are similar to SSRIs, including nausea, headache, insomnia, and sexual dysfunction. Additionally, due to noradrenergic effects, SNRIs can cause increased blood pressure, tachycardia, and sweating. Serotonin syndrome is a rare but serious risk. Hyponatraemia and increased bleeding risk are also concerns.
Monitoring
Blood pressure and heart rate should be monitored regularly, especially at the start of treatment and following dose adjustments, due to the noradrenergic effects. Baseline and periodic renal and liver function tests may be considered. Monitoring for clinical response and adverse effects is crucial.
Prescribing Safety (OSCE)
When prescribing SNRIs, always check for drug allergies and potential interactions, especially with MAOIs, triptans, or other drugs affecting blood pressure. Counsel patients on the potential for increased blood pressure and heart rate, and the importance of regular monitoring. Advise against abrupt cessation to avoid withdrawal symptoms.
MLA High-Yield Notes
Venlafaxine is a commonly used SNRI, with its noradrenergic effects becoming more prominent at higher doses. Duloxetine is particularly useful for neuropathic pain and fibromyalgia. SNRIs can be considered when SSRIs are ineffective or not tolerated, but their side effect profile, particularly cardiovascular, requires careful consideration. Discontinuation syndrome can be severe with venlafaxine.
Common SBA Themes
SBA questions often focus on the unique side effects of SNRIs, particularly hypertension and tachycardia, and how to manage them. Differentiating SNRIs from SSRIs based on their broader neurotransmitter effects and indications (e.g., neuropathic pain) is another common theme. Questions might also involve serotonin syndrome or discontinuation syndrome.
References
- BNF (British National Formulary)
- NICE (National Institute for Health and Care Excellence) Guidelines
- MHRA (Medicines and Healthcare products Regulatory Agency) Guidance