💊 Dopamine Agonists

Dopamine agonists are a class of drugs that directly stimulate dopamine receptors in the brain. They are used in the management of Parkinson's disease and restless legs syndrome.

Dopamine agonists directly activate postsynaptic dopamine receptors (primarily D2 and D3 receptors) in the striatum, mimicking the effects of endogenous dopamine. This bypasses the need for conversion by DOPA decarboxylase, unlike levodopa. They can be ergot-derived (e.g., bromocriptine) or non-ergot derived (e.g., ropinirole, pramipexole, rotigotine).

Dopamine agonists are used as monotherapy in early Parkinson's disease to delay the need for levodopa, or as adjunctive therapy with levodopa in advanced disease to reduce 'off' periods and dyskinesias. They are also indicated for the treatment of moderate to severe restless legs syndrome. Some, like bromocriptine, are used for hyperprolactinaemia.

Absolute contraindications include hypersensitivity to the specific agonist. Ergot-derived agonists are contraindicated in patients with a history of fibrotic reactions (e.g., pulmonary, retroperitoneal, cardiac valvular fibrosis) or uncontrolled hypertension. Non-ergot agonists require caution in patients with severe cardiovascular disease or psychosis.

Common side effects include nausea, vomiting, orthostatic hypotension, dizziness, and somnolence. Impulse control disorders (e.g., pathological gambling, hypersexuality, compulsive shopping) are a significant and serious adverse effect. Hallucinations and confusion can also occur, particularly in older patients. Ergot-derived agonists carry a risk of fibrotic reactions.

Patients should be monitored for the development of impulse control disorders, and their psychiatric status should be regularly assessed. Blood pressure should be monitored, especially during initiation, due to orthostatic hypotension. Renal and hepatic function may be monitored, particularly with certain agents. For ergot-derived agonists, regular cardiac and pulmonary assessments are necessary.

When prescribing, always screen for a history of impulse control disorders or psychosis. Counsel patients and their families about the risk of impulse control disorders and to report any changes in behaviour. Be aware of potential interactions with antipsychotics (which can antagonise dopamine effects) and antihypertensives (exacerbating hypotension). Advise on gradual dose titration to minimise side effects.

Dopamine agonists are often preferred in younger Parkinson's patients to delay levodopa initiation and its associated motor complications. A critical side effect is the development of impulse control disorders, which requires careful patient and family counselling. Non-ergot agonists are generally preferred due to the fibrotic risks associated with ergot derivatives.

SBAs frequently highlight impulse control disorders as a key adverse effect of dopamine agonists, often presenting a scenario where a patient develops new compulsive behaviours. Questions might also compare their efficacy and side effect profile with levodopa, or differentiate between ergot and non-ergot derived agonists. Orthostatic hypotension is another common theme.