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I was interested in the recent paper by Hussain et al1 describing the efficacy of sildenafil citrate for erectile dysfunction in patients with Parkinson's disease or multiple system atrophy (MSA). Their findings provide reassurance that this popular drug is both effective and safe in parkinsonian patients provided orthostatic hypotension is recognised as a potential side effect in MSA. Sildenafil inhibits cyclic GMP specific phosphodiesterase thereby enhancing nitric oxide mediated relaxation of the corpus cavernosum. The authors do not mention any effects of sildenafil on parkinsonian symptoms, although its mechanism of action would make this unlikely.
In addition to erectile dysfunction, many factors including motor symptoms contribute to sexual dysfunction and dissatisfaction in this population. I have previously reported penile erections in a significant proportion of patients following subcutaneous injections of the dopamine agonist apomorphine to treat motor fluctuations in Parkinson's disease.2 Some of these patients started using intermittent apomorphine injections specifically for erectile dysfunction. In contrast to sildenafil, dopamine agonists act centrally on dopamine receptors in the paraventricular nucleus of the hypothalamus to stimulate oxytocin release.3 The benefit to motor symptoms in Parkinson's disease is mediated through dopamine receptors in the striatum. Apomorphine also benefits motor disabilities in some patients with MSA although orthostatic hypotension may be exacerbated by stimulation of peripheral dopamine receptors and its role in erectile dysfunction in this group has not been explored.
In view of the additional benefits to parkinsonian motor symptoms, subcutaneous apomorphine should be regarded as an alternative to sildenafil in treating patients with Parkinson's disease and erectile dysfunction. Sublingual preparations of apomorphine have recently been developed for this indication.4