Phosphodiesterase inhibitors and the eye

Phosphodiesterase type 5 (PDE5) inhibitors are effective oral treatments for erectile dysfunction and have become one of the most widely prescribed medications worldwide. The mechanism of action is to reduce the degradation of cyclic GMP (cGMP) potentiating the effect of nitric oxide in the corpus cavernosum and allowing erectile function to occur by consequent relaxation of penile smooth muscle. Because of the presence of PDE5 in choroidal and retinal vessels these medications increase choroidal blood flow and cause vasodilation of the retinal vasculature. The most common symptoms are a blue tinge to vision and an increased sensitivity to light. There have been reports of non-arteritic anterior ischaemic optic neuropathy and serous macular detachment in users of PDE5 inhibitors, although a causal relationship has not been conclusively shown. Despite the role of cGMP in the production and drainage of aqueous humour these medications do not appear to alter intraocular pressure and are safe in patients with glaucoma. All PDE5 inhibitors weakly inhibit PDE6 located in rod and cone photoreceptors resulting in mild and transient visual symptoms that correlate with plasma concentrations. Psychophysical tests reveal no effect on visual acuity, visual fields or contrast sensitivity; however, some studies show a mild and reversible impairment of blue-green colour discrimination. PDE5 inhibitors transiently alter retinal function on electroretinogram testing but do not appear to be retinotoxic. Despite the role of cyclic nucleotides in tear production there is no detrimental effect on tear film quality. Based on the available evidence PDE5 inhibitors have a good ocular safety profile.