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Since 1998, phosphodiesterase type 5 (PDE5) inhibitors have been used extensively for the treatment of erectile dysfunction. These agents include sildenafil (Viagra, Pfizer), vardenafil (Levitra, Bayer AG) and tadalafil (Cialis, Lilly-ICOS LLC). Development of non-arteritic anterior ischaemic optic neuropathy (NA-AION) has been reported following the use of these drugs,1 mostly Viagra. I have discussed elsewhere the possible cause-and-effect relationship between the PDE5 inhibitors and NA-AION.1 To understand that relationship, it is absolutely essential that one must have an in-depth knowledge of the following related basic scientific facts:
blood supply of the optic nerve head: the factors that influence the ONH blood flow and its measurement;
pathogenesis of NA-AION;
systemic and ocular vascular effects of PDE5 inhibitors.
Following is a brief discussion of these issues.
BLOOD SUPPLY OF THE OPTIC NERVE HEAD (ONH): THE FACTORS THAT INFLUENCE THE ONH BLOOD FLOW AND ITS MEASUREMENT
My multifaceted studies showed that posterior ciliary circulation is the main source of ONH blood supply.2 3 The blood flow in the ONH is influenced by many factors, discussed at length elsewhere.4 These include systemic arterial blood pressure, endothelial derived vasoactive agents, autoregulation of blood flow in the ONH and intraocular pressure. Autoregulation can be deranged by many systemic and local causes, including ageing, arterial hypertension, diabetes mellitus, marked arterial hypotension, arteriosclerosis, atherosclerosis, hypercholesterolaemia, vasospasm and probably regional vascular endothelial disorders.4–7 Many of those diseases also interfere with the release of endothelial derived vasoactive agents.4
In spite of many claims, by using a variety of methods, unfortunately, we do not have as yet a reliable method of measuring blood flow in the ONH accurately;8 limitations of various advocated methods are discussed in detail elsewhere.8 There is a common misconception that since blood supply to the ONH is by the posterior ciliary artery circulation, measurements of blood flow in the short posterior ciliary arteries in the orbit …
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