Can lowering blood pressure mean higher risk for glaucomatous field defects?
Many of us recall horror stories about cases where a sudden lowering of BP resulted in loss of field. Probably these have been few since no systematic reports on the subject seem to exist. Still, as a local area authority on glaucoma (also a world authority), Malvolio, puts it:
As ophthalmologues we know that a blood flow sufficient for normal function of the eye requires a BP which sometimes may surpass what may seem acceptable to the modern cardiologist. As mentioned by Demailly (1989) a,o orthostatic BP decrease , surgical interventions with great blood loss and the use of betablockers aministrated generally may have harmful ocular effects. A correlation between low tension glaucoma and arterial hypotension has been noted. The question is not unequivocally settled. Investigations seem therefore motivated concerning a possible effect of BP reduction on ocular functions, such as threshold level of light sensitivity, visual field and amplitude of the ocular pulse wave.
So what is the reason to bring up just now the question of BP versus IOP? Because what was a few years ago considered an ”acceptable” level of BP for a person in the typical POAG age, 60-70 years, namely 160/100, is no longer acceptable. Particularly not in the popular press.
As a Swedish tabloid put it: An increase of 20 mm Hg in systolic pressure or 10 mm Hg in diastolic pressure above 115/75 mm Hg doubles the risk of death from cardiovascular disease. And who does not want to cut the risk of dying from cardiovascular disease by fifty per cent?
The risk figure comes from a position paper on BP by the Swedish Council on Technology Assessment in Health Care, SBU, ”Moderately elevated blood pressure”. The statement is correctly quoted, but for many reasons totally meaningless to use out of context. It is well worth reading the full text of the SBU-paper (see pdf.file).
Summing up, there is indeed all over the Western world a new policy on BP lowering, reemphasizing systolic BP (SBP). The aim level for the lowering ”should” henceforth be below 140/90; pressures considerably lower than earlier accepted. (A few BP-talibans, and the popular press ”require” even lower)
Let us see what Alon Harris, a well known ocular physiologist, writes about this. We take a piece from EVIDENCE THAT STUDYING VASCULAR CONSIDERATIONS CAN BENEFIT PATIENTS WITH GLAUCOMA (http://www.wills-glaucoma.org/2003symp/harris.htm)
In 1983, Framingham Eye Study participants with OAG were reported to have significantly low BP/IOP ratios. In addition, persons with definite glaucomatous visual field defects had lower ratios than those with suspect defects or no defects (11). Low perfusion pressure (PP) was also an OAG risk factor in the Baltimore Eye Survey, Egna-Neumarkt, Proyecto VER, and most recently, the Barbados Incidence Study of Eye Diseases (BISED) (12-15). Additionally, the Baltimore Eye study found diastolic PP (DPP, DPP = diastolic BP - IOP) of less than 30 mmHg to be strongly associated with OAG (risk ratio =6), whereas systolic PP (SPP, SPP = systolic BP - IOP,) and mean PP (MPP, MPP = mean BP – IOP) were only mildly associated (12). In the Egna-Neumarkt study, OAG prevalence increased progressively with decreased DPP (13). The Proyecto VER study found similar results at a low DPP (14). The BISED study found all three factors (DPP, SPP, and MPP) to be related to OAG (15). Low DPP, had the strongest correlation, approximately tripling the risk ratio of developing OAG. (15
The only vascular parameter that meets the criteria necessary to be considered clinically, based on evidence based medicine, is diastolic perfusion pressure. The relationship between perfusion pressure and glaucoma is not known, however the existence of a relationship begs the question: Are vascular deficits and ischemia involved in the pathogenesis and progression of glaucoma?
In 2006 SBU will publish an EBM-based report on Diagnosis and Treatment of Glaucoma (Eckerlund-Heijl). Perhaps this report will take into consideration also to the paper on Treatment of Moderately Elevated Blood Pressure.
Till then...be extra careful when you lower BP on POAGs!
Olle holm