BP refers to the pressure exerted by blood on the arterial walls as the heart beats. BP is maximal during ventricular contraction, i.e. systolic BP, and minimal during ventricular relaxation, i.e. diastolic BP. BP reduces in magnitude as it moves away from the heart. Clinically, systemic BP is measured at the brachial artery. BP is considered normal when systolic BP is <120 mmHg and diastolic BP is <80 mmHg. The diagnosis of pre-hypertension is when systolic BP is between 120 to 139 mmHg and diastolic BP is 80 to 89 mmHg. Stage I hypertension is when the systolic BP is 140-159 mmHg and diastolic BP is 90 to 99 mmHg, while stage II is >160 mmHg systolic BP and >100 mmHg diastolic BP.32 Mean arterial pressure can be calculated by29:
Equation 1.7
Any association of BP and glaucoma is controversial (Table 1.1). Some authors found a significant association with hypertension, while others found a significant association with hypotension. A few epidemiological studies have even associated glaucoma to both hypertension and hypotension.25, 33 Table 1.1 summarizes studies that have investigated any association between BP and glaucoma.
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Studies Findings
Thessaloniki Eye Study34
Early Manifest Glaucoma Trial23 The Barbados Eye Study24 Leske et al.35
Kaiser et al.36
Glaucoma is associated with arterial hypotension
Rotterdam Eye Study37 Blue Mountains Eye Study26 Beaver Dam Eye Study38
Egna-Neumarkt Glaucoma Study22
Glaucoma is associated with arterial hypertension
Baltimore Eye Survey33
Los Angeles Latino Eye Study25
Glaucoma is associated with both arterial hypotension and
hypertension
Table 1.1: Summary of studies on blood pressure association with glaucoma.
1.7.1 Arterial hypotension and glaucoma
From the vascular perspective, low BP leads to reduction of OPP, which in turn leads to ischemia and neuropathy. It has been demonstrated that aggressive systemic hypertensive treatment exacerbates glaucoma progression. A population study on a non-glaucomatous cohort reported an increase in cupping and cup-to-disc ratio in treated hypertensive subjects, compared to untreated and normal subjects.34 In another study on glaucoma patients, it was found that systemic diuretic usage was related to glaucoma development.39 A similar result was reported by a 6 month longitudinal study which reported that degenerative changes and impaired blood flow were evident among glaucoma patients who take antihypertensive medication in the evenings.40 This raises the concerns about any possible effect of hypertensive treatment on glaucoma.
13 1.7.2 Arterial hypertension and glaucoma
It is counter intuitive to associate hypertension to glaucoma, considering that high BP results in a high OPP.18 The Baltimore Eye Study33, 41 showed that relationship of hypertension to glaucoma is age dependent where it appears to be protective in younger patients but not in older patients. They speculated that blood vessels around the ONH experience atherosclerosis with age, becoming rigid and narrow, and thus compromising perfusion. Hypertension is therefore suggested to be a chronic risk factor that involves a long-term changes of the vascular beds which impairs perfusion.42
1.7.3 Effect of BP on tissue flow
It is expected that high arterial pressure will increase blood flow in tissues. The increase of perfusion pressure in a vessel will increase the blood velocity and at the same time will tend to distend the lumen size but this will be opposed by increase in tone, which in turn will increase flow. The relationship between BP and blood flow is exponential; increasing BP two fold will cause 4 to 6 folds of increase in flow.16 However, this is not evident in a peripheral system like in the eye as autoregulation ensures stable perfusion despite BP changes. It was reported that retinal flow remains stable during dynamic exercise.43 A study on healthy retinal arteriolar showed there was 2.5% arteriolar constriction with 21.8mmHg increase in MAP from baseline, suggesting a resistance to vasodilation in the retinal arteriolar vessels despite large change in MAP.44 Choroidal flow45, 46 and retrobulbar flow47 was also reported to remain stable during dynamic exercise.
14 1.7.4 Effect of BP on IOP
Epidemiological studies have consistently reported an association between high BP and IOP.22, 33, 37, 38
Animal studies have shown that the IOP is elevated upon stimulation of hypertension.48, 49 It is, however, unclear how hypertension increases IOP. It has been proposed that BP elevation tends to elevate ciliary artery pressure which leads to elevation of ultrafiltration of aqueous production and hence IOP elevation. Alternatively, increased arterial pressure may also lead to a small elevation to the venous pressure, which may reduce the aqueous outflow and then increase IOP.42 However, the change in IOP induced by BP is small (0.2-0.4mmHg IOP increase per 10mmHg BP increase),50 thus it is unlikely that there is an association of glaucoma with high BP.
1.7.5 Ambulatory blood pressure monitoring (ABPM)
BP is influenced by factors which include physical and emotional status, environmental circumstances, the autonomic nervous system, the renin-angiotensin-aldosterone system, and neurohumoral factors.51-53 BP measurement in a doctor’s office may mask the real BP value where a “white coat reaction” has been reported to cause an increase of 20% in the BP value in suspected-hypertensive and hypertensive patients.54 Masked hypertension on the other hand may also occur where a patient produces a normal office BP value despite having hypertension.51, 53 As a result, a continuous BP measurement is pivotal which is permissible using ambulatory device. The International Database in Ambulatory BP monitoring in relation to Cardiovascular Outcome (IDACO) suggested normal daytime ambulatory BP should be <130/85 mmHg, nighttime BP <110/70 mmHg, and 24-hour BP <125/75 mmHg.55
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