BLOOD PRESSURE – INTRAOCULAR PRESSURE & GLAUCOMA
In 1857 von Graefe presented a patient with glaucoma but normal intraocular pressure. Since then, scientists have been entertaining the hypothesis that glaucoma is not a result of increased eye pressure alone. In 1947 Wolff claimed that glaucoma is the consequence of optic nerve head ischemia (reduced blood supply) whereas Duke-Elder implicated arteriosclerosis as having a causal effect.
BLOOD PRESSURE – INTRAOCULAR PRESSURE & GLAUCOMA
Hypertension has been implicated as a risk factor for glaucoma during previous decades a claim that has been confirmed by the epidemiologic Baltimore Eye Study. However, the association appears to be complex. In the above study the risk of Primary Open Angle Glaucoma increased in patients with systolic or diastolic arterial hypertension, but the correlation was not linear and applied only for systolic blood pressures over 130mmHg. A similar association was documented by the Rotterdam Eye Study, whereas the Barbados Eye Study did not confirm an association of elevated systolic blood pressure with the risk for Primary Open Angle Glaucoma. More light was shed into this issue by the Framingham Study which demonstrated that patients with visual field abnormalities also had a decreased arterial to intraocular pressure ratio.
Normal organ function depends on adequate blood flow. Therefore, the relationship of arterial pressure which increases blood flow, to intraocular pressure which negatively impacts ocular perfusion, is of paramount importance.
In the eye ocular blood flow is a function of ocular perfusion pressure which is calculated as the difference between 2/3 of the mean arterial pressure minus mean intraocular pressure.
A drop in ocular perfusion pressure can cause ischemia and tissue damage. Maintenance of adequate ocular perfusion pressure depends on a precise homeostatic mechanism that is further affected by factors such as the use and timing of anti-hypertensive medications or arteriosclerosis. Low ocular perfusion pressure occurring most commonly at night can cause deterioration of the disease even in patients that appear to have well controlled intraocular pressures during office hours.
OCULAR PERFUSION PRESSURE AS A RISK FACTOR FOR GLAUCOMA
The association between ocular perfusion pressure and Primary Open Angle Glaucoma has been confirmed by many epidemiologic studies. Low diastolic perfusion pressure below 50-55mmHg has been identified as a risk factor for Primary Open Angle Glaucoma in studies conducted in Europe and the United States. Especially patients with diastolic ocular perfusion pressure (DOPP) below 30mmHg have a 6 times higher risk to manifest glaucoma compared to patients with DOPP ≥ 50mmHg.
The Barbados Eye Study supports the above conclusions.
OCULAR PERFUSION PRESSURE AS RISK FACTOR FOR GLAUCOMA PROGRESSION
Many epidemiologic studies conclude that vascular risk factors such as arteriosclerosis, affect the clinical course of glaucoma. Furthermore, fluctuations in the ocular perfusion pressure are associated with the severity of low-tension glaucoma.
Recently, the large prospective Early Manifest Glaucoma Trial has demonstrated that patients with low systolic ocular perfusion pressure progress faster compared to their counterparts with high systolic ocular perfusion pressure. Similar results were obtained for the diastolic ocular perfusion pressure. In the same longitudinal study, a medical history of cardiovascular disease was also associated with disease progression.
HOW IS BLOOD PRESSURE RELATED TO GLAUCOMA?
Many studies have determined that there is a weak association between blood pressure and intraocular pressure more so for the systolic blood pressure. It has been estimated that for each 10mmHg increase in systolic blood pressure there is a corresponding increase in intraocular pressure by 0,23-0,32mmHg (Barbados and Beaver Dam Eye studies). The association with the diastolic blood pressure is less clear cut.
Whereas blood pressure appears to have an effect on intraocular pressure the relationship of hypertension to Primary Open Angle Glaucoma is more complicated. Epidemiologic studies have provided mixed results. Some of them provide a positive correlation between blood pressure and Primary Open Angle Glaucoma, whereas other studies show the exact opposite.
In the Early Manifest Glaucoma Trial patients with hypertension demonstrated a smaller risk for disease progression.
In general, most recent multicenter studies support that hypertension is an important risk factor affecting the prevalence, the incidence and the progression of glaucoma.
ANTIHYPERTENSIVE TREATMENT & GLAUCOMA
As previously discussed, the association between ocular perfusion pressure and glaucoma in the Rotterdam Eye Study existed only in patients treated for hypertension. Similarly, in a relevant study conducted in Thessaloniki structural defects on the optic disc confirmed by a special instrument have been associated with diastolic blood pressures below 90mmHg only in patients receiving antihypertensive medications. A plausible explanation would be that hypertension affects blood vessels causing arteriosclerosis and if in such patients the blood pressure is lowered below a certain limit blood flow to the optic nerve is severely impeded.
Antihypertensive medications can thus affect the course of Primary Open Angle Glaucoma. The Rotterdam Eye Study confirmed increased risk for the disease in patients taking calcium channel blockers after a follow-up of 6.5 years. These medications lower blood pressure without affecting intraocular pressure reducing this way the ocular prefusion pressure. A review of the classes of medications that patients with glaucoma receive is consequently necessary in patients with glaucoma.
CONCLUSIONS
1. There is an association between blood pressure and intraocular pressure but the association between hypertension and glaucoma is far more complex.
2. Recent large epidemiologic studies have confirmed that an association exists between low ocular perfusion pressure and Primary Open Angle Glaucoma.
3. Primary Open Angle Glaucoma is a multifactorial disease in which vascular components and ocular perfusion pressure may be implicated.
4. Nocturnal hypotension especially as a result of excessive antihypertensive treatment may led to disease progression and visual field deterioration.
Dr. Theodoros Filippopoulos, MD
Ophthalmic Surgeon