Diurnal and Nocturnal Behavior of Episcleral Venous Pressure

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Daily Rhythms of Eye Pressure and Venous Pressure Our eyes have a natural 24-hour cycle of pressure changes. Both intraocular pressure (IOP) (the pressure of fluid inside the eye) and episcleral venous pressure (EVP) (the downstream pressure in the veins on the surface of the eye) tend to be highest in the early morning and lower by evening () (). In one recent study of healthy adults, the mean IOP and EVP were both highest at 8 AM and fell by late afternoon (). In other words, like a clock, IOP and EVP peak in the morning and wane later in the day () (). However, night‐time patterns are influenced by sleep posture. When we lie down on our back, blood settles differently, so both IOP and EVP rise. For example, one study found that as soon as a person lies down, EVP jumps by about 3–4 mmHg and stays high while supine (). This contributes to known findings that eye pressure measured at night (when a person is usually lying flat) tends to be higher than daytime sitting measurements () (). In one carefully controlled sleep-lab experiment, volunteers showed higher mean IOP at night partly due to increased EVP and fluid shifting when lying down (). Thus nighttime IOP often exceeds daytime levels because of the supine position and higher venous pressure. In general, IOP and EVP move together across the day. When one study compared them round‐the‐clock, changes in EVP closely paralleled changes in IOP () (). In both healthy people and those on blood-pressure medicines, higher EVP in the morning accompanied higher IOP, and both fell through the afternoon () (). This synchronization means factors that push IOP up (like lying down) also raise EVP, since EVP partly “holds up” IOP from falling below it () (). In short, EVP and IOP share daily rhythms with morning peaks and day/evening troughs () (), but staying flat in bed at night produces higher values for both. How Lifestyle and Body Factors Can Change EVP Several everyday factors affect eye pressures. Staying well hydrated, dietary choices, and nervous system activity all play a role: Hydration (Water Intake): Drinking lots of fluid quickly can raise eye pressure. In one classic study, healthy people who drank a liter of water saw their IOP jump by about 4.4 mmHg for over two hours (). This happens because extra fluid increases the blood and ocular fluid volume. By analogy, EVP likely rises a bit with high fluid intake, although direct EVP data is limited. In clinical practice, patients are sometimes advised to avoid gulping large volumes of water right before IOP checks. Salt Intake: Eating very salty food leads the body to retain water, raising blood volume and pressure. Recent research in a large population found that people with higher dietary salt (measured by urine sodium) had slightly higher IOP and more glaucoma (). The highest-salt group had IOP about 0.45 mmHg above the lowest-salt group. Scientists suggest this may be due to more fluid volume and higher episcleral venous pressure pushing fluid back into the eye (). In other words, excess salt can subtly elevate EVP (and thus IOP) and may increase glaucoma risk () (). Caffeine (Coffee): Caffeine is a mild stimulant that briefly raises IOP. In the same classic study, drinking caffeine led to about a 4.0 mmHg rise in IOP lasting around 95 minutes (). The mechanism likely involves caffeine’s vascular effects. We have less direct data on EVP after caffeine, but by raising overall ocular pressure, it may also raise EVP slightly. For patients sensitive to eye pressure changes, avoiding strong coffee or energy drinks before an eye exam can help avoid an artificial spike. Alcohol: Alcohol has the opposite effect. The 1986 study showed that drinking alcohol caused IOP to drop by up to 3.7 mmHg, with values returning to normal within about an hour (). Alcohol is a vasodilator (it relaxes blood vessels), which may lower both blood and venous pressures, including the episcleral veins. So moderate alcohol can transiently lower EVP and IOP, but this is not considered a therapy (and excessive drinking has many risks). For measurement, it implies having an alcoholic drink just before a pressure check might temporarily make one’s IOP/EVP look lower than usual. Autonomic (Stress and Nerves): The autonomic nervous system (our “fight-or-flight” vs “rest-and-digest” system) can adjust vessel tone throughout the body, including the episcleral vessels. Studies note that changes in autonomic activity can change EVP (). For example, being stressed or anxious (activating the sympathetic system) can constrict some eye vessels, whereas relaxation (parasympathetic) may dilate them. One observation: vigorous exercise caused an immediate drop in IOP of about 4.3 mmHg (). This might be partly due to changes in blood flow and venous tone. In practice, rapid heart rate or adrenaline can slightly alter EVP as well. It is wise to sit quietly before measuring eye pressure, to let things settle. Body Posture: Moving from sitting to lying increases EVP. Multiple studies show that IOP measured lying down is consistently ~2–4 mmHg higher than when sitting (). This is largely due to higher EVP when supine. Thus doctors usually check IOP in a seated position for consistency. But patients should remember: when they lie down (for sleep or rest), their eye pressures rise. Eyelid Closure: As it turns out, simply closing the eyelid (such as when dozing) does not significantly change EVP or IOP (). One study found no effect from keeping one eye closed overnight. So it’s the posture (supine) rather than blinking or shut eyelid that drives pressure changes at night. In summary, factors that boost blood/eye fluid (like too much salt or water, caffeine, lying flat) tend to raise EVP and IOP, while vasodilators or activity (alcohol, exercise) tend to lower them () (). Patients may be advised to minimize heavy salt, caffeine, and alcohol around the time of pressure checks. Implications for Monitoring and Treatment These rhythms and triggers have real-world impacts on glaucoma care. Because IOP (and EVP) peak in the morning, relying on a single afternoon office measurement can miss dangerous spikes () (). A patient whose IOP is “normal” at 2 PM might actually have had a higher pressure earlier that day. Therefore, doctors sometimes repeat IOP checks at different times, or even use extended monitoring. For example, one noninvasive device (the Triggerfish® contact lens sensor) records 24-hour ocular pressure patterns continuously, including while sleeping (). Studies show this lens can safely capture the ups and downs of IOP (and inferred EVP changes) around the clock (). If available, such monitoring can reveal night-time peaks or large swings that single visits miss. Without that technology, home tonometry (self-measuring IOP) or evening clinic visit can help find the highest pressures. Medication timing can also consider these patterns. Many glaucoma eye drops work over 24 hours, but some effects vary. For instance, carbonic anhydrase inhibitors and beta-blockers reduce fluid production, so giving them before the morning rush might blunt the rise. Prostaglandin analogs increase outflow and usually act over a full day, so they are often given at bedtime to cover the early morning period. In any case, discussing timing with one’s doctor is wise. A typical strategy is to try to have the maximum drug effect coincide with the known IOP peak (often morning) () (). (Some doctors note, for example, that beta-blockers like timolol may work best if dosed in the morning when sympathetic tone is higher.) There is no single rule for all patients, but understanding that EVP and IOP ebb and flow suggests chronotherapy (timed dosing) could optimize control. Practically speaking, patients should follow these tips: Record Multiple Readings: If possible, get IOP measurements at different times (morning and afternoon, or during a home period) to catch peaks. Consistent Posture: Always measure IOP sitting upright, both at home or clinic. Note that lying down (even for sleep) raises the pressure. Report Drinks or Meds: Let the doctor know about large water intake, caffeine, salt meals, or new medications (like oral decongestants or stimulants) around measurement time. Tailor Eye Drops: Ask the doctor if any medication timing should adjust for your daily schedule (e.g. take certain drops at night vs morning). By aligning treatment and monitoring with the eye’s daily cycle, one can better manage glaucoma risk. For example, if a patient’s pressure is highest upon waking, an evening dose of medication might be most effective, whereas a midday peak might call for a morning dose. There is ongoing research on “ocular chronotherapy,” but the key idea is clear: when and how often we measure and treat should reflect the clock-like behavior of eye pressure. Recommendations for Future Research Protocols To better understand EVP’s 24-hour behavior, future studies should use standardized, controlled protocols. Here are some suggestions: Controlled Environment: Use a sleep-lab or clinical research setting where lighting, temperature, and noise are kept constant. Keep subjects on a strict sleep-wake schedule (e.g. lights on at 7 AM, off at 11

Transcript

SPEAKER_00 0:00

Daily rhythms of eye pressure and venous pressure. Our eyes have a natural 24-hour cycle of pressure changes. Both intraocular pressure, IOP, the pressure of fluid inside the eye, and episcleral venous pressure, EVP, the downstream pressure in the veins on the surface of the eye, tend to be highest in the early morning and lower by evening. In one recent study of healthy adults, the mean IOP and EVP were both highest at 8 a.m. and fell by late afternoon. In other words, like a clock, IOP and EVP peak in the morning and wane later in the day. However, nighttime patterns are influenced by sleep posture. When we lie down on our back, blood settles differently, so both IOP and EVP rise. For example, one study found that as soon as a person lies down, EVP jumps by about 3-4 millimeters and stays high while supine. This contributes to known findings that eye pressure measured at night, when a person is usually lying flat tends to be higher than daytime sitting measurements. In one carefully controlled sleep lab experiment, volunteers showed higher mean IOP at night, partly due to increased EVP and fluid shifting when lying down. Thus, nighttime IOP often exceeds daytime levels because of the supine position and higher venous pressure. In general, IOP and EVP move together across the day. When one study compared them round the clock, changes in EVP closely paralleled changes in IOP. In both healthy people and those on blood pressure medicines, higher EVP in the morning accompanied higher IOP and both fell through the afternoon. This synchronization means factors that push IOP up, like lying down, also raise EVP, since EVP partly holds up IOP from falling below it. In short, EVP and IOP share daily rhythms with morning peaks and day-evening troughs, but staying flat in bed at night produces higher values for both. How lifestyle and body factors can change EVP. Several everyday factors affect eye pressures. Staying well hydrated, dietary choices, and nervous system activity all play a role. Hydration, water intake, drinking lots of fluid quickly can raise eye pressure. In one classic study, healthy people who drank a liter of water saw their IOP jump by about 4.4 millimeter grams for over two hours. This happens because extra fluid increases the blood and ocular fluid volume. By analogy, EVP likely rises a bit with high fluid intake, although direct EVP data is limited. In clinical practice, patients are sometimes advised to avoid gulping large volumes of water right before IOP checks. Salt intake, eating very salty food, leads the body to retain water, raising blood volume and pressure. Recent research in a large population found that people with higher dietary salt, measured by urine sodium, had slightly higher IOP and more glaucoma. The highest salt group had IOP about 0.45 millimetrams above the lowest salt group. Scientists suggest this may be due to more fluid volume and higher episclal venous pressure, pushing fluid back into the eye. In other words, excess salt can subtly elevate EVP and thus IOP and may increase glaucoma risk. Caffeine. Caffeine is a mild stimulant that briefly raises IOP. In the same classic study, drinking caffeine led to about a 4.0 millimeter grams rise in IOP lasting around 95 minutes. The mechanism likely involves caffeine's vascular effects. We have less direct data on EVP after caffeine, but by raising overall ocular pressure, it may also raise EVP slightly. For patients sensitive to eye pressure changes, avoiding strong coffee or energy drinks before an eye exam can help avoid an artificial spike. Alcohol. Alcohol has the opposite effect. The 1986 study showed that drinking alcohol caused IOP to drop by up to 3.7 mm, with values returning to normal within about an hour. Alcohol is a vasodilator, it relaxes blood vessels, which may lower both blood and venous pressures, including the episcleral veins. So moderate alcohol can transiently lower EVP and IOP, but this is not considered a therapy, and excessive drinking has many risks. For measurement, it implies having an alcoholic drink just before a pressure check might temporarily make one's IOP EVP look lower than usual. Autonomic stress and nerves. The autonomic nervous system, our fight or flight versus rest and digest system, can adjust vessel tone throughout the body, including the episcleral vessels. Studies note that changes in autonomic activity can change EVP. For example, being stressed or anxious, activating the sympathetic system, can constrict some eye vessels, whereas relaxation, parasympathetic, may dilate them. One observation vigorous exercise caused an immediate drop in IOP of about 4.3 mm her grams. This might be partly due to changes in blood flow and venous tone. In practice, rapid heart rate or adrenaline can slightly alter EVP as well. It is wise to sit quietly before measuring eye pressure to let things settle. Body posture. Moving from sitting to lying increases EVP. Multiple studies show that IOP measured lying down is consistently 2 to 4 mm hergrams higher than when sitting. This is largely due to higher EVP when supine. Thus, doctors usually check IOP in a seated position for consistency. But patients should remember, when they lie down for sleep or rest, their eye pressures rise. Eyelid closure. As it turns out, simply closing the eyelid, such as when dozing, does not significantly change EVP or IOP. One study found no effect from keeping one eye closed overnight, so it's the posture, supine rather than blinking or shut eyelid, that drives pressure changes at night. In summary, factors that boost blood eye fluid, like too much salt or water, caffeine lying flat, tend to raise EVP and IOP, while vasodilators or activity, alcohol, exercise tend to lower them. Patients may be advised to minimize heavy salt, caffeine, and alcohol around the time of pressure checks. Implications for monitoring and treatment. These rhythms and triggers have real-world impacts on glaucoma care. Because IOP and EVP peak in the morning, relying on a single afternoon office measurement can miss dangerous spikes. A patient whose IOP is normal at 2 p.m. might actually have had a higher pressure earlier that day. Therefore, doctors sometimes repeat IOP checks at different times or even use extended monitoring. For example, one non-invasive device, the triggerfish contact lens sensor, records 24-hour ocular pressure patterns continuously, including while sleeping. Studies show this lens can safely capture the ups and downs of IOP and inferred EVP changes around the clock. If available, such monitoring can reveal nighttime peaks or large swings that single visits miss. Without that technology, home telometry, self-measuring IOP, or evening clinic visit can help find the highest pressures. Medication timing can also consider these patterns. Many glaucoma eye drops work over 24 hours, but some effects vary. For instance, carbonic anhydrase inhibitors and beta blockers reduce fluid production, so giving them before the morning rush might blunt the rise. Prostaglandin analogues increase outflow and usually act over a full day, so they are often given at bedtime to cover the early morning period. In any case, discussing timing with one's doctor is wise. A typical strategy is to try to have the maximum drug effect coincide with the known IO peak, often morning. Some doctors note, for example, that beta blockers like Timolol may work best if dosed in the morning when sympathetic tone is higher. There is no single rule for all patients, but understanding that EVP and IOP ebb and flow suggests chronotherapy, time dosing could optimize control. Practically speaking, patients should follow these tips. Record multiple readings. If possible, get IOP measurements at different times, morning and afternoon, or during a home period, to catch peaks. Consistent posture. Always measure IOP sitting upright, both at home or clinic. Note that lying down even for sleep raises the pressure. Report drinks or meds. Let the doctor know about large water intake, caffeine, salt meals, or new medications, like oral decongestants or stimulants around measurement time. Tailor eye drops. Ask the doctor if any medication timing should adjust for your daily schedule, e.g., take certain drops at night versus morning. By aligning treatment and monitoring with the eye's daily cycle, one can better manage glaucoma risk. For example, if a patient's pressure is highest upon waking, an evening dose of medication might be most effective, whereas a midday peak might call for a morning dose. There is ongoing research on ocular chronotherapy, but the key idea is clear when and how often we measure and treat should reflect the clock-like behavior of eye pressure. Recommendations for future research protocols. To better understand EVP's 24-hour behavior, future studies should use standardized controlled protocols. Here are some suggestions. Controlled environment. Use a sleep lab or clinical research setting where lighting, temperature, and noise are kept constant. Keep subjects on a strict sleep-wake schedule, e.g., lights on at 7 a.m., off at 11 p.m. This avoids external cues that could disturb circadian patterns. Fixed posture. Standardize body position during measurements. For daytime, have subjects sit quietly for several minutes before each EVP reading. For nocturnal measurements, record whether the subject is supine, as usually happens during sleep. It would be ideal to measure EVP in both sitting and supine positions to separate postural effects from true circadian changes. Frequent time points. Take EVP and IOP measurements at multiple times across 24H. For example, measure every 2 to 3 hours, including during night and early morning. Previous work often measured at 6 to 8 a.m. noon and evening. Future studies could sample more intensively, e.g. every 3-4H, to define the full curve. Control intake of modifiers. Instruct participants to avoid or control salt, caffeine, alcohol, and large meals for at least 24H before and during the study. Hydration should be kept steady, no water bolus just before measurements. This minimizes noise from the modifiers discussed above. Record physiological data, along with EVP IOP, record blood pressure, heart rate, and perhaps visual fields or retinal blood flow. This helps correlate EVP changes with systemic circulatory factors. Use subjective measurement tools. Whenever possible, use automated or digital episcleral venomometers to reduce observer bias. Keep the same device and calibration throughout the study. Some protocols even videotape the vein collapse moment for more consistency. By following such protocols, future studies can reliably map EVP's daily cycle and how it aligns with IOP. They would clarify, for example, how much of the nocturnal IOP rise is due to EVP versus other factors. Ultimately, well-controlled chronobiology studies could lead to new insights, perhaps proving that targeting EVP directly, with new drugs or timing, might further stabilize IOP for glaucoma patients. Conclusion. Episcleral venous pressure is not static. It oscillates over the day, much like intraocular pressure. Both tend to rise upon waking and decline through the afternoon. At night, lying flat raises EVP and IOP beyond daytime levels. Daily habits, hydration, salt intake, caffeine, alcohol, and activity can shift these pressures up or down temporarily. For patients, the practical message is clear. Measuring and controlling eye pressure fairly requires respecting these rhythms. By timing medications and checkups in light of the clock-like behavior of EVP and IOP, we can better catch high pressure spikes and protect vision. All links to sources are available in the text version of this article. You can find the full article at VisualFieldTest.com. Thanks for listening. To check your visual field, click the link at the bottom of this article or visit visualfieldtest.com.