Neurological Tests and Procedures
(Carotid Patency Evaluation, Ocular Pressures)
What is oculoplethysmography?
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Oculoplethysmography (OPG) is used to detect the presence of a blockage in the carotid arteries. These arteries, located on each side of the neck, deliver oxygenated blood from the heart to the brain. If the carotid arteries are blocked or narrowed due to fatty deposits, the brain can become deprived of oxygen and stroke (damage to the brain tissue due to lack of blood flow) can occur.
OPG may be used to monitor the success of a carotid endarterectomy, a surgical procedure that treats narrowing of the carotid artery.The OPG test indirectly measures the blood flow in the ophthalmic artery, which branches off the carotid artery and supplies blood to the eye. Suction cups are placed on the eyes for the recording and sensors are placed on the earlobes.
If the pulse arrives at the ophthalmic arteries and the earlobes at the same time, this usually means that the carotid arteries are free of blockage. In contrast, if the pulse arrives at the eyes and the earlobes at different times, this may mean that blood flow in these arteries is restricted.
OPG only estimates the extent of blockage that may be present. If the results are abnormal, than a more precise test called angiography (x-ray evaluation of the artery after injection with an opaque dye) may be necessary.
As a result of ultrasound technology that can assess blood flow in the carotid artery much more easily, OPG is rarely performed anymore.
What is oculopneumoplethysmography?
OPG is often conducted in conjunction with a test called oculopneumoplethysmography (OPPG; OPP-Gee). Like OPG, OPPG can detect a narrowing or blockage of the carotid artery and is given for the same reasons, but the procedure is slightly different.
For OPPG, suction cups resembling contact lenses are placed on the eyes and a stronger vacuum than that used for OPG is applied (about 300 mmHg versus 50 mmHg for OPG). The strong suction temporarily halts the pulses in the ocular artery. When the force of the suction is reduced, the pulses reappear and are recorded. Blood pressure in the arm is also taken. Together, these measurements can be used to determine blood flow through the carotid artery.
Other related procedures that may be performed to evaluate the carotid artery and its branches are cerebral arteriogram and carotid artery duplex scan. Please see these procedures for more information.
Your physician may recommend OPG if you experience ataxia (inability to coordinate muscle movement), syncope (fainting due to lack of blood flow to the brain), or stroke. OPG may also be used to determine blood flow after a carotid endarterectomy (removal of a blockage in the carotid artery).
There may be other reasons for your physician to recommend OPG.
OPG is associated with a risk of corneal abrasion (scratches on the surface of the eye). Not rubbing the eyes or inserting contact lenses immediately after the test can reduce this risk.
OPG should not be performed on persons who have had eye surgery within the previous two to six months or in those with cataracts, conjunctivitis (pinkeye), diabetes mellitus, uncontrolled glaucoma, lens implantation, or a history of retinal detachment.
The test is usually not performed on persons allergic to local anesthetics.
Anesthetic eye drops used for the procedure may cause temporary blurred vision and a mild burning sensation when applied.
OPPG is contraindicated in patients using anticoagulant (blood-thinning) medications, due to the increased risk for conjunctival hemorrhage (bleeding that causes the white part of the eye to become red). OPPG may cause a temporary loss of vision.
There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.
Certain factors or conditions may interfere with the results of the test. These include, but are not limited to, the following:
- high blood pressure
- blinking and eye movements
- abnormal heart rhythm (arrhythmias)
- Your physician will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
- In most cases, you will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.
- Generally, no prior preparation, such as fasting or sedation, is required.
- Notify your physician if you have been diagnosed with high blood pressure or abnormal heart rhythm, as these can also influence the results.
- Notify your physician of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
- Notify your physician if you have a history of bleeding disorders or if you are taking any anticoagulant (blood-thinning) medications, aspirin, or other medications that affect blood clotting. It may be necessary for you to stop these medications prior to the procedure.
- If you wear contact lenses, you will be asked to remove them. You should bring your eyeglasses to the procedure, as you should not insert your contact lenses for several hours after the procedure.
- If you are being treated for glaucoma, consult your physician about taking your medication and eye drops.
- Due to the possibility of blurred vision after the procedure, you should plan to have someone drive you home afterwards.
- Based upon your medical condition, your physician may request other specific preparation.
OPG may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.
Generally, OPG follows this process:
- You will be asked to lie on your back on a table or a bed.
- You will be asked to avoid blinking or moving your eyes, as this can interfere with the test results.
- Anesthetic eye drops will be placed in your eyes.
- Earlobe pulse sensors will be placed on each earlobe.
- The blood flow in each earlobe will be recorded.
- Suction cups resembling contact lenses will be applied to both eyes.
- The blood flow in each eye will be recorded.
- The time difference in the pulse rate between each eye and the eyes and the ears will be measured. A delay in blood flow suggests that a blockage is present in the carotid artery.
Once the test is complete, you will be observed for any eye discomfort or light sensitivity, which may indicate corneal abrasion. You will also be instructed not to rub your eyes or insert contact lenses for a few hours.
Your vision may be temporarily blurred. However, once you are home you should contact your physician if blurred vision or any burning or discomfort continues.
Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.