The Dreaded Air Puff Test
Why do patients have to suffer through the air puff test? What is it measuring? Is there an easier way to get this measurement?
For many patients the dreaded air puff test is the most dreaded part of the eye exam. You're left sitting there in suspense and the moment you relax - BAM! - you're blasted by a shot of air right in the eye. The Eye Doctor isn't trying to torture you. The air puff test is done to measure the pressure in the eye to see if there is risk of vision loss from glaucoma or vascular occlusion.
Eye pressure is a measure of fluid pressure in the eye. Aqueous humor is the fluid that causes pressure to be high or low. If aqueous is over produced and/or not drained fast enough from the eye the result is a high eye pressure. Low eye pressure results from under production and/or over drainage of aqueous. Gonioscopy is done to examine the drainage structures in patients with abnormal eye pressure.
Eye pressure is a necessary thing, but when the pressure is too low or too high the eye can be severely and permanently damaged. When the pressure in the eye gets below 6 mm we worry about phthisis bulbi. In this condition the eye collapses in on itself due to a lack of pressure to support its internal structures.
When the pressure in the eye is above 20 mm Hg we start to worry about glaucoma. The appearance of the optic nerve is critical in determining if glaucoma is present. The optic nerve looks somewhat like a yellow/orange donut. It represents all of the nerve fibers collected from the millions of rods and cones in the retina.
The optic nerve also represents a soft spot (less dense surrounding structures) in the eye. When the pressure is above 20 mm Hg, the optic nerve is the structure that is more susceptible to damage by excavation and erosion of the optic nerve's neuroretinal rim (the ring of the donut).
When the Eye Doctor examines the optic nerve they look at the neuroretinal rim, the optic cup (hole in the donut) and surrounding blood vessels to check for damage to the optic nerve. If damage is present this is considered glaucoma, if no damage is present this is considered ocular hypertension. The Eye Doctor will decide if adfitional testing is needed to confirm glaucoma is present or if the patient just needs to be closely followed.
To treat glaucoma eye pressure needs to be reduced. Also, an eye pressure over 30 mm Hg requires treatment as it will result in glaucoma if it isn't caught early. Eye pressure can be reduced by drops, pills, laser treatment or surgery.
Eye pressure measurements (IOP) can be thrown off by many things. Corneal thickness is the most common variable to throw off IOP measurement. The cornea has an average thickness of 555 microns (about half a millimeter). If the cornea is thicker than average the IOP reading is higher than the patient's true IOP. The measurement is falsely low for a thin cornea.
Over wearing contact lenses can cause swelling in the cornea and produce a false low reading. Diuretics or excessive alcohol intake before an eye exam can also cause a false low reading. Avoid overwraring your contacts or partying too hard before your exam - it could prevent your Eye Doctor from knowing your true risk for vision loss.
Some key information on the air puff machine:
Air puff machine / NCT (non-contact tonometer)
Uses a rapid air pulse to flatten (applanate) the cornea. The amount of applanation and corresponding IOP are detected by an electro-optical system. The NCT is used by many practitioners because it is fast and easily done by a technician. It has improved accuracy, but still often overestimates IOP (especially with stiffer corneas).
Alternatives to the air puff test
Goldmann Applanation Tonometer (GAT)
Goldmann is the gold standard in IOP measurement. It requires an anesthetic to numb the eye. A prism head is placed against the cornea and a dial is turned to see how much weight is required to temporarily flatten 3.06 mm of the cornea via the flawed Imbert-Fick law (IOP = force / area).
Tono-pen or Accu-pen
The Tono-pen is a digital handheld tonometer with an outer ring that flattens the cornea and a central transducer to detect the pressure applied to flatten the cornea. It is often used because it is easy to carry and use. It does require anesthetic and isn't as accurate as the Goldmann often overestimating IOP.
The future of IOP measurement:
Pascal / Dynamic Contour Tonometry (DCT)
Pascal IOP measurement is state-of-the-art. It takes hundreds of readings per second and even accounts for corneal thickness in its measurements. Pascal provides an ocular pulse amplitude or rating of optic nerve blood flow and also a quality reading to assess the confidence of a measurement. It requires anesthetic and can be difficult to sit through. It requires eight seconds of contact between its probe and the cornea. It has been found to be more reproducible and consistent than GAT, but reports higher readings than GAT.
Diaton Transpalpebral Tonometer
The Diaton can take measurements through the eyelid over sclera (white of eye). It is the easiest to tolerate. No anesthetic, no air puff and no corneal contact. Unfortunately, it isn't accurate enough to replace other methods at this time.
Summary
The air puff test is medically necessary and potentially vision saving.
Optic nerve appearance determines if the eye pressure value is safe.
Don't overwear contact lenses or drink too much alcohol before an eye exam.
Any eye pressure reading over 30 mm Hg requires treatment.
Eye pressure can be reduced with drops, pills, laser, or surgery.
There are alternatives to the air puff test:
Air puff and Tono-pen - fast and easy
Goldmann - is the gold standard
Pascal - most data, hard to tolerate
Diaton - easiest to toletate, least accurate
For many patients the dreaded air puff test is the most dreaded part of the eye exam. You're left sitting there in suspense and the moment you relax - BAM! - you're blasted by a shot of air right in the eye. The Eye Doctor isn't trying to torture you. The air puff test is done to measure the pressure in the eye to see if there is risk of vision loss from glaucoma or vascular occlusion.
Eye pressure is a measure of fluid pressure in the eye. Aqueous humor is the fluid that causes pressure to be high or low. If aqueous is over produced and/or not drained fast enough from the eye the result is a high eye pressure. Low eye pressure results from under production and/or over drainage of aqueous. Gonioscopy is done to examine the drainage structures in patients with abnormal eye pressure.
Eye pressure is a necessary thing, but when the pressure is too low or too high the eye can be severely and permanently damaged. When the pressure in the eye gets below 6 mm we worry about phthisis bulbi. In this condition the eye collapses in on itself due to a lack of pressure to support its internal structures.
When the pressure in the eye is above 20 mm Hg we start to worry about glaucoma. The appearance of the optic nerve is critical in determining if glaucoma is present. The optic nerve looks somewhat like a yellow/orange donut. It represents all of the nerve fibers collected from the millions of rods and cones in the retina.
The optic nerve also represents a soft spot (less dense surrounding structures) in the eye. When the pressure is above 20 mm Hg, the optic nerve is the structure that is more susceptible to damage by excavation and erosion of the optic nerve's neuroretinal rim (the ring of the donut).
When the Eye Doctor examines the optic nerve they look at the neuroretinal rim, the optic cup (hole in the donut) and surrounding blood vessels to check for damage to the optic nerve. If damage is present this is considered glaucoma, if no damage is present this is considered ocular hypertension. The Eye Doctor will decide if adfitional testing is needed to confirm glaucoma is present or if the patient just needs to be closely followed.
To treat glaucoma eye pressure needs to be reduced. Also, an eye pressure over 30 mm Hg requires treatment as it will result in glaucoma if it isn't caught early. Eye pressure can be reduced by drops, pills, laser treatment or surgery.
Eye pressure measurements (IOP) can be thrown off by many things. Corneal thickness is the most common variable to throw off IOP measurement. The cornea has an average thickness of 555 microns (about half a millimeter). If the cornea is thicker than average the IOP reading is higher than the patient's true IOP. The measurement is falsely low for a thin cornea.
Over wearing contact lenses can cause swelling in the cornea and produce a false low reading. Diuretics or excessive alcohol intake before an eye exam can also cause a false low reading. Avoid overwraring your contacts or partying too hard before your exam - it could prevent your Eye Doctor from knowing your true risk for vision loss.
Some key information on the air puff machine:
Air puff machine / NCT (non-contact tonometer)
Uses a rapid air pulse to flatten (applanate) the cornea. The amount of applanation and corresponding IOP are detected by an electro-optical system. The NCT is used by many practitioners because it is fast and easily done by a technician. It has improved accuracy, but still often overestimates IOP (especially with stiffer corneas).
Alternatives to the air puff test
Goldmann Applanation Tonometer (GAT)
Goldmann is the gold standard in IOP measurement. It requires an anesthetic to numb the eye. A prism head is placed against the cornea and a dial is turned to see how much weight is required to temporarily flatten 3.06 mm of the cornea via the flawed Imbert-Fick law (IOP = force / area).
Tono-pen or Accu-pen
The Tono-pen is a digital handheld tonometer with an outer ring that flattens the cornea and a central transducer to detect the pressure applied to flatten the cornea. It is often used because it is easy to carry and use. It does require anesthetic and isn't as accurate as the Goldmann often overestimating IOP.
The future of IOP measurement:
Pascal / Dynamic Contour Tonometry (DCT)
Pascal IOP measurement is state-of-the-art. It takes hundreds of readings per second and even accounts for corneal thickness in its measurements. Pascal provides an ocular pulse amplitude or rating of optic nerve blood flow and also a quality reading to assess the confidence of a measurement. It requires anesthetic and can be difficult to sit through. It requires eight seconds of contact between its probe and the cornea. It has been found to be more reproducible and consistent than GAT, but reports higher readings than GAT.
Diaton Transpalpebral Tonometer
The Diaton can take measurements through the eyelid over sclera (white of eye). It is the easiest to tolerate. No anesthetic, no air puff and no corneal contact. Unfortunately, it isn't accurate enough to replace other methods at this time.
Summary
The air puff test is medically necessary and potentially vision saving.
Optic nerve appearance determines if the eye pressure value is safe.
Don't overwear contact lenses or drink too much alcohol before an eye exam.
Any eye pressure reading over 30 mm Hg requires treatment.
Eye pressure can be reduced with drops, pills, laser, or surgery.
There are alternatives to the air puff test:
Air puff and Tono-pen - fast and easy
Goldmann - is the gold standard
Pascal - most data, hard to tolerate
Diaton - easiest to toletate, least accurate
Air puff test made my eye bleed - normal?
ReplyDeleteis 20 puff of air in your eyes consider to many
DeleteWhite text on black background is very hard to read. Perhaps your web designer should speak to an eye Doctor ;-)
ReplyDelete