Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. [1] Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg).
Intraocular pressure is determined by the production and drainage of aqueous humour by the ciliary body and its drainage via the trabecular meshwork and uveoscleral outflow. The reason for this is because the vitreous humour in the posterior segment has a relatively fixed volume and thus does not affect intraocular pressure regulation.
An important quantitative relationship (Goldmann's equation) is as follows: [2]
Where:
The above factors are those that drive IOP.
Palpation is one of the oldest, simplest, and least expensive methods for approximate IOP measurement, however it is very inaccurate unless the pressure is very high. [3] Intraocular pressure is measured with a tonometer as part of a comprehensive eye examination. Contact lens sensors have also been used for continuous intraocular pressure monitoring. [4]
Measured values of intraocular pressure are influenced by corneal thickness and rigidity. [5] [6] As a result, some forms of refractive surgery (such as photorefractive keratectomy) can cause traditional intraocular pressure measurements to appear normal when in fact the pressure may be abnormally high. A newer transpalpebral and transscleral tonometry method is not influenced by corneal biomechanics and does not need to be adjusted for corneal irregularities as measurement is done over upper eyelid and sclera. [7]
Current consensus among ophthalmologists and optometrists defines normal intraocular pressure as that between 10 mmHg and 20 mmHg. [8] [9] [10] [11] The average value of intraocular pressure is 15.5 mmHg with fluctuations of about 2.75 mmHg. [12]
Ocular hypertension (OHT) is defined by intraocular pressure being higher than normal, in the absence of optic nerve damage or visual field loss. [13] [14]
Ocular hypotension, hypotony, or ocular hypotony, is typically defined as intraocular pressure equal to or less than 5 mmHg. [15] [16] Such low intraocular pressure could indicate fluid leakage and deflation of the eyeball.
Intraocular pressure varies throughout the night and day. The diurnal variation for normal eyes is between 3 and 6 mmHg and the variation may increase in glaucomatous eyes. During the night, intraocular pressure may not decrease [17] despite the slower production of aqueous humour. [18] Glaucoma patients' 24-hour IOP profiles may differ from those of healthy individuals. [19]
There is some inconclusive research that indicates that exercise could possibly affect IOP (some positively and some negatively). [20] [21] [13]
Playing some musical wind instruments has been linked to increases in intraocular pressure. A 2011 study focused on brass and woodwind instruments observed "temporary and sometimes dramatic elevations and fluctuations in IOP". [22] Another study found that the magnitude of increase in intraocular pressure correlates with the intraoral resistance associated with the instrument, and linked intermittent elevation of intraocular pressure from playing high-resistance wind instruments to incidence of visual field loss. [23] The range of intraoral pressure involved in various classes of ethnic wind instruments, such as Native American flutes, has been shown to be generally lower than Western classical wind instruments. [24]
Intraocular pressure also varies with a number of other factors such as heart rate, respiration, fluid intake, systemic medication and topical drugs. Alcohol and marijuana consumption leads to a transient decrease in intraocular pressure and caffeine may increase intraocular pressure. [25]
Taken orally, glycerol (often mixed with fruit juice to reduce its sweet taste) can cause a rapid, temporary decrease in intraocular pressure. This can be a useful initial emergency treatment of severely elevated pressure. [26]
The depolarising muscle relaxant succinylcholine, which is used in anaesthesia, transiently increases IOP by around 10 mmHg for a few minutes. This is significant for example if the patient requires anaesthesia for a trauma and has sustained an eye (globe) perforation. The mechanism is not clear but it is thought to involve contraction of tonic myofibrils and transient dilation of choroidal blood vessels. Ketamine also increases IOP. [27] [28]
Ocular hypertension is the most important risk factor for glaucoma.
Intraocular pressure has been measured as an outcome in a systematic review comparing the effect of neuroprotective agents in slowing the progression of open angle glaucoma. [29]
Differences in pressure between the two eyes are often clinically significant, and potentially associated with certain types of glaucoma, as well as iritis or retinal detachment.
Intraocular pressure may become elevated due to anatomical problems, inflammation of the eye, genetic factors, or as a side-effect from medication. Intraocular pressure laws follow fundamentally from physics. Any kinds of intraocular surgery should be done by considering the intraocular pressure fluctuation. Sudden increase of intraocular pressure can lead to intraocular micro barotrauma and cause ischemic effects and mechanical stress to retinal nerve fiber layer. Sudden intraocular pressure drop can lead to intraocular decompression that generates micro bubbles that potentially cause multiple micro emboli and leading to hypoxia, ischemia and retinal micro structure damage. [30]
Glaucoma is a group of eye diseases that lead to damage of the optic nerve, which transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of sight" because the loss of vision usually occurs slowly over a long period of time. A major risk factor for glaucoma is increased pressure within the eye, known as intraocular pressure (IOP). It is associated with old age, a family history of glaucoma, and certain medical conditions or medications. The word glaucoma comes from the Ancient Greek word γλαυκóς, meaning 'gleaming, blue-green, gray'.
Latanoprost, sold under the brand name Xalatan among others, is a medication used to treat increased pressure inside the eye. This includes ocular hypertension and open-angle glaucoma. Latanaprost is applied as eye drops to the eyes. Onset of effects is usually within four hours, and they last for up to a day.
Tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg), with the normal eye pressure range between 10 and 21 mmHg (13–28 hPa).
Ocular hypertension is the presence of elevated fluid pressure inside the eye, usually with no optic nerve damage or visual field loss.
A glaucoma valve is a medical shunt used in the treatment of glaucoma to reduce the eye's intraocular pressure (IOP).
Brinzolamide is a carbonic anhydrase inhibitor used to lower intraocular pressure in patients with open-angle glaucoma or ocular hypertension.
Dorzolamide, sold under the brand name Trusopt among others, is a medication used to treat high pressure inside the eye, including in cases of glaucoma. It is used as an eye drop. Effects begin within three hours and last for at least eight hours. It is also available as the combination dorzolamide/timolol.
Glaucoma is a group of diseases affecting the optic nerve that results in vision loss and is frequently characterized by raised intraocular pressure (IOP). There are many glaucoma surgeries, and variations or combinations of those surgeries, that facilitate the escape of excess aqueous humor from the eye to lower intraocular pressure, and a few that lower IOP by decreasing the production of aqueous humor.
Brimonidine/timolol, sold under the brand name Combigan among others, is a fixed-dose combination medication eye drop used for the treatment of glaucoma. It is a combination of brimonidine and timolol.
Canine glaucoma refers to a group of diseases in dogs that affect the optic nerve and involve a loss of retinal ganglion cells in a characteristic pattern. An intraocular pressure greater than 22 mmHg (2.9 kPa) is a significant risk factor for the development of glaucoma. Untreated glaucoma in dogs leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness.
Armand Imbert (1850-1922) and Adolf Fick (1829-1901) both demonstrated, independently of each other, that in ocular tonometry the tension of the wall can be neutralized when the application of the tonometer produces a flat surface instead of a convex one, and the reading of the tonometer (P) then equals (T) the IOP," whence all forces cancel each other.
Primary juvenile glaucoma is a subtype of primary congenital glaucoma that develops due to ocular hypertension and is diagnosed between three years of age and early adulthood. It is caused due to abnormalities in the anterior chamber angle development that obstruct aqueous outflow in the absence of systemic anomalies or other ocular malformation.
Normal tension glaucoma (NTG) is an eye disease, a neuropathy of the optic nerve, that shows all the characteristics of primary open angle glaucoma except one: the elevated intraocular pressure (IOP) - the classic hallmark of glaucoma - is missing. Normal tension glaucoma is in many cases closely associated with general issues of blood circulation and of organ perfusion like arterial hypotension, metabolic syndrome, and Flammer syndrome.
Brinzolamide/brimonidine, sold under the brand name Simbrinza, is a fixed-dose combination medication used to reduce intra-ocular pressure in adults with ocular hypertension or in those with an eye condition known as open-angle glaucoma. It contains brinzolamide and brimonidine tartrate. It is used as an eye drop.
Secondary glaucoma is a collection of progressive optic nerve disorders associated with a rise in intraocular pressure (IOP) which results in the loss of vision. In clinical settings, it is defined as the occurrence of IOP above 21 mmHg requiring the prescription of IOP-managing drugs. It can be broadly divided into two subtypes: secondary open-angle glaucoma and secondary angle-closure glaucoma, depending on the closure of the angle between the cornea and the iris. Principal causes of secondary glaucoma include optic nerve trauma or damage, eye disease, surgery, neovascularization, tumours and use of steroid and sulfa drugs. Risk factors for secondary glaucoma include uveitis, cataract surgery and also intraocular tumours. Common treatments are designed according to the type and the underlying causative condition, in addition to the consequent rise in IOP. These include drug therapy, the use of miotics, surgery or laser therapy.
Schwartz–Matsuo syndrome is a human eye disease characterised by rhegmatogenous retinal detachment, elevated intraocular pressure (IOP) and open angle of anterior chamber.
Ghost cell glaucoma (GCG) is a type of secondary glaucoma occurs due to long standing vitreous hemorrhage. The rigid and less pliable degenerated red blood cells block the trabecular meshwork and increase the pressure inside eyes.
Uveitic glaucoma is most commonly a progression stage of noninfectious anterior uveitis or iritis.
Ocular hypotony, or ocular hypotension, or shortly hypotony, is the medical condition in which intraocular pressure (IOP) of the eye is very low.
Hypotony maculopathy is maculopathy due to very low intraocular pressure known as ocular hypotony. Maculopathy occurs either due to increased outflow of aqueous humor through angle of anterior chamber or less commonly, due to decreased aqueous humor secretion by ciliary body.
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