pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
Tonometry
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 ...
is the method
eye care professional
An eye care professional (ECP) is an individual who provides a service related to the eyes or vision. It is any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level ...
s use to determine this. IOP is an important aspect in the evaluation of patients at risk of
glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
. Most tonometers are calibrated to measure pressure in millimeters of mercury ( mmHg).
Physiology
Intraocular pressure is determined by the production and drainage of
aqueous humour
The aqueous humour is a transparent water-like fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the poster ...
by the
ciliary body
The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliar ...
and its drainage via the
trabecular meshwork
The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the ...
and uveoscleral outflow. The reason for this is because the
vitreous humour
The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humor ...
in the
posterior segment
The posterior segment or posterior cavity is the back two-thirds of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve.
:
Where:
* is the IOP in millimeters of mercury (mmHg)
* the rate of aqueous humour formation in microliters per minute (μL/min)
* the resorption of aqueous humour through the uveoscleral route (μL/min)
* is the facility of outflow in microliters per minute per millimeter of mercury (μL/min/mmHg)
* the episcleral venous pressure in millimeters of mercury (mmHg).
The above factors are those that drive IOP.
Measurement
Palpation is one of the oldest, simplest, and least expensive methods for approximate IOP measurement. Intraocular pressure is measured with a
tonometer
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 calibrate ...
as part of a comprehensive
eye examination
An eye examination is a series of tests performed to assess vision and ability to focus on and discern objects. It also includes other tests and examinations pertaining to the eyes. Eye examinations are primarily performed by an optometrist, ...
.
Measured values of intraocular pressure are influenced by
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
l thickness and rigidity. As a result, some forms of refractive surgery (such as
photorefractive keratectomy
Photorefractive keratectomy (PRK) and laser-assisted sub-epithelial keratectomy (or laser epithelial keratomileusis) (LASEK) are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lense ...
) 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.
Classification
Current consensus among
ophthalmologists
Ophthalmology ( ) is a surgical subspecialty within medicine that deals with the diagnosis and treatment of eye disorders.
An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a medic ...
and
optometrists
Optometry is a specialized health care profession that involves examining the eyes and related structures for defects or abnormalities. Optometrists are health care professionals who typically provide comprehensive primary eye care.
In the Un ...
defines normal intraocular pressure as that between 10 mmHg and 20 mmHg. The average value of intraocular pressure is 15.5 mmHg with fluctuations of about 2.75 mmHg.
Ocular hypertension
Ocular hypertension is the presence of elevated fluid pressure inside the eye (intraocular pressure), usually with no optic nerve damage or visual field loss.
For most individuals, the normal range of intraocular pressure is between 10 mmHg and 2 ...
(OHT) is defined by intraocular pressure being higher than normal, in the absence of
optic nerve
In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...
damage or
visual field
The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...
loss.
Ocular hypotension, hypotony, or
ocular hypotony
Ocular hypotony, or ocular hypotension, or shortly hypotony, is the medical condition in which intraocular pressure (IOP) of the eye is very low.
Description
Normal IOP ranges between 10–20 mm Hg. The eye is considered hypotonous if the IOP is â ...
, is typically defined as intraocular pressure equal to or less than 5 mmHg. Such low intraocular pressure could indicate fluid leakage and deflation of the eyeball.
Influencing factors
Daily variation
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 despite the slower production of aqueous humour.
Glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
patients' 24-hour IOP profiles may differ from those of healthy individuals.
Fitness and exercise
There is some inconclusive research that indicates that exercise could possibly affect IOP (some positively and some negatively).
Musical instruments
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".
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.
The range of intraoral pressure involved in various classes of ethnic wind instruments, such as
Native American flute
The Native American flute is a flute that is held in front of the player, has open finger holes,
and has two chambers: one for collecting the breath of the player and a second chamber which creates sound.
The player breathes into one end of the ...
s, has been shown to be generally lower than Western classical wind instruments.
Drugs
Intraocular pressure also varies with a number of other factors such as
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
rate,
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
, fluid intake, systemic medication and topical drugs. Alcohol and
marijuana
Cannabis, also known as marijuana among other names, is a psychoactive drug from the cannabis plant. Native to Central or South Asia, the cannabis plant has been used as a drug for both recreational and entheogenic purposes and in various tra ...
consumption leads to a transient decrease in intraocular pressure and
caffeine
Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is mainly used recreationally as a cognitive enhancer, increasing alertness and attentional performance. Caffeine acts by blocking binding of adenosine to ...
may increase intraocular pressure.
Taken orally,
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
(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.
The depolarising muscle relaxant
succinylcholine
Suxamethonium chloride, also known as suxamethonium or succinylcholine, or simply sux by medical abbreviation, is a medication used to cause short-term paralysis as part of general anesthesia. This is done to help with tracheal intubation or ele ...
, which is used in
anaesthesia
Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), a ...
, transiently increases IOP by around 10 mmHg for a few minutes. This is significant for example if the patient requires anaesthesia for a
trauma
Trauma most often refers to:
* Major trauma, in physical medicine, severe physical injury caused by an external source
* Psychological trauma, a type of damage to the psyche that occurs as a result of a severely distressing event
*Traumatic i ...
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
The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye, and contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear ...
blood vessels.
Ketamine
Ketamine is a dissociative anesthetic used medically for induction and maintenance of anesthesia. It is also used as a recreational drug. It is one of the safest anesthetics, as, in contrast with opiates, ether, and propofol, it suppresses ...
also increases IOP.
Significance
Ocular hypertension is the most important risk factor for
glaucoma
Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...
.
Intraocular pressure has been measured as a reviewedary outcome in a systematic review comparing the effect of neuroprotective agents in slowing the progression of open angle glaucoma.
Differences in pressure between the two eyes are often clinically significant, and potentially associated with certain types of glaucoma, as well as
iritis
Uveitis () is inflammation of the uvea, the pigmented layer of the eye between the inner retina and the outer fibrous layer composed of the sclera and cornea. The uvea consists of the middle layer of pigmented vascular structures of the eye and ...
or retinal detachment.
Intraocular pressure may become elevated due to anatomical problems,
inflammation
Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
of the eye, genetic factors, or as a
side-effect
In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequence ...
from
medication
A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and re ...
. 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.