IntraOcular pressure

What is Ocular Pressure?

Ocular pressure is determined by the balance between the production and drainage of liquids within the eye. The method used to quantify it is called tonometry, which expresses the measurement in millimeters of mercury (mmHg). For the correct function of the eye and its structures, it is very important to maintain the intraocular pressure within values between 10 and 21 mmHg. The maintenance of pressure within this physiological range is necessary to preserve the optimal anatomical conditions for refraction, therefore to allow a correct vision. From the physiological point of view, in fact, the intraocular pressure helps to stabilize the shape of the eyeball and protects it from deformations that could be caused by the weight of the eyelids and the tone of the extraocular muscles. Moreover, it prevents the formation of swellings, through the drainage and the return to the blood circulation of the liquids containing the waste metabolites.

Several factors can temporarily influence the blood pressure level, such as diurnal deviation, heart rate, alcohol and caffeine consumption, exercise and fluid intake or some systemic and topical medications. A pathological alteration of eye pressure, on the other hand, can have unpleasant consequences for visual function and can occur without the patient being aware of it.

The high pressure inside the eye is an important indicator in the evaluation of glaucoma, of which it is a risk factor. This eye disease, in general, does not cause pain or particular symptoms, but produces characteristic alterations to the optic nerve and to the neural cells present in the retina. If glaucoma continues to progress and is not adequately treated, it can affect peripheral vision and cause irreversible damage to the optic nerve, which can lead to blindness. In most cases, pressure is harmful when greater than 21 mmHg, but some patients may suffer negative consequences for lower intraocular pressures (normotensive glaucoma). On the contrary, some people can tolerate higher than normal blood pressure without suffering damage to the optic nerve or loss of the visual field ( ocular hypertension ).

To better understand: the dynamics of aqueous humor

The eye is a closed spheroid formation, hollow inside.

In each eyeball, two cavities can be distinguished:

The anterior cavity, smaller, can in turn be divided into two chambers ( anterior : between iris and cornea; posterior : between iris and crystalline), both filled with aqueous humor (liquid).
The vitreous chamber is the ample posterior cavity, which contains the vitreous body (or vitreous humor), gelatinous and transparent. This acts as a support to the posterior portion of the lens and to the retina, pressing the neural layer against the pigmented layer. The vitreous body has a relatively fixed volume and is not involved in the regulation of eye pressure.

Both the vitreous body and the aqueous humor contribute to stabilize the shape and position of the eye.

To maintain a constant eye pressure, the eye continuously produces a small amount of aqueous humor, while an equal level of this fluid is drained through a complex network of cells and tissues located in the anterior chamber near the ciliary body. With its continuous circulation, the aqueous humor acts as a transporter of metabolites and waste substances.

The aqueous humor is produced as an interstitial fluid, mainly by mechanisms of active secretion, just behind the edge of the iris, in the ciliary bodies. A minor production path takes place for plasma ultrafiltration. The aqueous humor passes through the epithelial cells of the ciliary processes and flows from the posterior chamber onto the surface of the crystalline lens, overcomes the iris and circulates in the anterior chamber, where it is removed. Most resorption occurs through the trabecular meshwork and Schlemm's canal (or venous sinus of the sclera) in the corner between the cornea and the iris. The aqueous humor passes through the progressively smaller pores that make up the trabecular network and the cells that line the canal wall. The Schlemm's canal constitutes an outflow towards the venous circle of the eye: it communicates directly with the episcleral veins and the absorption by this pathway therefore depends on the gradient of the intraocular pressure.

The implications of the above are:

The production of aqueous humor is largely constant. Under normal conditions, an increase in eye pressure will be compensated to some extent by an increase in fluid drainage.
In the presence of pathological changes, intraocular pressure decreases when fluid production is lower or excessive drainage. On the other hand, if the aqueous humor is produced excessively and / or if it does not flow properly due to an insufficient permeability of the drainage system (as happens, for example, in glaucoma), the pressure of the liquid at the internal eye increases causing ocular hypertension .

Factors influencing eye pressure

In healthy people, eye pressure is generally between 10 and 21 mmHg (the average value is 15.5 mmHg with fluctuations of ± 2.75 mmHg). The range of physiological values is relatively wide and the individuality of the case must always be considered; however, values above or below these limits must be defined as "suspicious".

Although elevated intraocular pressure is not the only clinical sign in the diagnosis of glaucoma-related disorders, it is still one of the most important. Therefore, the clinical distinction between physiological, suspicious and pathological values is very important.

Mild changes in blood pressure levels are normal: they can occur from one season to another or even during the day and night. The diurnal variation in healthy subjects is between 3 and 6 mmHg, while it may increase in glaucomatous patients and with ocular hypertension.

The values of eye pressure are higher in the morning, particularly immediately after waking up and tend to decrease over the course of the day. However, this is only valid for about 80% of people and is a factor to consider when trying to find the real values of eye pressure for a particular patient (it would be ideal if the measurements were performed at different times during the day). Ocular pressure also depends on the thickness of the cornea. This last parameter is measured in any patient to correctly interpret the meaning of the data found.

Changes in eye pressure can be caused by anatomical problems, inflammation, trauma or eye infections, genetic factors and the use of certain drugs. The pressure level of the eye varies with changes in heart or respiratory rate and can also be influenced by exercise and fluid intake. Coughing, vomiting and lifting heavy objects can also cause temporary changes in eye pressure. Alcohol consumption induces a transient reduction, while caffeine can increase blood pressure. Recently, this effect was also found among players of some wind instruments.

A significant and persistent change in eye pressure, not adequately treated, can cause vision problems and induce the onset of eye diseases. Abnormal ocular pressure values usually do not cause symptoms. For this reason, it is important to regularly undergo eye examinations to detect changes.

Causes

Pathological changes in eye pressure can be caused by:

Production of excess or defective fluids;
Inadequate or increased drainage;
Long-term use of some drugs: for example corticosteroids used to treat asthma and other conditions have been shown to increase the risk of ocular hypertension in some individuals;
Eye trauma: an eye injury can affect the balance between production and drainage of intraocular fluids. Sometimes, this consequence can occur months or years after the eye trauma;
Other eye diseases: hypertension has been associated with a number of other ocular pathologies, including pseudoexfoliation syndrome and pigment dispersion syndrome. According to the researchers, even people with thinner corneal thickness may be exposed to a greater risk of ocular hypertension and glaucoma.
Corrective eye surgery: the measured intraocular pressure values are influenced by the corneal thickness and stiffness. As a result, some forms of refractive surgery (such as photorefractive keratectomy) can give a normal outcome, when in reality the pressure can be high.

Ocular hypertension

The term ocular hypertension refers to any situation where the pressure inside the eye is higher than normal. Although its definition has evolved over the years, this condition commonly presents the following criteria:

An eye is considered hypertensive if the pressure is constantly equal to 21 mmHg or higher (measured two or more times in both eyes);
The optic nerve appears normal;
No sign of glaucoma is evident from an examination of the visual field (assesses peripheral vision);
No signs of other eye diseases are present.

Ocular hypertension is used to describe individuals who should be carefully monitored for the onset of glaucoma. For this reason, another term that may refer to an increase in eye pressure is "suspected glaucoma".

Ocular hypotonia

Intraocular pressure is typically defined as equal to or less than 5 mmHg. This low blood pressure could indicate excessive drainage or fluid leakage from the eyeball. When the eye pressure is too low it can cause vision problems, although the manifestations may vary: some people may experience visual symptoms at 10 mmHg, others remain asymptomatic even at 6 mmHg. Hypotonia can be treated with a variety of techniques, depending on the cause.

Diagnosis

The most commonly used diagnostic technique of determining the ocular tone, ie the internal pressure of the eye, is tonometry .

The ophthalmologist can use one of the following tonometry methods to measure intraocular pressure:

Applanation tonometry: measures the mechanical force necessary to temporarily flatten a small part of the corneal surface. Applanation tonometry requires the use of a slit lamp, with the aid of fluorescein under topical anesthesia. Measurements are taken for both eyes on at least 2-3 occasions. The values thus obtained must be compared to the results of central tachymetry (which measures the corneal thickness).
Non-contact tonometry (or air jet tonometry) : calculates the intraocular pressure using a jet of air directed at the corneal surface. The impulse is able to determine a rapid corneal applanation. This technique is safe, as no device comes into contact with the eye and topical anesthesia is not required. However, some specialists consider tonometry without contact less precise than the previous one.

If abnormal intraocular pressures are detected, further tests may be needed for diagnostic confirmation. These tests may include ophthalmoscopy, visual field examination, and pachymetry.

Gonioscopy. Gonioscopy is a diagnostic technique used to examine the space between the iris and the cornea, at the level of the anatomical angle, where the aqueous humor outflow channels are present. The ophthalmologist is not able to see the drainage angle by looking directly at the front of the eye, but can use a mirror lens. This test is important to determine if the drainage angles are open, reduced or closed and to rule out other conditions that could cause high eye pressure.
Ophthalmoscopy . The ophthalmologist examines the structures present in the back of the eye (ocular fundus). The ophthalmoscope is an instrument that projects a beam of light onto the retina, through the pupil and provides detailed information on the internal ocular structures, with particular attention to the optic nerve.
Examinations of the visual field. A visual field test checks peripheral (or lateral) vision and allows the exclusion of visual field defects (another sign of glaucoma).
Pachymetry. Corneal thickness may influence the accuracy of ocular pressure values encountered during tonometric measurements. A thin cornea can give false low pressure readings, while a thick cornea can give an incorrect result compatible with hypertension. During the procedure, a probe, called a pachymeter, is gently placed on the cornea to measure its thickness.

Treatment

Depending on the individual case and the extent of the disorder, the ophthalmologist may decide not to start therapy immediately, merely monitoring eye pressure through regularly scheduled tests: variations that do not affect vision may not require treatment, to unless the patient is at risk for glaucoma. In other cases, the doctor may decide to prescribe one or more drugs to reduce ocular hypertension.

Topical therapy is often the first line of treatment to achieve blood pressure reduction. Using eye drops can improve the condition, but to be effective it is important to follow the prescribed regimen. Adherence to the indicated treatment, in fact, can represent a problem for people with ocular hypertension, often asymptomatic. For this reason it is important to be aware that the lack of therapeutic intervention could lead to a further increase in intraocular pressure, which in turn can induce damage to the optic nerve and permanent loss of vision. The ophthalmologist, in general, to define the effectiveness of the treatment plan in treatment, plans a visit over several weeks to measure the parameter again. Lowering eye pressure, either by using eye drops or surgery, can prevent the progression of glaucoma.