Intraocular Pressure

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Joe Dispenza
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What is Ocular Pressure?

La eye pressure it is determined by the balance between the production and drainage of fluids 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 keep the intraocular pressure within values ​​between 10 and 21 mmHg. Maintaining the pressure within this physiological range is necessary to maintain the optimal anatomical conditions for refraction, therefore to allow correct vision. From a physiological point of view, in fact, 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. Furthermore, it prevents the formation of swelling, by draining and returning to the blood circulation of the liquids containing the waste metabolites.



Several factors can transiently affect blood pressure, such as diurnal deviation, heart rate, alcohol and caffeine consumption, exercise, and fluid intake or certain systemic and topical medications. A pathological change in ocular pressure, on the other hand, can have unpleasant consequences for visual function and can occur without the patient being aware of it.


The elevated pressure inside the eye is an important indicator in the evaluation of the glaucoma, of which it constitutes a risk factor. This eye disease generally does not cause pain or particular symptoms, but produces characteristic changes in the optic nerve and neural cells in the retina. If glaucoma continues to progress and is not properly treated, it can affect peripheral vision and cause irreversible damage to the optic nerve, which can lead to blindness. In most cases, blood pressure is detrimental when greater than 21 mmHg, but some patients may experience adverse consequences at lower intraocular pressures (normotensive glaucoma). Conversely, some people can tolerate higher than normal blood pressure levels without reporting optic nerve damage or visual field loss (ocular hypertension).


To better understand: the dynamics of aqueous humor

The eye is a closed spheroidal formation, hollow inside.
In each eyeball, two cavities can be distinguished:

  • La anterior cavity, smaller, can in turn be divided into two chambers (front: between iris and cornea; back: between iris and crystalline), both filled with aqueous humor (liquid).
  • La glass room is the large posterior cavity, which contains the vitreous body (or vitreous humor), gelatinous and transparent. This acts as a support for the posterior portion of the lens and 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 ocular pressure.

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


To maintain constant eye pressure, the eye continually 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.
Aqueous humor is produced as interstitial fluid, predominantly by active secretion mechanisms, just behind the iris margin, in the ciliary bodies. A minor production path occurs for plasma ultrafiltration. The aqueous humor passes through the epithelial cells of the ciliary processes and flows from the posterior chamber to the surface of the lens, passes the iris and circulates to the anterior chamber, where it is removed. Most resorption occurs via the trabeculae and Schlemm's canal (or venous sinus of the sclera) at 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. Schlemm's canal constitutes an outflow path towards the venous circulation of the eye: it communicates directly with the episcleral veins and absorption through this path 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 drainage is excessive. On the other hand, if the aqueous humor is produced to an excessive extent and / or if it does not flow properly due to insufficient permeability of the drainage system (as happens, for example, in glaucoma), the pressure of the fluid at the inside of the eye increases causing ocular hypertension.

Factors affecting eye pressure

In healthy people, eye pressure is generally between 10 and 21 mmHg (the mean 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 should be defined as "suspicious".


While 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.
Slight changes in blood pressure 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 patients with glaucoma and ocular hypertension.
Eye pressure values ​​are highest in the morning, particularly immediately after waking up, and tend to decrease throughout the day. However, this only applies to around 80% of people and is a factor to consider when trying to find the true eye pressure values ​​for a particular patient (it would be ideal if the measurements were taken 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 infections of the eye, by genetic factors, and by the use of certain medications. The pressure level of the eye varies with changes in heart or respiratory rate and can also be affected 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 has also been found among the players of some wind musical instruments.

A significant and persistent change in eye pressure that is not properly treated can cause vision problems and lead to eye disease. Abnormal eye pressure values ​​usually cause no symptoms. For this reason, it is important to have regular eye exams to detect any changes.

Causes

Pathological changes in eye pressure can be caused by:

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

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 features the following criteria:

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

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

Ocular hypotonia

Intraocular pressure is typically defined as 5 mmHg or less. This low blood pressure level could indicate excessive drainage or fluid leaking from the eyeball. When the eye pressure is too low it can cause vision problems, although the manifestations can 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 to determine the ocular tone, that is the internal pressure of the eye, is the Tonometry.
The ophthalmologist may use one of the following tonometry methods to measure intraocular pressure:

  • Tonometria ad applanazione: measures the mechanical force required 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 related to the results of the central tachymetry (which measures the corneal thickness).
  • Non-contact tonometry (or air jet tonometry): calculates intraocular pressure using a jet of air directed to the corneal surface. The impulse is able to determine a rapid corneal applanation. This technique is safe, as no devices come into contact with the eye and topical anesthesia is not required. However, some specialists consider non-contact tonometry to be less accurate than the previous one.

If abnormal intraocular pressures are detected, further tests may be required 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 anatomical angle, where the aqueous humor outflow channels are present. The ophthalmologist cannot 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 drainage angles are open, narrow or closed and to rule out other conditions that could cause elevated eye pressure.
  • Ophthalmoscopy. The ophthalmologist examines the structures 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.
  • Visual field tests. A visual field test checks peripheral (or lateral) vision and helps rule out any visual field defects (another sign of glaucoma).
  • Pachimetria. Corneal thickness can affect the accuracy of ocular pressure values ​​found 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, limiting himself to monitoring the eye pressure through regularly scheduled tests: changes that do not affect vision may not require treatment, due 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 for it 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. In order to define the effectiveness of the therapeutic plan in the treatment, the ophthalmologist usually schedules a visit over the course of several weeks to measure the parameter again. Lowering eye pressure, whether using eye drops or surgery, can prevent the progression of glaucoma.


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