TEAR OSMOLARITY AND GLAUCOMA

Tear osmolarity is a very reliable index to assess the damage caused by the medical therapy of glaucoma in the ocular surface.

Medical therapy is the first line treatment in open-angle glaucoma (80% of glaucoma). However, we know that its prolonged use causes multiple alterations in the ocular surface (OSD), especially when drugs with preservatives (1) and even more if it’s Benzalconium Chloride (BAK) (2).

Epidemiological studies show the relationship between antiglaucoma treatment and significant changes on the ocular surface (3-8), with the presence of symptoms in 60% of cases (5), percentage which increases in proportion to the number of drugs used and whose maximum is reached when it’s a three-drug treatment (8) and when it expands over time (9).

Cases with changes on the ocular surface ((OSD), are usually recognised by the tear hiperosmolarity (the most frequent and the most relevant sign), independent to the origin. The tear hiperosmolarity induces apoptosis and inflammatory processes of the ocular surface, delivering cytokines and some other inflammatory mediators.

The literature review (15-19) shows that 60% of patients who suffer from glaucoma and are treated medically, had changes consistent with ocular surface disease (Figure 1), 67.5% had alterations in the stability of the tear film (BUT reduced) and 47.5% had a higher tear osmolarity than 308 mOsm / L. normal limit value.

Figura 1.- Superficial punctata queratitis in a glaucomatous patient with medical therapy. Slit lamp picture (A) and with fluorescein staining (B).

Figura 1.- Superficial punctata queratitis in a glaucomatous patient with medical therapy. Slit lamp picture (A) and with fluorescein staining (B).

In a recent study (20), statistical analysis shows a high correlation between the values ​​of osmolality and the rate OSDI, which measures the symptomatology of the ocular surface’s damage: r: 0.486; P = 0.002, as well as the BUT, which measures the stability of the tear film: r = −0.49; P = 0.009. A high correlation between osmolarity and the number of drugs used (r = 0.409, P = .009), the number of instillations (r = 0.405, P = 0.01), and the number of instillations of drugs with preservatives (r = 0.629, P, 0.0001) is also shown. No correlation was observed between osmolarity and Schirmer test and with vital dyes as well.

It is evident that the degree of severity and symptoms of involvement of the ocular surface is strongly correlated with tear osmolarity (21-28). The more pain felt, the higher the osmolarity will be. (P = 0.016) (20).

Symptoms of discomfort which goes with the treatment of glaucoma (generally due to dry eye), is one of the main factors which explain the poor adherence to treatment or disengagement (26). That’s why it’s really important to evaluate the ocular surface before starting treatment and during this, too, to highlight any changes which need to be treated.

It is important to know that the tests used in the assessment of dry eye have a different value in these patients. The Schirmer test and vital dyes are hardly correlated with the values of ​​OSDI in glaucomatous patients, as it’s not a real dry eye, is due to the toxic effect of the drugs, and even more when preservatives are used.

Benzalconium Chloride (BAK) is one of the most frequent preservatives used in ophthalmic drugs. BAK causes an elevation of the tear osmolarity as it has a detergent effect (quaternary ammonium). BAK acts on lipid layer reducing the stability of the tear film, increasing the evaporative rate and thus increasing the osmolarity, the consequence is an evaporative type dry syndrome. The increase in osmolarity is a fact reflected in the BUT, with a really high correlation between them: Osmolarity increases and BUT is reduced. As the dryness of the eye is not caused by the deficit of the tear secretion, there’s no correlation with the Schirmer test, which keeps with normal values.

Once shown these dates, we think that a patient who suffers from glaucoma, (whether the person has just been diagnosed and the medical treatment is planed, and also if he or she has already been treated), the evaluation of the ocular surface is really important as complementary treatments will have to be approached to any alterations seen, so that it gets better or even to be operated on as an alternative, just not to damage anymore the ocular surface and ovoid the possible breach of the treatment.

To evaluate the status of the ocular surface, especially in patients who are already being medically treated, the test of tear osmolarity is shown as the most significant factor to make decisions. It consists in a no painful test easy to perform and highly accurate, especially now with the new systems as TearLab, based on detection with a chip that requires only 50 nL (Figure 2). (21-22).

Figura 2.- Tear osmolarity measurement with Tear Lab system

Figura 2.- Tear osmolarity measurement with Tear Lab system

The analysis of the tear osmolarity is thought to be a very important test to value the medical treatment applied to patients who suffer from glaucoma and their possible harmful effect on the ocular surface, causing or aggravating the syndrome of the evaporative dry eye.

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