Accuracy of Schiotz Tonometry in Measuring the Intraocular Pressure After Corneal Refractive Surgery

Abstract

PURPOSE:To test the accuracy of Schiotz tonometry (ST) in patients that underwent photorefractive keratectomy (PRK). METHODS: 175 eyes previously treated with PRK for myopia, myopic astigmatism or mixed astigmatism, with a mean preoperative spherical equivalent (SE) of -4.27±2.83 diopters (range: -13 to -0.25 diopters), had their intraocular pressure (IOP) measured, at least 3 months after PRK, with Goldmann applanation tonometry (GAT) and ST with both 5.5-g and 10-g load and also utilizing the Friedenwald nomogram. These measurements were retrospectively compared to the IOP readings obtained with GAT before surgery. RESULTS: Before PRK the mean IOP, as measured with GAT, was 14.7±2.63 mmHg. After PRK, the mean IOP readings were 10.9± 2.85 mmHg (P<0.001) with GAT, 13.58±3.42 mmHg (P<0.001) with 5.5-g load ST; 12.32±3.67 mmHg (P<0.001) with 10-g load ST, and 16.19±5.63 mmHg (P<0.001) with the Friedenwald nomogram. The different methods showed a poor agreement, the worst one being between GAT before surgery and the Friedenwald nomogram (r 2 =0.05). Moreover, ST with both 5.5 g and 10 g load showed an underestimation (more pronounced for the 10-g than for the 5.5-g load) that increased with the amount of treatment. CONCLUSIONS: Although ST with a 5.5 g load seems to be more accurate to measure the true IOP in patients that have undergone PRK compared to the Goldmann technique, it cannot be considered to be accurate in a sufficient number of cases. IOP measurement after refractive surgery is still a problem, and further studies are needed to overcome it. RESUMEN OBJETIVO:Evaluar el grado de precisión de la tonometría de Schiotz (TS) en pacientes que se han sometido a una operación de queratectomía fotorrefractiva (en inglés, PRK). MÉTODOS: Se midió la presión intraocular (PIO) de 175 ojos que habían sido previamente tratados con PRK de miopía con o sin astigmatismo, con una refracción preoperativa (equivalente esférico, o SE) de -4.27±2.83 (rango: -13 a -0,25 dioptrías). Las medidas, realizadas al menos transcurridos 3 meses desde la operación de PRK, utilizaron las técnicas de tonometría por aplanación de Goldmann (TAG) y la TS, esta última con cargas de 5,5 g, de 10 g y utilizando un nomograma de Friedenwald. Estas medidas se compararon retrospectivamente a los valores de PIO obtenidos con TAG antes del procedimiento quirúrgico. RESULTADOS: Antes de la PRK, la PIO media obtenida con TAG fue de 14,7±2,63 mmHg. Después de la PRK, la PIO resultante fue de 10,9±2,85 mmHg (P<0,001) con TAG; de 13,58±3,42 mmHg (P<0,001) con TS y una carga de 5,5 g; de 12,32±3,67 mmHg (P<0,001) con TS y una carga de 10 g; y de 16,19±5,63 mmHg (P<0,001) con TS y utilizando un nomograma de Friedenwald. Los resultados revelan una mala concordancia, siendo las medidas con TAG antes de la operación y las que utilizaron un nomograma de Friedenwald (r 2 =0,05) las que produjeron la peor concordancia. Además, la TS tanto con carga de 5,5 g como con carga de 10 g produjeron estimaciones de la PIO por debajo del valor real (con carga de ...

Intraocular Pressure and Trabecular Meshwork Outflow Facility After Descemet Stripping Endothelial Keratoplasty

“…28,29 Schioetz tonometry is known to be more dependent on biomechanical factors, such as ocular rigidity, 15,30 suffers from a high variability even in normal corneas, and can give false high IOP values in eyes with corneal scars, PK, or after superficial keratectomy. 28,[31][32][33] Grant 34 showed that glaucomatous eyes typically have a reduced trabecular meshwork outflow facility and this was confirmed for eyes after PK too. 35,36 After DSEK we did at no time measure a pathologic outflow facility (C-value).…”

Commentary on “Intraocular Pressure After LASEK”

“…Basically, the result of both theirs [1] and our paper [2] is similar, i.e., Schiotz indentation tonometry (ST) is less affected by previous surface ablation when evaluating the IOP than Goldmann tonometry (GAT).…”

“…Another major difference between the paper by Rosa et al [1] and ours [2] is that we did include normal eyes that were evaluated by the same examiner and using the same three tonometers that were used in the operated eyes, while Rosa et al only performed GAT tonometry in the preoperative exam. By doing so, they have missed the chance to compare GAT and ST readings in normal, unoperated corneas.…”

Results of First In Vivo Trial of an Acoustic Self-Tonometer

“…8,9 These methods have disadvantages similar to those for the GAT and show a higher measurement uncertainty than the GAT. 8,9 The iCare rebound tonometer (iCare Finland Oy, Vantaa, Finland), which shoots a small plastic-covered metal pin onto the cornea from a short distance and measures the rebound velocity, is also contact based but does not require an anesthetic. The measurement uncertainty is higher in comparison to the GAT, with a confidence interval of ±6.6 mm Hg (k p = 2) within the physiological normal pressure range.…”