Laser in situ keratomileusis (LASIK) has gained popularity over recent decades and become a widely accepted type of corneal refractive surgery. During myopic LASIK, corneal stromal ablation with the excimer laser results in reduced corneal thickness and curvature. Such changes affect the measurement of intraocular pressure (IOP).
Noncontact pneumatic tonometry (NCT) is a simple and safe device for routine IOP measurements. Previous data have shown that NCT can produce accurate IOP assessment comparable to Goldmann tonometry, which is the gold standard. However, NCT has been shown to underestimate IOP measurements in patients with myopic LASIK in various studies, and different methods have been proposed to determine the actual IOP.
Before a better device can be designed, it is important to identify the factors that cause the underestimation in LASIK patients. Although numerous studies have shown that corneal thickness plays an important role, the effect of corneal curvature is not conclusive.
In this study, we retrospectively evaluated the measurements of IOP using NCT before and after LASIK surgery and their relationship with the postoperative corneal thickness and corneal curvature.
DISCUSSION
The measurement of IOP can be based on the indentation method or the applanation method. With either technique, measurement is performed by deforming the globe and correlating the force responsible for the deformation with the pressure within the eye. Errors in both measuring techniques arise from the variations of corneal rigidity and corneal curvature. NCT deforms the corneal apex by means of a jet of air. The force of the air jet, which is generated by a solenoid-activated piston, increases linearly over time. The time required to flatten the cornea is directly related to the force of the jet necessary to flatten the cornea and correspondingly to the intraocular pressure. It has been shown to be as accurate as applanation in many studies over the normal range.
In a busy refractive clinic, NCT has become a very effective screening tool for the assessment of IOP. However, it has also been shown to underestimate IOP in patients with myopic LASIK. Therefore, it is important to know the effect of LASIK on IOP measurement by NCT.
In myopic LASIK, the corneal thickness is reduced. With less corneal tissue producing counterpressure, less force is required to deform the cornea. At the same time, myopic LASIK also flattens the cornea. With a flatter cornea, the anterior corneal surface does not need to deform as much to reach the applanation area.
Although many studies found the association of corneal thickness with manifest IOP, the results of the association between the IOP and corneal curvature are conflicting. One of the reasons is that previous studies used the direct keratometry reading obtained from corneal topography or keratometer for the assessment. However, direct keratometry readings from the device are known to be inaccurate in patients after corneal refractive surgery like LASIK.
With existing keratometers and videokeratoscopes, the radius of curvature of the anterior corneal surface is what is actually measured. The keratometric diopters are derived form radius of curvature using an effective refractive index in a paraxial formula where K = (n - 1)/r. The refractive index between air and the anterior corneal surface is 1.376. Therefore, the refractive power of the anterior corneal surface should be 0.376/r. However, these devices are calibrated to give the true corneal power. The assessment of the true corneal power is based on the assumption that the relationship between the anterior and posterior curvature and the distance between them is a constant. Based on the Gullstrand eye model, the 2 refracting surfaces can be considered as 1 with a fictitious single refractive index of 1.3375. This is the refractive index that most keratometers and videokeratoscopes use.
After refractive surgery, the basic assumption no longer holds because the anterior corneal curvature changes and the posterior curvature remains constant. The distance between the 2 refractive surfaces is also significantly reduced. Therefore, the basic assumption of the Gullstrand eye model is no longer valid. The direct keratometry readings from these devices are therefore inaccurate.
Furthermore, when the tonometer applanates the cornea, or when the jet of air is blown onto the cornea, it is the deformation of the anterior cornea that gives the endpoint of the assessment. Therefore, anterior curvature data should be used in the evaluation of the underestimation of NCT readings. In our study, we found a significant association of both the corneal thickness and the anterior curvature with the actual IOP.
However, adding anterior corneal curvature as a second variable can explain only an additional 3% of the variability in the model even though the difference is significant. In other words, the contribution of anterior curvature to the manifest postoperative IOP is small and may be significant in high corrections. With a large number of cases among the high myopia range, this may be one of the reasons we are able to show the significance.
Although we are able to show a significant association between the IOP and the corneal thickness and anterior curvature, our study is limited by a relatively small sample size. Our model can account for only 47% of the variability in the actual postoperative IOP. This raises the possibility that factors other than postoperative corneal thickness and postoperative anterior curvature may affect the manifest IOP. This may also be because the IOP we are using for comparison is the preoperative value, which is not necessary be the same as the actual postoperative pressure. Further studies using direct postoperative IOP measurement will be required to identify some of these factors.
Last, although a correction factor can be added to the manifest IOP to assess the actual IOP, other IOP measurement techniques such as pressure phosphene tonometry should also be considered. Pressure phosphene tomometry is a device that measures IOP independent of corneal rigidity and the corneal curvature and theoretically can give better results in LASIK patients.
No comments:
Post a Comment