Using Noncontact Tonometry in Patients After Myopic LASIK Surgery  

Cornea. 2006 Jan;25(1):26-28.  

Cheng AC, Fan D, Tang E, Lam DS.  

PURPOSE:: To evaluate the effect of corneal curvature and corneal thickness on the assessment of intraocular pressure (IOP) using noncontact tonometry (NCT) in patients after myopic LASIK surgery.  

METHODS:: All patients who had myopic LASIK in a university-based eye clinic between February 2002 and May 2002 were retrospectively analyzed. Preoperative NCT was compared with postoperative NCT, postoperative corneal thickness, and postoperative corneal curvature.  

RESULTS:: The difference between the mean preoperative NCT (15.46 +/- 2.50 mm Hg) and postoperative NCT (6.30 +/- 1.57 mm Hg) was significant (9.16 +/- 1.96 mm Hg, P < 0.010). Preoperative NCT significantly correlated with postoperative NCT (P < 0.001), postoperative corneal thickness (P = 0.006), and postoperative anterior corneal curvature (P < 0.010).  

CONCLUSIONS:: Both corneal thickness and anterior corneal curvature affect IOP assessment in patients with myopic LASIK. Although correction formulas can be used to estimate the actual IOP, alternative methods should be investigated to assess IOP independent of corneal thickness and curvature.  

"Our model can account for only 47% of the variability in the actual postoperative IOP." 


Excerpts from the full text of the article that was posted above:  

"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).1-6 

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,7-9 which is the gold standard. However, NCT has been shown to underestimate IOP measurements in patients with myopic LASIK in various studies,1-6 and different methods have been proposed to determine the actual IOP.10-13 

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,4-9 the effect of corneal curvature is not conclusive.4-9"  

"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.4-8 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,5,10,12,14,15 the results of the association between the IOP and corneal curvature are conflicting.7,9,13,16-20 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.21,22  

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.23  

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."