Patients are not informed that IOP measurements after LASIK are not accurate. This can have serious implications for patients. High IOP must be treated in order to avoid permanent vision loss.

http://www.ncbi.nlm.nih.gov/entrez/quer ... query_hl=1

Cornea. 2006 Jan;25(1):26-28.


Effect of Corneal Curvature and Corneal Thickness on the Assessment of Intraocular Pressure Using Noncontact Tonometry in Patients After Myopic LASIK Surgery.

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.

Graefes Arch Clin Exp Ophthalmol. 2005 Dec;243(12):1218-20. Epub 2005 Jul 8.

Changes in corneal thickness and curvature after different excimer laser photorefractive procedures and their impact on intraocular pressure measurements.

Svedberg H, Chen E, Hamberg-Nystrom H.

St Erik's Eye Hospital, Karolinska Institutet, Polhemsgatan 50, 112 82, Stockholm, Sweden, enping.chen@sankterik.se.

BACKGROUND: Excimer laser refractive surgery alters the shape and thickness of the cornea by removing central corneal tissue with submicrometer precision. The aim of the study was to analyze the changes in central corneal thickness (CCT) and curvature before and after different excimer laser photorefractive procedures and their possible impact on intraocular pressure (IOP) estimations with Goldmann applanation tonometry.

METHODS: Data on CCT, corneal curvature and IOP readings with Goldmann applanation tonometry before and after excimer laser photorefractive surgery were analyzed retrospectively. The data was further analyzed separately in two subgroups; the photorefractive keratectomy /laser-assisted subepithelial keratomileusis (PRK/LASEK) group and the laser in situ keratomileusis (LASIK) group.

RESULTS: The overall post-operative IOP readings were significantly lower than pre-operative values. There was a significant difference in the lowering of the IOP readings between the two subgroups: LASIK caused a lower IOP reading than PRK/LASEK.

CONCLUSION: The change in corneal thickness and curvature affects the estimation of IOP with Goldmann applanation tonometry after excimer laser photorefractive surgery. The amount of reduction in IOP reading might be influenced by the specific laser surgical procedure. This is of clinical importance in the evaluation of any future glaucoma in the increasing number of patients who undergo photorefractive laser surgery.

Am J Ophthalmol. 2006 Mar;141(3):530-536.


A predictive model for postoperative intraocular pressure among patients undergoing laser in situ keratomileusis (LASIK).

Yang CC, Wang IJ, Chang YC, Lin LL, Chen TH.

Department of Ophthalmology, Taipei County San-Chung Hospital, Taipei, Taiwan.

PURPOSE: The aim of this study was to develop a predictive model based on preoperative variables for estimating postoperative intraocular pressure (IOP) of those eyes undergoing LASIK surgery, to predict the amount of underestimated IOP after LASIK for myopia and myopic astigmatism.

DESIGN: Pretest-post-test longitudinal study.

METHODS: Both eyes of 193 eligible subjects who underwent LASIK procedures at the Department of Ophthalmology, National Taiwan University Hospital, from July 2000 to December 2002 for myopia and myopic astigmatism were identified to build up the predictive models. IOPs were measured with noncontact air-puff tonometry. Information on age, gender, preoperative central corneal thickness (CCT), preoperative central corneal curvature (CCK), preoperative spherical equivalent refractive error, and ablation depth was collected and applied for predicting postoperative IOP after LASIK based on linear mixed model.

RESULTS: Significant predictors for postoperative IOP after myopic LASIK procedures included age, gender, preoperative IOP, ablation depth, preoperative CCT, and preoperative spherical equivalent refractive errors. The linear mixed model, taking into account these significant preoperative correlates and the correlation of IOPs between both eyes of the same patient, explained 91% of the variation of postoperative IOP.

CONCLUSIONS: A statistical model was developed for predicting the amount of underestimated IOP after LASIK for myopia and myopic astigmatism, which is of clinical importance to uncover ocular hypertension among patients whose information on postoperative IOP immediately after LASIK is not available.

Surv Ophthalmol. 2005 May-Jun;50(3):245-51.


Comment in:
Surv Ophthalmol. 2005 Nov-Dec;50(6):611-2; author reply 612.

Considerations of glaucoma in patients undergoing corneal refractive surgery.

Bashford KP, Shafranov G, Tauber S, Shields MB.

Department of Ophthalmology and Visual Sciences, Yale University School of Medicine, New Haven, Connecticut; and Glaucoma Consultants of Colorado, P.C., Littleton, Colorado, USA.

Glaucoma patients present a unique set of challenges to physicians performing corneal refractive surgery. Corneal thickness, which is modified during corneal refractive surgery, plays an important role in monitoring glaucoma patients because of its effect on the measured intraocular pressure. Patients undergo a transient but significant rise in intraocular pressure during the laser-assisted in situ keratomileusis (LASIK) procedure with risk of further optic nerve damage or retinal vein occlusion. Glaucoma patients with filtering blebs are also at risk of damage to the bleb by the suction ring. Steroids, typically used after refractive surgery, can increase intraocular pressure in steroid responders, which is more prevalent among glaucoma patients. Flap interface fluid after LASIK, causing an artificially low pressure reading and masking an elevated pressure has been reported. The refractive surgeon's awareness of these potential complications and challenges will better prepare them for proper management of glaucoma patients who request corneal refractive surgery.

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

Cornea. 2006 Jan;25(1):26-28.


Effect of Corneal Curvature and Corneal Thickness on the Assessment of Intraocular Pressure Using Noncontact Tonometry in Patients After Myopic LASIK Surgery.



"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."


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

J Cataract Refract Surg. 2002 Feb;28(2):356-9.

Progression to end-stage glaucoma after laser in situ keratomileusis.

Shaikh NM, Shaikh S, Singh K, Manche E.

Department of Ophthalmology, Stanford University School of Medicine, Stanford, California 94305, USA.

We describe 2 patients, one a glaucoma suspect because of family history and the other with juvenile glaucoma. Both patients developed complications after laser in situ keratomileusis that required frequent topical steroids, leading to steroid-induced glaucoma. In both cases, corneal edema from the acute rise in intraocular pressure (IOP) caused inaccurate IOP measurement by standard methods. The inability to recognize glaucoma early may have resulted in significant irreversible vision loss.

J Biomech Eng. 2006 Feb;128(1):150-60.


Biomechanical modeling of refractive corneal surgery.

Alastrue V, Calvo B, Pena E, Doblare M.

Group of Structural Mechanics and Material Modelling, Aragon Institute of Engineering Research (13A), University of Zaragoza, Spain.

The aim of refractive corneal surgery is to modify the curvature of the cornea to improve its dioptric properties. With that goal, the surgeon has to define the appropriate values of the surgical parameters in order to get the best clinical results, i.e., laser and geometric parameters such as depth and location of the incision, for each specific patient. A biomechanical study before surgery is therefore very convenient to assess quantitatively the effect of each parameter on the optical outcome. A mechanical model of the human cornea is here proposed and implemented under a finite element context to simulate the effects of some usual surgical procedures, such as photorefractive keratectomy (PRK), and limbal relaxing incisions (LRI). This model considers a nonlinear anisotropic hyperelastic behavior of the cornea that strongly depends on the physiological collagen fibril distribution. We evaluate the effect of the incision variables on the change of curvature of the cornea to correct myopia and astigmatism. The obtained results provided reasonable and useful information in the procedures analyzed. We can conclude from those results that this model reasonably approximates the corneal response to increasing pressure. We also show that tonometry measures of the IOP underpredicts its actual value after PRK or LASIK surgery.

Biomechanics of Corneal Refractive Surgery

JOURNAL OF REFRACTIVE SURGERY
Vol. 22 No. 3 March 2006

Dan Z. Reinstein, MD, MA(Cantab), FRCSC; Cynthia Roberts, PhD

Excerpt:

"Some of the evidence pointing to the impact of corneal
biomechanical properties on surgical outcomes lies
in the measurement of intraocular pressure (IOP), both
before and after refractive surgery. It is well known that
measured IOP is reduced, on average, following a refractive
procedure. It has been assumed that this is the
result of reduced curvature and thickness in myopic
procedures. However, Chang and Stulting5 performed
a retrospective review of over 8000 myopic LASIK
patients, and determined that although measured
pressure was reduced on average by approximately
2 mmHg, the range of change was approximately
+10 to -15 mmHg. Every patient in this population
had reduced thickness and curvature, and yet almost
half of them had an increase in measured IOP. Clearly,
the artifact in IOP measurement cannot be explained
by thickness alone, and “correction” of measured IOP
postoperatively using a linear correction factor based
on thickness is problematic.6 This leads to the conclusion
that refractive surgery likely alters the fundamental
biomechanical properties of the cornea.

JOURNAL OF REFRACTIVE SURGERY
Vol. 22 No. 3 March 2006

A Correction Formula for the Real Intraocular Pressure After LASIK for the Correction of Myopic Astigmatism

Markus Kohlhaas, MD; Eberhard Spoerl, PhD; Andreas G. Boehm, MD; Katharina Pollack, MD


PURPOSE
To create a correction formula to determine the real intraocular pressure (IOP) after LASIK considering the altered corneal thickness, corneal curvature, and corneal stability.

METHODS
This prospective clinical trial comprised 101 eyes of 59 patients (34 women and 25 men) that underwent LASIK with a mean preoperative spherical equivalent refraction of –6.3±2.17 diopters (D) (–3.0 to –11.5 D). Mean patient age was 32±9 years. Preoperatively and 6 months postoperatively, IOP (by Goldmann applanation tonometry), keratometry (by topography), and central corneal thickness (CCT) (by ultrasound pachymetry) were evaluated. These parameters were measured in all patients between 8 and 11 o’clock in the morning.

RESULTS
Due to the LASIK procedure, IOP was reduced from 16.5±2.1 mmHg (range: 12 to 22 mmHg) to 12.9±1.9 mmHg (range: 8 to 16 mmHg). Multiple linear regression analysis of the IOP values before and after LASIK showed a significant correlation between the measured IOP and CCT and keratometry values (R2=0.631; P<.001). After LASIK, the biomechanical bending strength of the cornea is reduced by the cut so that the measured IOP must be additionally corrected by 0.75 mmHg. An equation containing all three changes is given: IOP (real) = IOP (measured) + (540-CCT)/71 + (43 - K-value)/2.7 + 0.75 mmHg.

CONCLUSIONS
Intraocular pressure measurements after LASIK for the correction of myopia are inaccurate as a consequence of changes in CCT, corneal curvature, and corneal flap stability. After LASIK, the measured IOP should be corrected to avoid false low IOP applanation readings. [J Refract Surg. 2006;22:263-267.]

J Huazhong Univ Sci Technolog Med Sci. 2006;26(3):372-3, 377.

Measurement of intraocular pressure after LASIK by dynamic contour tonometry.

Liu L, Lei C, Li X, Dong J.

Refract Surgery Center Tongji Hospital, Tongji Medical College, Huazhong university of Science and Technology, Wuhan 430030, China.

Changes of corneal properties induced by laser in situ keratomileusis (LASIK) results in low inaccurate intraocular pressure (IOP) readings by Goldmann applanation tonometry (GAT). Before and after LASIK, the applied value of IOP, measured by dynamic contour tonometry (DCT) in comparison to GAT, was evaluated. Before and 1, 4 weeks after LASIK, the IOP in 30 cases (60 eyes) was measured by GAT and DCT respectively. The obtained results were statistically processed by SPSS11.5 statistical software. The results showed that central corneal thickness (CCT) could affect GAT measurements but not DCT measurements. The comparison of IOP one and 4 weeks after LASIK revealed that the readings from GAT was separately decreased by 5.00 +/- 1.12 and 5.45 +/- 1.13 mmHg as compared with those before LASIK, while those from DCT had no significant difference. It was concluded that LASIK-induced changes of CCT could influence the accuracy of GAT measurements, but had no influence on those from DCT. DCT was more beneficial to the measurements of IOP in normal eyes and those subject to LASIK surgery.

I have designed a web based refractive surgery database website called

www.safeguardyoursight.com.

The website is designed to store preoperative,operative and postoperative information for refractive surgery patients. This information is important in glaucoma screening and management as well as in calculating intraocular lenses in cataract surgery in post refractive surgery patients.

I think people who visit your site may be interested in the information/services on my site.

please see "new solutions to long term problems with LRS" in the general discussion section for more information on this topic

If Dr. Craig Berger MD were genuinely interested in saving your sight he would be out preventing his colleagues from performing harmful surgeries, and would refrain from performing them himself. Instead he's trying to make a buck off known damage related to corneal refractive surgery that will cause you future medical concerns.

Keep your money out of the pockets of those involved with the LASIK industry, and store a copy of your own medical records at HOME.

Pressure Phosphene Tonometry Versus Goldmann Applanation Tonometry for Measuring Intraocular Pressure Before and After LASIK

Journal of Refractive Surgery Vol. 23 No. 4 April 2007

Gabi Shemesh, MD; Oran Man, MBBCh; Adi Michaeli, MD; David Varssano, MD; Moshe Lazar, MD

Undetected Development of Glaucoma After Radial Keratotomy

Journal of Refractive Surgery Vol. 24 No. 1 January 2008
Alexander Friedrich Scheuerle, MD; Michael Martin, MD; Hans Eberhard Voelcker, MD; Gerd Auffarth, MD

PURPOSE
To report a case of advanced glaucomatous optic atrophy years after bilateral radial keratotomy.

METHODS
Multiple intraocular pressure (IOP) measurements of both eyes in a 40-year-old woman who underwent previous bilateral radial keratotomy were obtained using Goldmann applanation tonometry as well as air-puff and Schiotz tonometry. In addition to regular eye examinations, corneal thickness, surface, and shape were examined using Orbscan and C-Scan.

RESULTS
The cornea of both eyes did not show signs of corneal thinning, but flattening of the corneal surface was observed. The decreased corneal curvatures precipitated a misjudgment of IOP readings measured by central applanantion tonometry (12 to 18 mmHg), whereas impression and non-contact tonometry revealed elevated IOP values (21 to 27 mmHg).

CONCLUSIONS
Changes of the corneal shape without corneal thinning can lead to falsely low IOP values. Therefore, in eyes that have undergone corneal refractive surgery, non-Goldmann measurement of IOP and continued examination of the optic nerve and possibly visual fields are recommended. [J Refract Surg. 2008;24:51-54.]

J Glaucoma. 2008 Dec;17(8):611-8.

Impact and Permanence of LASIK-induced Structural Changes in the Cornea on Pneumotonometric Measurements: Contributions of Flap Cutting and Stromal Ablation.

Sánchez-Navés J, Furfaro L, Piro O, Balle S.
*Department of Physics Instituto Balear de Oftalmología, Ctra. La Vileta, Palma de Mallorca daggerDepartmento de Física Interdisciplinar, Institut Mediterrani d'Estudis Avançats (CSIC-UIB), C/Miquel Marqués, Esporles, Spain.

PURPOSE: To determine the factors that lead to changes in intraocular pressure (IOP) measurements after laser-assisted in situ keratomileusis (LASIK) and their long-term stability.

PATIENTS AND METHODS: Five hundred twenty-two myopic eyes and 296 hyperopic eyes were enrolled in the study. Pneumotonometry was used to measure IOP once in the preoperative stage and twice in the postoperative stage-1 month after the operation and 1 year later. Ultrasonic pachymetry was used to determine preoperative and intraoperative corneal thicknesses and axial length of the eye, whereas optical pachymetry was used in the preoperative stage and 1 month after surgery. Corneal topography was used to determine the preoperative and postoperative mean curvature of the anterior surface of the cornea over 3 and 5-mm diameter regions. Comparative statistical analysis of the retrospective data series was performed.

RESULTS: A highly significant reduction of IOP readings is found after LASIK for both myopic and hyperopic eyes. The reduction is stable 1 year after LASIK. In the case of myopic eyes, the reduction has a highly significant linear correlation with the amount of tissue ablated in the central region of the cornea.

CONCLUSIONS: Pneumotonometric IOP readings after LASIK are reduced, without recovering preoperative values even 1 year after surgery, because of flap cutting and tissue removal in the central region of the cornea. The contribution of flap cutting is estimated to be (1.6+/-0.8) mm Hg, whereas ablation contributes an additional (0.029+/-0.003) mm Hg/mum of removed tissue. This effect should be considered when evaluating the accuracy of IOP measurements in LASIK patients who are at risk for developing glaucoma.

From the full text:

Pneumotonometric IOP measurements after LASIK exhibit a highly significant reduction for both myopic and hyperopic eyes. The reduction in IOP readings arises from both the cutting of the flap and the thinning of the central cornea, because peripheral ablation seems not to affect post-LASIK tonometry. The cut of the flap reduces IOP readings by 1.6±0.8 mm Hg, whereas tissue ablation in the central cornea induces an additional reduction by 0.029±0.003 mm Hg/µm of the ablated tissue. This IOP reduction is stable even 1 year after LASIK, so it seems that healing cannot recover the normal stiffness of the cornea. The observed reduction in IOP measurements after LASIK implies that IOP readings that would not be considered risky in nonoperated eyes may instead indicate high risk of suffering glaucoma in operated patients. This effect is most prominent in operated myopic eyes, when patients with apparently normal post-LASIK IOPs should be considered at risk. An approximate compensation of the reduction in IOP readings after LASIK is obtained by adding 1 mm Hg per corrected diopter plus 1.5 mm Hg owing to flap removal.