Analysis of Postoperative Intraocular Pressure Underestimation Measured with Non Contact Tonometry after Corneal Refractive Surgery

Wook Kyum Kim; Eun Young Cho; Hee Sun Kim; Hee Kyung Lee; Jin Kuk Kim

doi:10.3341/jkos.2014.55.2.167

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Kim, Cho, Kim, Lee, and Kim: Analysis of Postoperative Intraocular Pressure Underestimation Measured with Non Contact Tonometry after Corneal Refractive Surgery

Original Article

J Korean Ophthalmol Soc 2014;55(2):167-172.

Published online: 7 September 2014

DOI: https://doi.org/10.3341/jkos.2014.55.2.167

Analysis of Postoperative Intraocular Pressure Underestimation Measured with Non Contact Tonometry after Corneal Refractive Surgery

Wook Kyum Kim, MD, Eun Young Cho, MD, Hee Sun Kim, MD, Hee Kyung Lee, MD, Jin Kuk Kim, MD

B&VIIT Eye Center

Address reprint requests to Jin Kuk Kim, MD B&VIIT Eye Center, #3 Seocho-daero 77-gil, 14F Ala Tower, Seocho-gu, Seoul 137-856, Korea, Tel: 82-2-501-6800, Fax: 82-2-501-6435, E-mail: kimjinkuk@hanmail.net

Received 27 June 201319 August 2013 Accepted 28 January 2014

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose

To analyze the postoperative intraocular pressure (IOP) underestimation measured with non-contact tonometry after corneal refractive surgery.

Methods

The postoperative IOP decrease measured with non-contact tonometry (NCT), regarded as IOP underestimation, was calculated in 253 LASIK patients and 281 LASEK patients. Multiple regression analysis was performed to determine the preoperative factors which affect postoperative IOP underestimation. The right eye results were reported in this paper.

Results

The postoperative IOP decrease was affected by age (r = -0.0420, p = 0.03), corneal ablation depth (r = 0.0466, p < 0.01), and operation method (LASIK or LASEK) (r = 0.6006, p < 0.01). For every 100 μm decrease of corneal thickness by LASIK, the IOP decreased 6.29 ± 2.40 mm Hg in patients under 26 years of age and 6.12 ± 2.53 mm Hg in patients above 26 years of age (p = 0.05). For every 100 μm decrease of corneal thickness by LASEK, the IOP decreased 5.77 ± 2.37 mm Hg in patients under 26 years of age and 5.44 ± 2.62 mm Hg in patients above 26 years of age (p = 0.05).

Conclusions

The postoperative IOP underestimation measured with NCT was more prominent in younger-aged patients after LASIK than LASEK with deeper ablation depth.

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Keywords: Central corneal thickness, Intraocular pressure, LASEK, LASIK, Non-contact tonometer

References

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Table 1.

General characteristics of 534 eyes of 534 patients who received LASIK or LASEK

Characteristics	Values
LASIK : LASEK (eyes)	253 (48%) : 281 (52%)
Age at operation (years)	25.1 ± 4.8
Preoperative myopia (diopters)	-5.08 ± 1.79
Preoperative astigmatism (diopters)	-1.11 ± 0.78
Preoperative BCVA (log MAR)	-0.03 ± 0.05
Preoperative K average (diopters)	43.59 ± 1.26
Preoperative central cornea thickness (μm)	529.1 ± 30.3
Corneal ablation depth (μm)	92.1 ± 21.8
Postoperative corneal thickness (μm)	437.1 ± 34.5
Preoperative IOP (mm Hg)	15.14 ± 2.63
Postoperative IOP (mm Hg)	9.74 ± 2.00
IOP change (mm Hg)	5.40 ± 2.37
IOP change/ablation depth (mm Hg/μm)	0.059 ± 0.025
Preoperative white to white (mm)	11.74 ± 0.37
Postoperative sphere (diopters)	+0.10 ± 0.57
Postoperative astigmatism (diopters)	-0.40 ± 0.26
Postoperative UCVA (log MAR)	-0.12 ± 1.42

Values are presented as mean ± SD.

BCVA = best corrected visual acuity; IOP = intraocular pressure; UCVA = uncorrected visual acuity.

Table 2.

Correlation coefficient of several preoperative factors with postoperative intraocular pressure decrease

Preoperative factors	Correlation coefficient	p-value
Age (years)	-0.1280	<0.01
Sex	-0.0500	0.25
Average Keratometry (mm)	0.0417	0.34
Ablation depth (μm)	0.4442	<0.01
Operation methods (LASIK/LASEK)	0.1596	<0.01

Pearson’s correlation was done.

Table 3.

The multivariate regression analysis of preoperative factors that could affect postoperative IOP change using NCT after corneal refractive surgery

Preoperative factors	IOP change after refractive surgery
Preoperative factors	Correlation coefficient r	t Statistic	p-value
Age (years)	-0.0420	-2.215	0.03
Sex	-0.1890	-0.847	0.40
Ablation depth (μm)	0.0466	11.117	<0.01
LASIK or LASEK	0.6006	3.287	<0.01
K average (diopters)	0.1166	1.529	0.13

IOP = intraocular pressure; NCT = non contract tonometer.

Table 4.

The comparison of preoperative factors and postoperative results between LASIK and LASEK patients

	LASIK (253 eyes)	LASEK (281 eyes)	p-value^*
Age at operation (years)	25.0 ± 5.3	25.3 ± 4.4	0.44
Preoperative myopia (diopters)	-5.13 ± 2.00	-5.04 ± 1.58	0.58
Preoperative astigmatism (diopters)	-1.20 ± 0.87	-1.03 ± 0.68	0.01
Preoperative K average (diopters)	43.65 ± 1.25	43.58 ± 1.18	0.47
Preoperative central cornea thickness (μm)	544.5 ± 25.6	515.3 ± 27.4	<0.01
Corneal ablation depth (μm)	93.5 ± 23.5	90.9 ± 20.0	0.18
Postoperative corneal thickness (μm)	451.1 ± 30.6	424.4 ± 32.9	<0.01
Preoperative IOP (mm Hg)	15.8 ± 2.6	14.6 ± 2.5	<0.01
Postoperative IOP (mm Hg)	10.0 ± 2.1	9.5 ± 1.9	<0.01
IOP change (mm Hg)	5.8 ± 2.4	5.0 ± 2.2	<0.01
IOP change/ablation depth (mm Hg/μm)	0.062 ± 0.024	0.056 ± 0.025	<0.01

Values are presented as mean ± SD.

^* Student’s t-test was done.

Table 5.

The change of intraocular pressure in every 100 μm ablation after corneal refractive surgery according to age group and operation methods

		IOP change per 100 μm ablation		p-value^*
		After LASIK	After LASEK	p-value^*
Age group	<26 years	6.29 ± 2.40 (153 eyes)	5.77 ± 2.37 (157 eyes)	0.05
	≥26 years	6.12 ± 2.53 (100 eyes)	5.44 ± 2.62 (124 eyes)	0.05

Values are presented as mean ± SD.

^* Student’s t-test was done.

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