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Year : 2020  |  Volume : 15  |  Issue : 2  |  Page : 197-201

Compare and study intraocular pressure measured by noncontact tonometer and contact tonometer (Goldmann's applanation tonometer and schiotz) and their correlation with central corneal thickness

Department of Ophthalmology, Jawaharlal Nehru Medical College, Datta Meghe Institute of Medical Sciences (Deemed to be University), Sawangi(Meghe), Wardha, Maharashtra, India

Date of Submission14-Jan-2020
Date of Decision18-Jan-2020
Date of Acceptance31-Jan-2020
Date of Web Publication21-Dec-2020

Correspondence Address:
Dr. Sohan Lohiya
Department of Ophthalmology, DMIMS (DU), Wardha - 442 001, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jdmimsu.jdmimsu_8_20

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Background: To compare the intraocular pressure (IOP) measurements with Non Contact Tonometry (NCT) and Goldman Applanation tonometry (GAT) and to compare NCT IOP and GAT IOP and Schiotz IOP among various central corneal thickness (CCT) group. Aims: To compare the intraocular pressure (IOP) measurements with Non Contact Tonometry (NCT) and Goldman Applanation tonometry (GAT) and to compare NCT IOP and GAT IOP and Schiotz IOP among various central corneal thickness (CCT) groups. Settings and Design: This was a cross sectional, observational study. 120 patients were sequentially collected after taking inclusion and exclusion criteria into consideration. Methods and Material: IOP measurements were done by NCT and then by GAT followed by Schiotz. Them followed by CCT. All IOP readings were taken in the sitting position over fifteen minutes. 120 patients were included in this study. Statistical analysis used: Statistical analysis was done by using descriptive and inferential statistics using Pearson's Correlation Coefficient and software used in the analysis was SPSS 24.0 version and p<0.05 is considered as level of significance Results: The IOP measured with both GAT and NCT showed no significant change with increasing CCT. The difference between the means increases with increasing CCT upto 600 microns. At lower IOPs ≤ 20 mm Hg, GAT measures are higher than NCT and this relationship is reversed at high IOPs. Conclusions: Both the tonometers showed a significant correlation with the gold standard technique (Goldman's applanation tonometer) over a range of IOP and CCT with the Schiotz tonometer better than the NCT.

Keywords: Goldmann's applanation tonometry, intraocular pressure, noncontact tonometry, schiotz

How to cite this article:
Lohiya S, Pardasani R. Compare and study intraocular pressure measured by noncontact tonometer and contact tonometer (Goldmann's applanation tonometer and schiotz) and their correlation with central corneal thickness. J Datta Meghe Inst Med Sci Univ 2020;15:197-201

How to cite this URL:
Lohiya S, Pardasani R. Compare and study intraocular pressure measured by noncontact tonometer and contact tonometer (Goldmann's applanation tonometer and schiotz) and their correlation with central corneal thickness. J Datta Meghe Inst Med Sci Univ [serial online] 2020 [cited 2021 Jan 20];15:197-201. Available from: http://www.journaldmims.com/text.asp?2020/15/2/197/304269

  Introduction Top

Glaucoma is the second leading cause of blindness worldwide. Intraocular pressure (IOP) is the only known modifiable risk factor that has been shown to delay progression in both ocular hypertension and glaucoma patients. IOP measurement has an important role in case detection and management of primary open angle glaucoma.[1],[2],[3]

Clinical measurement of IOP has undergone several technical advances from the initial digital tension measurements, through indentation tonometry, to applanation tonometry and noncontact tonometry (NCT).

Central corneal thickness (CCT) is known to affect the accuracy of IOP (IOP) measurements. A thicker cornea requires greater force to applanate and, conversely, a thinner cornea is more easily flattened. A thin cornea is a significant risk factor for the development of glaucoma and it has yet to be determined whether this is an independent effect or a result of the influence of CCT on IOP measurements.

Goldmann's applanation tonometer has received a great importance because this method is independent of ocular rigidity; it is little influenced by variations in corneal curvature and it records the IOP directly by applanating the cornea. In Goldmann's applanation tonometry, surface tension of tear film and the force required to bend the cornea cancel each other, thus making Imbert-Fick-law applicable to this method. Goldmann's applanation tonometer shows no topographic effect and there by gives reproducible measurements on repeated measurements.

Noncontact (also called air-puff) tonometer uses a puff of air to applanate the cornea. IOP is measured by the amount of force by air puff required to flatten the cornea to a fixed level.

Indentation (Schiotz) tonometry: It measures IOP by finding how much cornea is indented by plunger of a fixed weight.[4],[5],[6],[7]

  Subjects and Methods Top

Study design

This was a cross-sectional, observational study.

Study location

This was a tertiary care teaching hospital-based study done in Department of Ophthalmology, AVBRH, Sawangi, Wardha, Maharashtra.

Duration of study

Six months.

Sample size

120 patients were sequentially collected after taking inclusion and exclusion criteria into consideration.

Subjects and selection methods

All patients who underwent uncomplicated cataract extraction at Acharya Vinobha Bhave Rural Hospital and now presented to the hospital with posterior capsular opacification were selected for the study after taking the inclusion and exclusion criteria into consideration.

Procedure for test

  1. The study adhered to the tenets of the Declaration of Helsinki and was approved by an institutional ethics committee of DMIMSU
  2. Patients who presented to the eye outpatient department and met the inclusion criteria and did not fall under the exclusion criteria were selected
  3. Informed consent was obtained from all subjects after the nature of the study was explained to them
  4. After getting informed consent, participants underwent a complete ophthalmic examination including visual acuity using Snellen's chart, refraction, slit-lamp examination, IOP, and CCT measurement
  5. The IOP was measured by a single investigator using the noncontact tonometer, Goldmann's applanation tonometer, and Schiotz indentation tonometer in that order to prevent lowering of IOP induced by contact. In all cases, a 5 min interval was ensured between any two methods of IOP measurement and an average of three measurements was taken as the final IOP obtained by that method
  6. First, the patient was seated at the tabletop model of Canon TX-10 Noncontact Tonometer (Canon U.S.A., Inc., One Canon Park, Melville, NY) and asked to fix at the target. The examiner aligned the cornea by superimposing the reflection of the target from the patient's cornea on a stationary ring. An air puff was automatically triggered when alignment was satisfactory
  7. Then patient's cornea was anaesthetized with topical application of 0.5% proparacaine hydrochloride and the tear film stained with sodium fluorescein using paper strips impregnated with fluorescein. With the patient in a sitting position, under cobalt blue light illumination, the biprism Goldmann's tonometer was brought into gentle contact with the center of the cornea. The fluorescein semicircles were viewed through the biprism, and the calibrated dial was adjusted till the inner edges overlapped. The reading on the dial was multiplied by ten for the IOP value
  8. The patient was placed in a supine position and asked to fix at a target. Zero error of Schiotz indentation tonometer was taken by placing the footplate on the test block provided. The eyelids were separated by hand without exerting pressure on the globe, and the tonometer foot plate was placed on the anaesthetized cornea so that the plunger moved freely vertically. The scale reading was noted. The 5.5 g weight was used, The subsequent readings were taken These readings were converted to IOP measurement in mm of Hg using Friedenwald's table.

Inclusion criteria

  1. Age above 30 years
  2. IOP within 10-50 mm Hg measured by NCT
  3. Astigmatism ≤3D cylinder.

Exclusion criteria

  1. Previous history of ocular surgery
  2. Patients with significant corneal astigmatism (3D) are excluded, so as to eliminate the requirement for alteration of the orientation of the tonometer prism from the convention of the horizontal meridian
  3. Physical difficulty in performing applanation (infirmity preventing suitable positioning at a slit lamp, or inability to cooperate with eye opening)
  4. Pathology of the cornea that could affect biomechanics and, thus, the size and appearance of the tear meniscus semicircles, like keratoconus, corneal scarring, previous corneal surgery, corneal infection, microphthalmos, buphthalmos, and blepharospasm
  5. Patients diagnosed with glaucoma.

Statistical analysis used

Statistical analysis was done using descriptive and inferential statistics using Pearson's Correlation Coefficient and software used in the analysis was SPSS 24.0 version (Chicago, Illinois USA) and P < 0.05 is considered as level of significance.

  Discussion Top

In seeking to evaluate a new instrument for clinical physiological measurement, it is necessary to compare its accuracy with that of the current standard and to determine in what manner of operation such accuracy is obtained.

With an array of tonometers available today, the tonometer used for screening should be feasible in screening setup.

Baseline values of the IOP will help the clinician in monitoring progress of the disease and response to treatment. While a number of tonometers are available for measuring the IOP, each has its own advantages and disadvantages. The increased costs and the need for specialized training for optimal utilization of modern tonometers preclude the use of such tonometers in the rural camp setting and outreach mass screening program.

It is well known that Goldmann's applanation tonometry (GAT) underestimates IOP in thin corneas and overestimates IOP in thick corneas. Applanation tonometry measures IOP by subjecting the eye to a force that flattens the cornea. It assumes that the Imbert-Fick law is applicable to the eye. This law states that the pressure within a sphere is approximately equal to the external force needed to flatten a portion of the sphere divided by the area of the sphere that is flattened [Table 2] and [Graph 2].
Table 1: Correlation of central corneal thickness with noncontact tonometer

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Table 2: Correlation of central corneal thickness with Goldmann's applanation tonometer

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Ethical Approval

Ethical approval for this study (DMIMS(DU)/IEC/2019-20/7981) was provided by the Ethical Committee of Datta Meghe Institute of Medical Sciences (Deemed to be University) on 20/04/19.

  Results Top

The noncontact tonometer (NCT) is a user-friendly instrument that lends well to use by the ophthalmology trainee as well as by the optometrist. The NCT has the potential advantage that it uses an air puff to indent the cornea, thereby reducing the possible risk of epithelial trauma and cross infection which can be of tremendous advantage while in use in mass screening camp setting. The NCT is often difficult to perform in patients with poor fixation and has also been found to significantly underestimate GAT measurements at lower IOP and to overestimate those at higher IOP [Table 1] and [Graph 1].

The Schiotz tonometer is another user-friendly instrument available for use by both the ophthalmology trainee and the optometrist with twin advantages of portability and affordability. However, the results of Schiotz tonometry are known to be affected by factors such as scleral rigidity, compressibility of the vascular content of the eye, the ease with which the fluid is expressed through the drainage channels and “Moses effect” [Table 3] and [Graph 3].
Table 3: Correlation of central corneal thickness with schiotz tonometer

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With the advent of applanation tonometers, there has been some doubt on the accuracy of the Schiotz tonometer. Several workers have compared the efficacy of tonometers with some showing good correlation between applanation tonometers and indentation tonometers and others finding only a moderate agreement between NCT and applanation tonometer [Table 4] and [Graph 4].
Table 4: Distribution of patients according to their age in years

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In this study, we have compared both Schiotz indentation tonometer and NCT to Goldmann's tonometer in the same set of patients and determined their respective agreement to Goldmann's tonometer [Table 5] and [Graph 5].
Table 5: Distribution of patients according to their gender

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The noncontact tonometer was found to read slightly higher reading than Goldmann's applanation tonometer. This study observed that overall IOP as measured by NCT was significantly higher than GAT (P = 0.004).

However, Salim et al. found no significant difference between NCT and GAT measurements (P = 0.64). Tonnu et al. reported GAT to be greater than NCT.

Most studies showed that NCT overestimates at low pressures and underestimates at high pressure when IOP readings are compared with GAT.

However, Tonnu et al. showed that NCT underestimates IOP at lower ranges and overestimates at higher IOP ranges. Our study showed similar results.

The mean pressure noted by Schiotz was 1–2 mm lower than mean obtained by Goldmann's applanation tonometer, indicating that tonometer tend to read lower than Goldmann's tonometer.

The difference between the two measurements in our study was greatest in patients with thick corneas, gradually lessening as CCT decreased. Although this finding was not significant (P = 0.809), CCT may contribute to the relative IOP overestimation at higher IOP levels.

An essential criteria for a good screening test is high specificity and high sensitivity. The noncontact tonometer has both high specificity and high sensitivity. It is thus a good screening tool. It gains further credentials as it is easy to use without any observer bias as it records pressure automatically. Its only drawback is cost.

Change in CCT with age can affect IOP as measured by Goldmann's tonometer.

  Conclusion Top

Our finding that NCT was found to be least accurate when CCT was greater than 600 micrometers is similar to the observation of Tonnu et al. that the readings by NCT are far more affected by changes in CCT than those of GAT.[8],[9],[10],[11],[12],[13],[14],[15]

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Allingham RR, Damji K, Freedman S, Moroi SE, Rhee DJ, Shields MB. Intraocular pressure and Tonometry. In: Shields Textbook Of Glaucoma.6th ed. New Delhi: Wolters Kluwer/Lippincott Williams & Wilkins. 2011. p. 24-40.  Back to cited text no. 1
Fisher JH, Watson PG, Spaeth G. A new handheld air impulse tonometer. Eye (Lond) 1988;2 (Pt 3):238-42.  Back to cited text no. 2
Pearce CD, Kohl P, Yolton RL. Clinical evaluation of the Keeler PULSAIR 2000 tonometer. J Am Optom Assoc 1992;63:106-10.  Back to cited text no. 3
Gupta V, Sony P, Agarwal HC, Sihota R, Sharma A. Inter-instrument agreement and influence of central corneal thickness on measurements with Goldmann, pneumotonometer and noncontact tonometer in glaucomatous eyes. Indian J Ophthalmol 2006;54:261-5.  Back to cited text no. 4
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Tonnu PA, Ho T, Newson T, El Sheikh A, Sharma K, White E, et al. The influence of central corneal thickness and age on intraocular pressure measured by pneumotonometry, noncontact tonometry, the Tono-Pen XL, and Goldmann applanation tonometry. Br J Ophthalmol 2005;89:851-4.  Back to cited text no. 5
Matsumoto T, Makino H, Uozato H, Saishin M, Miyamoto S. The influence of corneal thickness and curvature on the difference between intraocular pressure measurements obtained with a non-contact tonometer and those with a Goldmann applanation Tonometer. Jpn J Ophthalmol 2000;44:691.  Back to cited text no. 6
Jorge J, Díaz-Rey JA, González-Méijome JM, Almeida JB, Parafita MA. Clinical performance of the reichert AT550: A new non-contact tonometer. Ophthalmic Physiol Opt 2002;22:560-4.  Back to cited text no. 7
Ramakrishnan R, Krishnadas SR, Khurana M, Robin AL. Diagnosis and Management of Glaucoma. 1st ed. New Delhi: Jaypee brothers; 2013. p. 66-9.  Back to cited text no. 8
Thomas R. Glaucoma in India: Current status and the road ahead. Indian J Ophthalmol 2011;59 Suppl: S3-4.  Back to cited text no. 9
Thomas R, Parikh R, Paul P, Muliyil J. Population-based screening versus case detection. Indian J Ophthalmol 2002;50:233-7.  Back to cited text no. 10
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Aditi P, Khandekar A, Acharya S, Tidake P, Shukla S. Correlation between Nephropathy and Ophthalmic Complications in Cases of Sickle Cell Anemia: An Entangled Association. Indian J Med Res Spec 2019;10:72-5. Available from: https://doi.org/10.4103/INJMS.INJMS_4_19. [Last accessed on 2019 Dec 17].  Back to cited text no. 11
Prasad M, Daigavane S, Kalode V. Visual Outcome after Cataract Surgery in Rural Hospital of Wardha District: A Prospective Study. J Clin Diagn Res 2020;14. Available from: https://doi.org/10.7860/JCDR/2020/42643.13528. [Last accessed on 2019 Dec 17].  Back to cited text no. 12
Daigavane, Sachin, and Prarthana Patkar. To Compare the Changes in the Corneal Endothelium Post Phacoemulsification Surgery with Balanced Salt Solution vs. Viscoelastic Device. J Clin Diagn Res 2019;13:1-4. Available from: https://doi.org/10.7860/JCDR/2019/42723.13371. [Last accessed on 2019 Dec 17].  Back to cited text no. 13
Sana B, Lohiya S. Prevalence of Refractive Errors and Colour Blindness in School Going Children of Wardha Tehsil: A Prospective Study. J Clin Diagn Res 2020;14:1-4. Available from: https://doi.org/10.7860/JCDR/2020/43299.1-4. [Last accessed on 2019 Dec 17].  Back to cited text no. 14
Patkar P, Sune P, Sune P. Evaluation of Tear Film Functions Preoperatively and Postoperatively in Cases with Pterygium: A Case Control Study. J Clin Diagn Res 2020;14:NC10-13. Available from: https://doi.org/10.7860/JCDR/2020/43113.13461. [Last accessed on 2019 Dec 17].  Back to cited text no. 15


  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]


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