|Year : 2014 | Volume
| Issue : 3 | Page : 110-115
Comparison of transpalpebral tonometer with Goldmann applanation tonometer
Ashim Kumar Chakraborty, Mousumi Majumder, Santanu Sen
CSIR-Central Glass and Ceramic Research Institute, Kolkata, India
|Date of Web Publication||2-Sep-2014|
Ashim Kumar Chakraborty
Materials Characterization Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata 700032
Source of Support: None, Conflict of Interest: None
Transpalpebral tonometer, a new generation intraocular pressure (IOP) measuring instrument is nowadays used for self-tonometry. It is convenient and noninvasive and seems suitable for IOP measurement at home, as recommended by several authors. Apart from its use for self-tonometry, it has been reported that transpalpebral tonometer is more accurate in determining the IOP in thinned cornea after photorefractive procedures when compared with Goldmann applanation tonometer (GAT). However, several other studies have revealed that their sensitivity in detecting IOP in glaucomatous eye is low compared with standard GAT. The aim of this study is to review the results of several studies that have compared IOP measurements obtained by the transpalpebral tonometer and GAT.
Keywords: Goldmann applanation tonometer, intraocular pressure, transpalpebral tonometer
|How to cite this article:|
Chakraborty AK, Majumder M, Sen S. Comparison of transpalpebral tonometer with Goldmann applanation tonometer. Taiwan J Ophthalmol 2014;4:110-5
|How to cite this URL:|
Chakraborty AK, Majumder M, Sen S. Comparison of transpalpebral tonometer with Goldmann applanation tonometer. Taiwan J Ophthalmol [serial online] 2014 [cited 2022 Nov 29];4:110-5. Available from: https://www.e-tjo.org/text.asp?2014/4/3/110/204038
| 1. Introduction|| |
Glaucoma is the second leading cause after cataract, which ultimately leads to blindness and disproportionately affects both women and men. According to an estimate, approximately 67 million people worldwide were affected by primary glaucoma in 2000. Another estimate is that approximately 60.5 million people worldwide were affected by open-angle glaucoma and angle-closure glaucoma in 2010. Approximately 11.2 million Indians suffer from glaucoma after the age of 40 years with over 90% of the cases being diagnosed only after sufficient vision loss has occurred. Glaucoma accounts for 12% of blindness and 11.4% of low vision in the Indian subcontinent. Therefore, improved methods of screening and therapy for glaucoma are urgently needed. Different methodologies are proposed by scientists for diagnosis and management of glaucoma. The hydrostatic pressure inside the eye or variations in intraocular pressure (IOP) remain as one of the main characteristics of this disease. Reduction of IOP is currently the only treatment available for decreasing the risk of progression of glaucoma, and therefore, it is important to have an accurate reading of the true IOP. Although manometry is the most precise method for measuring IOP, its invasive nature and ethical issues related to the use of living eyes have restricted its use only in cadaver eye or in living eye undergoing enucleation or intraocular surgery. Most of the tonometers including Goldmann applanation tonometer (GAT) provide an IOP reading at a given instant. It is now known that IOP varies diurnally in both healthy individuals and glaucomatous patients, while the variation is higher in the latter group,, and to know this variation, one needs to make round-the-clock measurements of IOP. Necessity of a more practical home tonometer capable of taking multiple measurements was therefore felt for a long time. Subsequently, transpalpebral tonometers such as TGDc-01, Diaton, and pressure phosphene tonometer (PPT) were developed, which are portable devices and measures the IOP through the eyelid. The aim of this study was to evaluate the accuracy of measurement performed using different types of transpalpebral tonometers by the patient and/or by the examiner, and compare it with GAT.
| 2. Brief overview of IOP measurement using GAT and transpalpebral tonometer|| |
The IOP evaluation instruments are designed based on three different methods [Figure 1]. These are (1) manometry, (2) tonometry, and (3) transpalpebral IOP measurement. The classification of IOP measuring instruments is shown in [Figure 1]. Among these, the principle of GAT and transpalpebral tonometer is briefly discussed in the following sections.
|Figure 1: Classification of intraocular pressure (IOP) measuring instruments.|
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The working principle of GAT is based on the Imberte–Fick law, which states that if a plane surface is applied with a force F on to a thin spherical membrane within which a pressure P exists, then for an equilibrium condition, the expression P is given by the equation p = F/A, where P is the IOP, A is the area of the applied force (3.06 mm diameter), and F is the weight applied on the eye (1–10 g). A representation of the different forces acting on the globe of the eye according to Imbert and Fick law is shown in [Figure 2]. In this method, the instrument is mounted on a clinical slit lamp biomicroscope in which the applanation surface is placed in the center of a plastic cylinder (probe), which is attached to a spring-loaded arm with a knob. The probe is impressed upon the cornea to flatten a circular area of diameter 3.06 mm using the knob. The force imparted on the eye through the probe is varied between 1 g and 10 g to measure IOP between 5 mmHg and 50 mmHg within the eye. In GAT, anesthesia is required to be applied in the eye together with a fluorescein dye. The GAT readings are influenced by central corneal thickness (CCT). Pachymetry is needed to determine the CCT, and the tonometer readings are corrected for true IOP using the CCT. Edematous or scarred condition of the cornea also influences the accuracy of the GAT reading., Despite these limitations, GAT is the most widely used tonometer against which all other tonometers are compared to date.
|Figure 2: Representation of forces involved in applanation tonometry. W = tonometer force; s = surface tension of precorneal tear film; P = intraocular pressure; A = area of applanation; b = corneal rigidity/resistance to bending.|
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Transpalpebral tonometry refers to the method of measuring IOP through the eyelid of the sclera (noncorneal). Diaton (BiCOM Inc., Long Beach, NY, USA) is the commercially available version of this type of tonometer. The working principle of transpalpebral tonometer is based on determining the acceleration of a freely falling rod as it rebounds against the tarsal plate of the eyelid through the sclera. The advantages of this type of tonometer are that during IOP evaluation it does not make contact with the cornea and does not require application of anesthesia. [Figure 3] shows the schematic diagram of the position of rod during the measurement of IOP, and [Figure 4] shows the photo of actual measurement of IOP of a patient using Diaton. TGDc-01 tonometer (Ryazan State Instrument Making, Ryazan, Russia) is a commercially available transpalpebral tonometer, whereas the Diaton tonometer is an updated version of the TGDc-01 tonometer, which incorporates a vertical alignment sensor and an in-built software for the provision of an average IOP reading.
|Figure 4: Measurement of intraocular pressure using Diaton (Courtesy BiCOM Inc., Long Beach, NY, USA).|
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Proview PPT is another type of tonometer under this category, which is marketed by Bausch and Lomb. It was first described by Fresco who had developed a method and a device based on the principle that when pressure is applied on the sclera, it generates a phosphene spot, which can be perceived by the patients themselves. The probe of this pencil-shaped instrument is pressed against the upper eyelid with increasing pressure until visual phenomenons (phosphene) appear opposite to where the pressure was applied. As soon as the phosphene is detected, the tonometer is removed from the eyelid and the measurement is noted. The photograph of a PPT is shown in [Figure 5]. The PPT is convenient and noninvasive and suitable for home IOP measurement, as reported by Naruse et al. The PPT is more accurate in determining the IOP in thinned cornea after photorefractive procedures such as laser-assisted in situ keratomileusis (LASIK) when compared with GAT. However, another group of studies has revealed that the sensitivity of Diaton TGDc-01 or PPT in detecting IOP in glaucomatous eye is low compared with GAT.,
|Figure 5: Photograph of pressure phosphene tonometer. Note. Reprinted with permission from Reference 39.|
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| 3. Comparison of transpalpebral IOP measurement with GAT|| |
Although GAT has established itself as the gold standard for IOP measurement, this device measures IOP by coming in physical contact with the cornea and so GAT is not the device of choice in the cases of corneal disorder or corneal surgery.,, In such cases, a device that bypasses the cornea during IOP measurement such as the TGDc-01, Diaton, or Proview PPT may be more suitable. [Table 1] and [Table 2] show the results of 16 studies that compared GAT with TGDc-01 and PPT, respectively.
| 4. Discussion|| |
Most of the authors in this review have recommended transpalpebral tonometer to be a suitable alternative to GAT in cases where the latter is unsuitable for reasons cited earlier and for screening purposes. As this type of tonometer does not touch the cornea, it is less susceptible to spread infection from one patient to another during IOP measurement. Some studies reported that transpalpebral tonometer such as the PPT may be a more reliable method for measuring IOP after LASIK. In case of pediatric patients, PPT has been found to be suitable as a noninvasive, portable screener and is also well tolerated by children. Long use of PPT by patients themselves was found to lessen their anxiety about their glaucoma. General acceptability of PPT is more than GAT in the patient population. However, compared with GAT, TGDc-01 or Diaton tonometer has been found to overestimate and underestimate the IOP in the lower and higher IOP ranges, respectively. In addition, the intraobserver and interobserver variations were found to be always higher in TGDc-01 or Diaton than in GAT. Performance of Transpalpebral tonometer is also found to depend on whether difference between the mean IOP readings or limits of agreement between the IOP readings obtained by the two methods is considered clinically significant. In some studies, wide variability of IOP readings was observed between the different body postures, attributed to various reasons such as poor contact with the flat side of the tip with the anterior rib of the upper lid, verticality of the tonometer, and variation of the placement of the tonometer tip from the upper eyelid margin.
4.1. Future scope
There are several areas left out in the use of transpalpebral-type tonometers that demand further work. Ideal body posture during IOP measurement by Diaton or TGDc-01 is unclear. A few studies have shown that supine IOP is more than sitting IOP but none have been able to recommend the ideal position. Determination of the correct placement position of the tonometer on the upper eyelid is another area that remains unclear. These factors along with several other factors such as CCT, satisfactory contact of the tonometer tip with eyelid, to keep the tonometer vertical during measurement, movement of the eyeball during measurement, eyelid thickness that varies from one patient to another, and muscle tone of the patient may be responsible for the wide variability of data obtained by TGDc-01, Diaton, or PPT in several studies. Future studies should focus on these areas, so that after controlling these variables, satisfactory performance of the transpalpebral-type tonometers becomes possible.
| 5. Conclusion|| |
In this review, we have shown the scope and results of different studies involving the comparison of transpalpebral-type tonometers TGDc-01, Diaton, and PPT with GAT while measuring IOP of glaucoma, glaucoma suspect, normal, and pediatric patients. The review has clearly shown the different advantages and disadvantageous of using transpalpebral-type tonometers in this population. The “Discussion” section summarized our observations on the results of different findings of the studies highlighting the shortcomings. Future scope of work needed to make this type of tonometer more useful is also shown in this review.
This work is supported by the Council of Scientific and Industrial Research, India. The authors are grateful to all the members of staff of Materials Characterization Division of CSIR-Central Glass and Ceramic Research Institute (CGCRI), Kolkata, India. The authors are also grateful to the Director of CGCRI for permitting the publication of this work.
Conflicts of interest: The authors declare that they have no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]