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Intraocular gases and techniques for air–fluid exchange
Published in A Peyman MD Gholam, A Meffert MD Stephen, D Conway MD FACS Mandi, Chiasson Trisha, Vitreoretinal Surgical Techniques, 2019
The presence of an intraocular gas bubble results in errors in IOP measurement, so this must be considered when evaluating the IOP in a gas-filled eye postoperatively. The bubble causes a change in scleral rigidity, which in turn alters the accuracy of IOP determination. Previous studies have shown that the Goldmann tonometer gives accurate IOP measurements in gas-filled eyes. The Tonopen underestimates the IOP by about 10 mmHg or more when the IOP is 30 mmHg or more.24 The Schiotz tonometer also substantially underestimates the true IOP when the pressure is elevated in a gas-filled eye.25
Aqueous Humor Dynamics
Published in Neil T. Choplin, Carlo E. Traverso, Atlas of Glaucoma, 2014
There are three ways to measure facility of trabecular outflow: tonography, perfusion, and fluorophotometry. Tonography has been the most widely used method, and, although it is no longer used as a routine clinical test, its use has provided a large body of information about the pathophysiology of glaucoma and the mechanism of action of treatment modalities. It is still used in the research setting. Tonography is accomplished by placing a Schiotz tonometer (Figure 3.14) or electronic tonometer on the cornea of the supine patient for 2 or 4 min. The calibrated weight of the tonometer increases IOP and, hence, aqueous drainage. The IOP declines over the time interval, and with the aid of the Friedenwald tables, the change in the IOP allows one to infer the volume of aqueous humor displaced from the eye. If it is assumed that the displacement of fluid from the eye across the trabecular meshwork by the weight of the tonometer, ΔV, is the only factor to account for the IOP decrease, then
Write short notes on the principles of intraocular pressure measurement
Published in Nathaniel Knox Cartwright, Petros Carvounis, Short Answer Questions for the MRCOphth Part 1, 2018
Nathaniel Knox Cartwright, Petros Carvounis
Indirect measurement is used clinically:– in applanation tonometry, the cornea is flattened and IOP is determined by measuring the applanating force and the area flattened (P = F/A). The Goldmann tonometer measures the force required to flatten an area of cornea 3.06 mm in diameter. This is the diameter at which the resistance of an average thickness cornea is counterbalanced by the capillary forces of the tear film. Fluorescein in the tear film is used to allow precise estimation of the area of flattening. The Goldmann applanation tonometer is used with a slitlamp and is the most accurate method currently available. The Perkins tonometer is a portable tonometer which uses applanation to measure the IOP in the supine or erect patient. Portable electronic applanation devices employ a similar principle but flatten a smaller area of cornea– non-contact tonometers (air-puff tonometers) determine the IOP by measuring the time needed for a given force of air to flatten a known area of cornea– indentation tonometry is based on the calculation of IOP from the degree of corneal deformation produced by a given force. Indentation is measured by the degree of excursion of the instrument’s plunger, and converted to IOP values by means of a calibrated scale on the instrument or from a conversion table. The Schiotz tonometer is the most commonly used such instrument.
Introduction and Clinical Validation of an Updated Biomechanically Corrected Intraocular Pressure bIOP (v2)
Published in Current Eye Research, 2023
Ashkan Eliasy, Bernardo T. Lopes, Junjie Wang, Ahmed Abass, Riccardo Vinciguerra, Paolo Vinciguerra, Fang-jun Bao, Ahmed Elsheikh
Most intraocular pressure (IOP) measurement methods (or tonometry techniques), whether contact or non-contact, are based on a simple concept involving the application of a mechanical pressure to the cornea and relating the resulting deformation to the value of IOP. This concept is applied in contact methods including applanation tonometers (e.g. the Goladmann Applanation Tonometer, GAT),4 and indentation methods such as the Schiotz tonometer.5 It is also adopted in non-contact methods such as the Ocular Response Analyzer (ORA) and the Corvis ST tonometers where air pressure is used to deform the cornea.6,7 In all of these methods, a small deformation, or a high resistance to deformation, is considered an indication of a high IOP, and vice versa.
Current methods and new approaches to assess aqueous humor dynamics
Published in Expert Review of Ophthalmology, 2021
Carol B. Toris, Meghal Gagrani, Deepta Ghate
A noninvasive measurement technique used in humans and nonhuman primates to assess outflow facility is tonography (Figure 7). It is the recording of changes in IOP from sustained pressure on the globe. Pressures can be applied to the cornea by a weighted probe or to the limbal region by a suction cup. For the former, an electronic Schiotz tonometer or a pneumatonometer with tonography setting (Reichert, Depew, NY) are the instruments of choice. The Schiotz tonometer was developed in the 1950s and used for decades but is no longer commercially available. It has been replaced by the pneumatonometer. A comparison of the Schiotz and pneumatonometer for outflow facility assessment finds comparable results [70]. Tonography is explained by a series of equations that were described in the 1930s and refined during the next four decades. Jonas Friedenwald, Morton Grant, and Maurice Langham were the main contributors to the development of this technique [68,71–73]. Tonography was originally designed to be a diagnostic tool for glaucoma and it was used in clinical practice throughout the 1960s and 1970s. However, it is rarely used today for diagnostic purposes because of its failure to reliably identify cases of glaucoma. Nevertheless, tonography continues to be of value in the carefully controlled research environment when done by experienced technicians. Differences in C among and between experimental groups can be detected provided the number of subjects and the differences in C are large enough.
Formulation and investigation of pilocarpine hydrochloride niosomal gels for the treatment of glaucoma: intraocular pressure measurement in white albino rabbits
Published in Drug Delivery, 2020
Neelam Jain, Anurag Verma, Neeraj Jain
Rabbits were randomly divided into three groups (six rabbits in each group). Group I served as control while Group II and Group III were treated with G2 niosomal gel formulation and marketed Pilopine HS® gel respectively (Preethi & Kunal, 2016; Erfani et al., 2013). Glaucoma disease was induced by Bonomi et al., 1978 method. Rabbits were treated with subconjunctival injections of 0.25 ml Betamethasone injection (Betamethasone sodium 4 mg/ml) every week for three successive weeks in left and right eyes. Local anesthetic eye drops (Benox®) were used prior to subconjunctival injection. The activity was confirmed by noticing a bulge formation at the site of injection. The right eye of each rabbit was kept as control and the left eye was treated for glaucoma using 40 µl (equivalent to 40 µg of pilocarpine HCl) of selected formulation (G2) and marketed Pilopine HS® gel for group II and group III, respectively. The intraocular pressure (IOP) readings were measured using Schiotz Tonometer, before drug administration and 2 h, 4 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h and 24 h after drug administration. IOP was measured three times at each time interval and the means were recorded.