John A. McDermott
Schiøtz developed the first device that quantified intraocular pressure (IOP) with relative reproducibility. In combination with its simplicity and economy of design the Schiøtz tonometer could be used in the primary physician's office. A second major advance was the application of the principle of applanation by Goldmann, which improved on Schiøtz's device, both simplifying the examination further and improving on the validity and reproducibility of Schiøtz's instrument. As opposed to a manometer, which measures pressure directly and is impossible in an intact eye, both methods measure the pressure indirectly by deforming to some degree the surface of the globe and by 'converting' this deformation into the IOP.
INDENTATION TONOMETRY
With the Schiøtz tonometer, a series of known, standard weights are applied to the cornea via a plunger (Fig. 195.1). The plunger indents the cornea, and a scale records the deformation of the globe. These two values are then used to determine the IOP. The plunger moves in a vertical fashion in the center of the instrument and passes through a curved footplate that sits on top of the cornea with the patient in the supine position. A 'holder' fixes the footplate on the cornea but allows free movement of the plunger and the attached weights in the vertical direction. When held properly, the only force acting on the plunger and weights is the opposing force of the IOP (except for a negligible force of friction between the shaft of the holder and the plunger). A movement of the plunger from its 'zero' position toward the cornea can be correlated with deformation of the cornea. Since the excursion of the plunger is relatively small and would be difficult to read, a lever magnifies the excursion along a more readable calibrated scale. The greater the scale reading with a given weight, the greater will be the excursion of the plunger and the deformation of the globe. The IOP will therefore be lower.
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FIGURE 195.1 Schiøtz tonometer. |
THEORETICAL BASIS
Unfortunately, by applying a weight to the globe, one is not measuring the true 'steady-state' IOP. When the plunger indents the cornea, deforming the globe, the steady state IOP (P0) is raised to a higher pressure that is the pressure induced by the tonometer (Pt). Schiøtz performed experiments using a manometer to accurately measure the IOP of enucleated eyes. The experimental design allowed manipulation of the IOP in the eye. Thus P0 and Pt could be correlated with a given weight and a given scale reading. By changing the weight on the tonometer and by repeating the experiment, a chart could be constructed that would indicate the P0 for any given weight and scale reading. From these data, and by repeating Schiøtz's technique in his own experiments, Friedenwald derived a formula that more accurately determined the IOP.[1]
When the tonometer is placed on the eye, the indentation of the cornea results in a distention of the globe. Thus the scale reading, along with indicating the indentation of the cornea, also reflects this same distention. Friedenwald's formula relates this distention to the IOP. The formula required a constant 'K' or the 'coefficient of ocular rigidity', which is a measure of the resistance of the eye to the distending forces of the tonometer. Friedenwald's formula allowed more accurate tables to be established. Friedenwald first published his tables in 1948, and an updated version, which Friedenwald thought to be more accurate, is known as the 1955 tables. Comparison with applanation tonometry suggests that the 1948 tables are more precise. Unfortunately, not all eyes behave in the same fashion to external pressure, and the tables established by Friedenwald are based on a single K value (0.0245 for the 1948 tables, and 0.0215 for the 1955 tables). Friedenwald determined that the value of K for an individual eye could be calculated from two tonometric scale readings using different weights. Friedenwald's 'nomogram' allows one to graphically determine K from these two values. At present, simplified tables exist that obviate the need for calculation and provide both the P0 and the K values from the paired scale readings on the involved eye.
CLINICAL TECHNIQUE
The patient is supine with the cornea anesthetized. The fingers of the examiner spread the lids carefully to avoid putting pressure on the globe. The patient is asked to fixate while the tonometer footplate is applied to the cornea, and the handle is positioned to keep the tonometer vertical and to allow free movement of the plunger to indent the cornea. The needle will oscillate with the ocular pulse, and the midpoint of the excursion is used as the scale reading. If the value is not greater than four units, an additional weight is added. In recording the measurement, the scale reading, the weight used, and the IOP (as read from the appropriate tables) are noted.
LIMITATIONS
The commonly available conversion tables use an average K value to calculate the IOP. If the true K value of the eye is higher than the average K value, the table will overestimate the true IOP. Similarly, a falsely low IOP will result if the true K value is less than the average K value. High ocular rigidity has been reported in patients with high hyperopia,[2] extreme myopia,[1] chronic glaucoma,[3] and vasoconstrictor therapy.[1]
Low ocular rigidity may occur with high myopia,[2] miotic therapy (especially cholinesterase inhibitors)[2] after retinal detachment surgery,[3] intravitreal injection of gas,[4] and vasodilator therapy.[1]
Falsely high IOP readings may be obtained with thick corneas or very steep corneas.[5] Given significant corneal pathology, and on an irregular surface, Schiøtz's measurements are unreliable.[6]
Since tables exist to overcome the rigidity problem, this alone has not resulted in a decline of Schiøtz tonometry; rather, the ease and accuracy of applanation tonometry, without the need for a supine patient, multiple readings, and reference to tables, have allowed applanation tonometry to replace Schiøtz tonometry with few exceptions. The Schiøtz tonometer, however, is still the basic ingredient in tonography and is described later in this chapter.
APPLANATION TONOMETRY
THEORETICAL BASIS
Applanation tonometry is based on the Inbert-Fick principle, which states that for an ideal sphere, the pressure (P) inside the sphere is equal to the force (F) required to applanate (flatten) its surface, divided by the area (A) of flattening:
The ideal sphere is dry, thin-walled, and readily flexible. The cornea, which is not a true sphere, also has none of these three characteristics. Consequently, there are two other significant forces that should be considered. The force of capillary attraction (T) between the tonometer head and the tear film is additive to the external force. In addition, a force (C), independent of IOP, is required to flatten the relatively inflexible cornea. Thus,
becomes
The variable A, about which we are concerned, is located on the interior surface of the cornea. The Goldmann applanator is designed so that A is equal to 7.35 mm2. To achieve this, the diameter of flattening of the cornea is 3.06 mm. With this value for A, the opposing forces of capillary attraction and corneal inflexibility cancel out.
In addition, using this value for A, the IOP in millimeters of mercury is equal to 10 times the force applied to the cornea in grams, which is a convenient conversion. Since only 0.5 ?L is displaced from the eye and the additional increase in pressure induced in the eye from its steady state by the tonometer tip is negligible, applanation tonometery is not significantly affected by ocular rigidity.
GOLDMANN APPLANATOR
The tonometer 'tip', a tapered plastic cylinder containing a biprism, is the contact point with the cornea. The tip is connected via a rod to the body of the tonometer, which contains an adjustable spring that provides the appropriate applanating force (Fig. 195.2). The force is adjusted manually via a knob that contains a scale indicating the force applied in grams. When the endpoint is reached, the reading in grams is multiplied by 10 to convert to millimeters of mercury (Fig. 195.3).
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FIGURE 195.2 Tonometer tip approaches the eye. |
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FIGURE 195.3 Goldmann's applanation tonometer. |
The biprism splits the image of the circle of contact into two semicircles. When the inner margins of these semicircles just touch (Fig. 195.2), a 3.06-mm-diameter circle of cornea is applanated. The instrument is attached to the slit lamp, aligning the axis of the tip with the ocular and allowing visualization of the semicircles or mires.
CLINICAL MEASUREMENT
The patient is positioned at the slit lamp in the usual fashion after instilling topical anesthetic and sodium fluorescein into the tear film. The patient is instructed to fixate in the distance, to relax, and to breathe normally. If necessary, the lids are separated (without pressure). As the tip is advanced toward the cornea, gross horizontal and vertical adjustments are made by the examiner without using the oculars. As the instrument approximates the cornea, the cobalt-blue filter is inserted into the slit-lamp illuminator, and maximal illumination is used. The 1-g position is used before each measurement. Generally, it is more accurate to increase rather than decrease the force of applanation.[7] When contact is imminent, the examiner uses the ocular to observe the mires, which will appear green against a blue background. If the mires are of unequal size, vertical adjustment is made. The tonometer knob is rotated until the endpoint is achieved (Fig. 195.4). Ocular pulsations are noted, and the midpoint of the excursion of the internal margin of each semicircle is aligned. For accurate readings, certain precautions must be met. Valsalva maneuvers, or breath-holding by the patient, must be avoided. The semicircles should be clear with distinct margins. Wider, blurred semicircles result in falsely high readings as does vertical misalignment.[8] Measurements without the use of fluorescein underestimate the true IOP.[9]
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FIGURE 195.4 Optical endpoint of applanation. (a) Correct endpoint. (b) Overestimate of IOP. (c) Underestimate of IOP. (d) Vertical misalignment. |
Corneal astigmatism may result in false pressure readings. The error has been calculated at 1 mm for every 4 D (underestimated for with-the-rule; overestimated for against-the-rule). The biprism should be rotated in its housing so that the axis of least corneal curvature aligns with the red line on the prism holder; alternatively, an average of the pressure obtained with semicircles aligned horizontally and then vertically may be used.[10]
Prolonged contact of the applanator to the cornea should be avoided. Damage to the cornea may occur with fluorescein staining and distortion of the mires.[10] One may also observe a gradual decrease in the tonometer reading as contact is maintained on the cornea.[11]
TONOMETRY AND INFECTION
Contaminated tonometers are well-established vectors of infection. Along with the more common bacteria and viruses that cause ocular infection, hepatitis-B surface antigen can be isolated from the tonometer tip after applanation of infected patients. The human immunodeficiency virus has been isolated from human tears, although no cases of transmission from contaminated tonometers have been reported. Meticulous sterilization may reduce the risk of these pathogens.
The Centers for Disease Control and Prevention (CDC) suggests a 5-10 min soaking in 3% hydrogen peroxide or 70% ethanol or isopropanol. The tip should then be washed under running water and dried thoroughly before reuse.[12] A 'clinical alert' issued by the American Academy of Ophthalmology in 1989 states that rubbing the prism tip with a 70% isopropyl alcohol wipe was an acceptable procedure for disinfection. Care should be taken to thoroughly remove any disinfectant from the patient contact surface because these agents will cause corneal epithelial defects.[13]
REPRODUCIBILITY OF GOLDMANN APPLANATION MEASUREMENTS
How reproducible are Goldmann applanation measurements? In one study of untreated glaucoma patients and glaucoma patients on medicine, repeat measurements were made within a few minutes of each other. In the 145 eyes, differences of 2 mmHg or more occurred in 35%. Differences of 3 mmHg or greater occurred in only 7% of readings.[11]
CORNEA FACTORS AFFECTING ACCURACY OF APPLANATION MEASUREMENTS
For the effect of cornea factors on clinical applications, see also section on Miscellaneous Clinical Applications - Which Tonometer?
Astigmatism
The effect and correction for astigmatism has been discussed under Clinical Measurement.
Cornea Curvature
Corneal curvature appears to influence applanation tonometry readings. In 200 patients undergoing 'routine eye examination', whose vital statistics and mean IOP demonstrated the group to be a representative sample of the general population, there was a positive correlation between corneal curvature and tonometer readings. For each 3-D increase in corneal power in this sample, the average IOP increased 1 mmHg.[14] A more recent study minimizes the effect of curvature and suggests that the effect is not clinically significant.[15]
Central Corneal Thickness
The present discussion is limited to corneal thickness in corneas with normal stroma. Goldmann applanation tonometry in diseased corneas, especially in corneas thickened from edema, where applanation tonometry tends to underestimate the IOP, is dealt with elsewhere.
Although Goldmann and Schmidt,[8] recognized the effect of variations in corneal thickness on the outcome of applanation tonometry in their groundbreaking 1957 article, significant variation from average corneal thickness was thought to be rare. When optical pachymetry became available, a positive correlation between CCT and IOP measurements was documented.[16] Experimentally, this correlation was confirmed by Ehlers, who cannulated 'normal' eyes undergoing cataract surgery. Manometric and applanated pressures were most similar when the central corneal thickness (CCT) was 0.520 mm. Discrepancies as high as 7 mmHg per 0.100 mm of deviation from 0.520 mm were measured, with thinner corneas resulting in an underestimation of IOP by applanation and thicker corneas overestimating IOP.[17]
Subsequent studies document a wide variation in corneal thickness in normal eyes. A meta-analysis of over 30 years of published studies, analyzing 300 data sets found an average CCT of 0.534 mm (0.530 ± 0.029 mm for optical pachymetry vs 0.544±0.034 mm for ultrasonic pachymetry).[18] Where diurnal variation could be determined, the average coefficient of variation was 5.8% of mean CCT measurements.[18] CCT has been determined to be unrelated to cornea dimensions.[18] Although CCT appears to be unrelated to age in whites, the data regarding nonwhites is more equivocal.[18] African-Americans have been shown to have thinner corneas than Caucasians.[19,20] Some studies have shown Hispanic patients fall somewhere between African-Americans and Caucasians[21], while other studies suggests that Hispanics are equivalent to Caucasians and Asians with no difference among these three groups.[22]
Differences in CCT have been documented by many studies among various glaucoma syndromes, with thinner cornea measurements in patients with normal tension glaucoma (NTG) and thicker corneas among ocular hypertensives (OHT), with patients with primary open angle glaucoma (POAG) in the middle of the range.[23-26] Shah et al found that 85% of eyes with NTG, but only 36% of POAG had CCTs of <.540 mm. 42% of eyes with OHT had CCTs greater than 0.585 mm, but only 13% of eyes with POAG had similar values. Copt et al,[26] using a correction factor of 5 mmHg per 0.070 mm of deviation from the mean CCT for normal controls, found that 31% of eyes with NTG could be reclassified as POAG, while 56% of eyes with OHT could be reclassified as normal.
Any doubt regarding the import of CCT for the management of glaucoma, or more specifically, for glaucoma suspects, was eliminated by the landmark ocular hypertension treatment study (OHTS), the first study to prospectively document that a thin CCT was a risk factor, and a potent one, for development of POAG. The study compared the development of glaucoma in treated versus untreated OHT. Subjects with a CCT of 0.550 mm or less were three times as likely to develop glaucoma than subjects with a CCT ? 0.588 mm. Within the subgroup of the observation arm of the study that had the highest pressure, (mean IOP of 27.9 mmHg), 36% of participants with a CCT of less than 0.555 mm, went on to develop POAG, as opposed to only 6% of participants with a CCT of greater than 0.588 mm.[27] The impact of the study was so clear, that the American Academy of Ophthalmology published an early revision of its Preferred Practice Patterns for management of the glaucoma suspect. The guidelines include the results of the OHTS study and recommend that CCT be performed on all glaucoma suspects.[28]
The question of how to use the results of pachymetry has not been definitively resolved. Correction factors have been suggested by various investigators dating back to Ehler's manometric experiments, but none have been validated. In their meta-analysis, Dowdy and Zamen suggested an overall correction factor of 3.5 mmHg per 0.050 mm of CCT deviation from the mean. The correction factor varied, however, with a correction factor of 1.1 mmHg per 0.050 mm for 'normal eyes', to 2.5 mmHg per 0.050 mm for eyes with 'chronic disease', such as glaucoma, to as much as 10.5 mmHg per 0.050 mm for eyes suffering 'acute' pathology.[18] In the American Academy of Ophthalmology's Basic and Clinical Science Course, Vol. X, Glaucoma, suggested a correction factor of 0.5 mmHg for every 0.010 mm of deviation from a mean CCT 0.542 mmHg. The authors describe this as a 'rough' guide to adjust IOP. The assumption that CCT affects the glaucoma by distorting the results of applanation tonometry may only partially explain its impact as a risk factor for glaucoma. Other investigators have suggested biologic correlates other than IOP (or in addition to IOP). Lesk et al[29] demonstrated, with the confocal scanning laser, greater forward displacement via the lamina cribrosa after pressure lowering in eyes with thinner corneas. The authors of the OHTS study even caution that '. we cannot exclude the possibility that corneal thickness is related to other factors (besides IOP) affecting susceptibility to glaucomatous damage.'[27]
LASER REFRACTIVE SURGERY
The increasing number of individuals undergoing refractive surgery presents a potential problem in the future for the diagnosis and management of glaucoma. Since the bulk of these individuals are young myopes, already genetically susceptible to glaucoma, and since the most popular procedures, photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), result in thinning of the cornea, the number of 'missed' cases of glaucoma could be substantial. Duch et al,[30] in a prospective study, compared Goldmann applanation tonometry and pneumotonometry in 118 eyes, pre- and 3 months post-LASIK. Preoperatively, a good correlation was measured between the two techniques. Postoperatively, the correlation was poor. Essentially, pneumotonometer readings remained the same, while Goldmann readings decreased significantly (2.9 mmHg per 0.070 mm reduction in cornea thickness). Park et al,[31] 6 months after LASIK in 83 patients, found that Goldmann applanator readings were, on average, 3.9 mmHg lower (25.2%) than pre-LASIK readings. Although corrective factors have been applied to LASIK-thinned corneas, as with corrective factors applied to unaltered corneas, none have been validated. In the early postop period, elevated pressures may be missed, when concerns about disturbing a flap or a healing epithelium defer measurement. Besides various postop corneal syndromes that may be assumed to elevate IOP, corticosteroid-induced glaucoma is a real danger. Pneumotonometer readings or Tono-Pen measurements of the peripheral cornea have been shown to more accurately reflect actual IOP in these scenarios.[32]
Since refractive surgery patients are usually young, many years might intervene between their refractive surgery and the issue of glaucoma. Since detection of ablation of the cornea at the slit lamp is problematic, a history offered by the patient might be the only clue to an underestimation of the IOP. The development of tonometers independent of CCT might alleviate this problem, and some devices are already in development. Until technology provides this solution, an argument can be made for pachymetry becoming a significant element of the initial ophthalmic examination even in an asymptomatic patient.
OTHER APPLANATION TONOMETERS
The Perkins applanation tonometer uses the same biprism as the Goldmann applanator. The light source is powered by battery, and a counterbalance enables the instrument to be used in both the vertical and the horizontal positions (Fig. 195.5). The readings are consistent and compare well with the Goldmann applanator. It is especially useful in the operating room for examinations under anesthesia and for invalid patients, infants, or children who cannot sit at the slit lamp.[33] The Draeger tonometer is similar to both the Perkins and the Goldmann tonometers except that a different biprism is used. However, this device is also portable and can be used in any position, similar to the Perkins tonometer.[7]
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FIGURE 195.5 Perkins' applanation tonometer. |
The MacKay-Marg tonometer applanates the cornea via a plunger that moves within a sleeve, in a fashion similar to a Schiøtz tonometer. The excursion of the plunger is electronically coupled to a transducer and graphically records the movement of the plunger on a moving strip of paper. The plunger first indents the cornea, recording on the graph paper, the sum of the force required to flatten the cornea and the IOP. As the tonometer advances, the sleeve abuts the cornea, transferring the force required to flatten the cornea to the sleeve. The pressure tracing then decreases to a level that represents the IOP. Because the tonometer records instantaneously, multiple readings should be averaged in order to adjust for fluctuation in pressure due to the ocular pulsation.[34] It is especially useful in edematous or irregular corneas.[35]
The principle of the pneumotonometer is similar to that of the MacKay-Marg tonometer. Corneal contact of the pencil-like tip records both the IOP and the force required to bend the cornea. Further advancement of the tip transfers the latter force to the surrounding 'collar'. In this case, the 'plunger' is replaced by a column of air and the contact surface is a polymeric silicone (Silastic) membrane. The air column is continually vented via a port. Changes in pressure in the column resulting from the applanation are recorded, via a transducer, on a moving strip of paper. Similar to the MacKay-Marg unit, this instrument is especially useful with edematous and irregular corneas.[36]
The Tono-Pen, which is a miniature, hand-held tonometer, works on a principle similar to that of the MacKay-Marg tonometer (Fig. 195.6). The instrument is 18 cm in length and weighs only 60 g. The MacKay-Marg waveform is analyzed internally by a microprocessor. About 4-10 estimations of the pressure, each of which is obtained by a brief touch to the cornea, are averaged, and the digital readout displays both the mean of the accepted estimations and their coefficients of variance (5%, 10%, 20%, <20%).
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FIGURE 195.6 Tono-Pen, hand-held tonometer. |
In manometric studies with eye bank eyes, the Tono-Pen was highly accurate from pressures of 10-50 mmHg. Clinically, the same study reported no statistically significant difference between Goldmann and Tono-Pen readings in the range of 10-35 mmHg.[37] In another clinical study of 270 eyes, the Tono-Pen measured an average of 1.7 mm higher than the Goldmann tonometer for pressures from 6 to 24 mmHg. Above 24 mmHg, the readings were similar. Large discrepancies (<6 mmHg) were found in only 18 of 270 eyes. In all except five eyes, obvious causes such as astigmatism or corneal disease could explain the discrepancy.[38] In another large study (142 eyes) comparing the Goldmann tonometer and the Tono-Pen, 63% of Tono-Pen readings were within ±2 mmHg of the Goldmann readings (77% within ±3 mmHg). The best correspondence occurred in the 11-20 mmHg range. Although the Tono-Pen tended to overestimate pressures in the 4-10 mmHg range, and underestimate in the 21-30 mmHg range, the authors concluded that even in these ranges, the discrepancy was small enough to consider the Tono-Pen 'clinically useful'.[39] Other studies have confirmed this tendency to overestimate at lower pressures and underestimate at higher pressures.[38,40]
The noncontact tonometer applanates the cornea by means of a jet of air. Once the instrument is properly aligned with the patient's eye, a fixed distance separates the cornea from the instrument. An optical system measures the time that it takes for the air puff to flatten the cornea. The time required to flatten the cornea is directly related to the force of the air jet and thus correlates with IOP.[41] Mean IOP readings compare favorably with Goldmann tonometry, although relatively large discrepancies could be found in some patients. This device can be used without anesthesia but is more accurate when anesthesia is used. Patients should be warned about the force of the air jet to reduce the potential of startling them.
MISCELLANEOUS CLINICAL APPLICATIONS - WHICH TONOMETER?
There is no question that Goldmann applanation tonometry is the most accurate, reproducible, and convenient method for routine tonometry in patients with corneas amenable to this method. Although the Tono-Pen shows good correlation with Goldmann readings (see previous section) it should not be used routinely, especially in known glaucoma patients or those in whom glaucoma is suspected. For patients who cannot assume the normal position at the slit lamp, such as bedridden patients, the Perkins tonometer gives accurate readings but is subject to the same limitations as the Goldmann tonometer in regard to corneal thickness.
In eyes with scarred or edematous corneas, various studies confirm the accuracy of the MacKay-Marg and pneumatonometer.[42-44] In one study including eyes with irregular corneas and eyes that had recently undergone keratoplasty or epikeratophakia, the Tono-Pen was equivalent to the MacKay-Marg in accuracy.[45] However, in another study of postkeratoplasty eyes (where the graft did not preclude Goldmann tonometry), in 57% of the eyes Goldmann and Tono-Pen readings varied by 3 mmHg or more, with the Tono-Pen usually showing higher readings.[46]
Because of its convenience and portability, many studies have attempted to validate the accuracy and reproducibility of the Tono-Pen in diverse clinical situations. Manometric and clinical studies demonstrate that the Tono-Pen is reliable and accurate in eyes with bandage plano T lenses (this, however, was not the case for soft contact lenses of different powers).[47] In vitrectomized, gas-containing eye bank eyes, Tono-Pen values showed a high correlation with manometric readings up to 25 mmHg. Above 30 mmHg, the Tono-Pen underestimated the manometric readings by an average of 21%.[48] In a pediatric population ranging in age from 1 to 60 months, under general anesthesia, there was a high correlation between Perkins and Tono-Pen readings in the range of 0-30 mmHg (Schiøtz tonometry significantly overestimated pressures and its use was not recommended).[49] In manometric studies of eye bank eyes with flat anterior chambers (lens-cornea apposition), applanation pressures measured by Goldmann, pneumotonometer, and Tono-Pen were so inaccurate and unpredictable as to render all three methods useless. The authors recommended tactile estimation to help clarify these clinical situations.[50]
The issue of tonometry after refractive surgery has been discussed earlier in this chapter (see section on Laser Refractive Surgery).
TONOGRAPHY
Schiøtz noted that repeated tonometry within a relatively short period resulted in a lowered IOP measurement. The rate at which IOP decreased seemed to be slower in eyes with glaucoma than in normal eyes.
When external pressure is applied to the eye, aqueous humor is expressed through the outflow channels, resulting in this lowering of IOP. Because of its deranged outflow function, this occurred more slowly in the glaucomatous eye. In 1950, Grant described tonography, a technique to measure the decrease in IOP that occurs when an external weight is applied to the eye. The formulas thus derived would allow quantification of the rate at which aqueous humor could be forced through the outflow channels by the weight of the tonometer. Grant called this newly derived characteristic of the eye 'the facility of aqueous outflow'.[51]
Theoretical Basis
Grant ingeniously used a paper strip recorder (such as that found in an electrocardiograph) connected to an electronic tonometer to record a continuous tracing of the changes in scale units that occurred once the tonometer was resting on the eye (Fig. 195.7).[51]
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FIGURE 195.7 Tonography unit. |
In the normal eye, there is a gradual decrease in IOP, resulting in a tracing with a gentle downward slope. In the glaucomatous eye, which has an increased resistance to expression of fluid through the outflow channels, there is less of a change in the IOP (indicated by a smaller change in the Schiøtz scale units), with the resultant tracing having a flatter slope (Fig. 195.8). From the tracing, the value of the facility of aqueous outflow can be determined from Grant's equations.
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FIGURE 195.8 Tonography tracings. (a) Typical tracing, 'normal' C value. (b) 'Flat' tracing in an eye with a glaucomatous C value. |
The softening of the globe is due to the expression of a certain volume of fluid from the eye, ?V. The greater the pressure applied to the eye, and the longer the time interval that the pressure is applied (T), the greater will be the value of ?V:
This proportion becomes a mathematical equation by the addition of a 'proportionality constant', which Grant calls 'C' or the facility of aqueous outflow:
The derivation of ?V and DP from the tracing requires an understanding of the mechanics by which IOP is measured with the Schiøtz tonometer. When the tonometer is placed on the eye, the plunger indents the cornea and the weight of the instrument distends the globe. The IOP is elevated from its 'steady state' pretonometer value, P0, to a higher pressure, i.e., the pressure induced by the tonometer Pt. For each scale reading, Friedenwald measured not only the volume of the corneal indentation and the volume of the distention of the globe but also the P0 and Pt (see section on Indentation Tonometry).[52,53] As the tonometer rests on the eye, the IOP decreases, the corneal indentation increases, and the distention of the ocular coat decreases. The value of ?V, which is the amount of fluid expressed from the eye at the end of time T, would be the difference between the volume of corneal indentation and the volume of ocular distention at time T. Using Friedenwald's data, Grant devised tables that provide the value of ?V, based on the initial and final Schiøtz scale readings during the tracing.
Since P0 and Pt for any scale reading could be determined from Friedenwald's tables, D P, the pressure induced by the tonometer above the steady state is equal to Pt - P0. However, since the IOP decreases during the tracing, DP is constantly changing. Grant calculated that this changing DP could be represented with minimal error by averaging the values of DP (i.e., Pt - P0) at each half-minute of the tracing. In the standard tracing, T is equal to 4 min. With the values for DP, ?V, and T, the facility of outflow C can therefore be determined in ?L/min per mmHg. In clinical practice, Grant's formula is incorporated into standard tonography tables; thus, the value of C may be determined by recording the initial and final Schiøtz scale readings and the tonometer weight applied during the 4-min tracing.
As with Schiøtz tonometry, the calculation of outflow facility is affected by ocular rigidity (see section on Indentation Tonometry). Tonography tables are based on a normal coefficient of ocular rigidity of 0.0215. With low ocular rigidity (e.g., high myopia), Schiøtz tonometry underestimates the true IOP, and the resultant C value is falsely low. Routinely, however, applanation tonometry is performed just before the application of the Schiøtz tonometer during tonography. A discrepancy between the two values allows an accurate calculation of C via the Friedenwald nomogram.
Test Performance
After applanation tonometry has been performed, the patient lies supine in a quiet setting. Both eyes are anesthetized and the electronic Schiøtz tonometer, which has been calibrated previously, is applied to the eye while the patient fixates on a ceiling target with the uninvolved eye. The proper Schiøtz weight used during tonography is determined by the initial applanation, and the standard tracing of 4 min is obtained. Proper tonography requires attention to detail, and several excellent manuals have been written to help one eliminate the numerous sources of error that can occur during the performance of the test.[54,55]
An acceptable tracing has a smooth gradual slope, with small oscillations indicating the ocular pulse and somewhat less apparent cycles of longer duration due to respirations. Any Valsalva maneuver, such as coughing or sneezing, will invalidate the tracing. When an appropriate tracing is obtained, the technician draws a line through the tracing, approximating the slope that allows one to read the scale units at time 0 and at 4 min. These are then used to determine C from the tonography tables or the Friedenwald nomogram.
Clinical Implications
Grant's initial paper, involving repeated examinations on normal eyes, determined C values with a range of 0.15-0.34 ?L/min per mmHg, with a mean of 0.243. Subsequent studies confirmed this normal C value.[56,57] In a given eye, outflow facility is fairly consistent and compares favorably with that obtained by perfusion experiments in enucleated eyes.[58,59] No gender differences have been detected. Cvalues appear to decrease gradually with age. After initially describing the techniques of tonography, Grant reported his results in more than 1000 tonograms on 600 normal and glaucomatous eyes.[60] His findings resolve the issue of whether the elevated pressure in glaucoma was caused by increased aqueous production or by decreased aqueous outflow. Without exception, reduced outflow facility could account for the elevation of IOP in the glaucomatous eyes. Lower C values, some of which were 0.0, occurred during attacks of angle-closure glaucoma. In chronic angle-closure glaucoma, C values would decrease proportional to the degree of closure of the angle. Administration of topical miotics increased outflow facility, thus establishing the mechanism of these drugs.
In the initial studies, tonography appeared to demonstrate a clear demarcation between normal and glaucomatous eyes. The C value in normal eyes ranges from 0.11 to 0.44 ?L/min per mmHg. In the glaucomatous eyes, C values did not exceed 0.11 ?L/min per mmHg. It was anticipated that in patients with suspected glaucoma (i.e., patients with normal optic nerves and visual fields), those patients with C values in the glaucomatous range would be especially likely to acquire optic nerve damage and thus be candidates for early intervention. Subsequent studies, however, did not demonstrate such a clear-cut distinction between glaucomatous and normal eyes. There is a broad overlap in these two groups between C values of 0.10 and 0.20.[61] In another study involving more than 1300 eyes, fully 35% of the glaucomatous eyes had values greater than 0.18.[58]
To better separate normal from glaucomatous eyes, Leydecker and Becker suggested a ratio of the IOP to the C value.[62,63] The higher the IOP and the smaller the C value, the greater will be the P0/C ratio. Although there was still a considerable overlap, this approach seemed to enhance the separation of the two groups compared with using the C value alone. Using a P0/C of 100 as the demarcation, 71% of glaucoma patients exceeded this value, whereas only 2% of normal individuals fell into this range.[46] P0/C ratios are usually included in the standard tonographic report.
Several longitudinal studies investigated tonography and its prognostic value in predicting glaucomatous optic nerve damage in populations of individuals in whom glaucoma was suspected. The results of these studies were equivocal.[63-67] In known glaucoma patients, attempts were made to use tonography as a prognosticator for the progression of disease. Most of these indicated that tonography added no additional information over tonometry in predicting in which patients disease would progress.
Interest in tonography waned as it became apparent that it would offer no easy solution in predicting which patient with suspected glaucoma would experience visual-field defects. It is apparent that at any given pressure, different eyes have different susceptibilities to optic nerve damage. Advances in ocular imaging have diverted attention toward the posterior segment in determining guidelines for the initiation of treatment and documentation of stability. Although the clinical use of tonography has decreased, it is still a key research tool, especially in determining the mechanism of action of new therapeutic agents in the treatment of glaucoma.
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