Ординатура / Офтальмология / Учебные материалы / Atlas of Glaucoma Second Edition Choplin Lundy 2007

APPLANATION TONOMETRY

Applanation tonometry is the procedure of choice in most clinical situations, and tonometers have been developed for use both at the slit lamp and handheld for use outside the examining lane. Included in this category are the Goldmann tonometer, the Perkins tonometer, the Mackay–Marg tonometer, the pneumatic tonometer, and the Tonopen. The underlying principle is similar for each instrument. Intraocular pressure is determined by measuring the force necessary to applanate, or flatten, a known surface area of the cornea.

Complete descriptions and instructions for use of each of the following instruments should be found in the manufacturer’s documentation provided with the instrument.

GOLDMANN TONOMETER

The Goldmann tonometer (Figure 4.7) is probably the most widely used tonometer and is the international standard for measuring intraocular pressure. In developing this tonometer, a number of assumptions had to be made. The first was that the human eye could be approximated to a per-

fectly elastic and infinitely thin sphere, and that its internal pressure could be measured by applying an external force that would be resisted by that pressure. It was assumed also that the cornea had a central thickness of approximately 520 m. However, human corneas vary considerably in thickness among individuals, and this is now known to affect the accuracy of Goldmann tonometry in measuring intraocular pressure. As early as 1975 it was shown, by measuring intraocular pressure manometrically at the time of cataract surgery, that eyes with thicker corneas measured higher than actual, and eyes with thinner corneas measured lower than actual. Measurements of intraocular pressure following laser vision correction are usually lower than the preoperative levels, now correctly attributed to the thinning of the cornea by laser ablation. Applying this principle to ocular hypertension, it has been demonstrated that eyes with ocular hypertension tend to have corneas that are thicker than average, and that eyes with thinner corneas are at higher risk for ‘converting’ to glaucoma at a given level of pressure. For example, in the Ocular Hypertension Treatment Study, eyes with pressures greater than 26 mmHg and central corneal thickness greater than 588 m had a 6% chance of converting to glaucoma within 5 years, while those with central

corneal thickness less than 555 m had a 36% chance. Similarly, many eyes with ‘normal tension glaucoma’ have been shown to have thinner than average corneas, and eyes with thinner than average corneas have also been show to be at higher risk for glaucoma progression. Clinically, it is essential to put intraocular pressure measurements within the context of central corneal thickness measurements in order to properly assess a patient’s risk for developing glaucoma or of it progressing. Although the exact relationship between true intraocular pressure, central corneal thickness, and intraocular

pressure as measured by the Goldmann applanation tonometer is not known, it has been estimated that the pressure varies by 1 mmHg for each 20 m of thickness above or below 540 m. This is summarized in Table 4.2. It is possible that in the near future we will have new tonometers available that will better approximate true intraocular pressure, either by incorporating central corneal thickness into their measurements, or by applying new measuring principles. One such tonometer under investigation as of this writing is the dynamic contour tonometer.

Figure 4.7 The Goldmann tonometer. Mounted on a slit lamp, the Goldmann tonometer consists of a tip attached by a rod to a coiled spring contained within the instrument housing. The tension on the spring, determined by the position of the calibrated knob, supplies the force used to flatten 3.06 mm of the central cornea. The tip contains a prism which splits the circular image of the flattened corneal surface into two semicircles, one above the other.

In using the Goldmann applanation tonometer, the flattening force to the cornea is supplied by a coiled spring contained in the housing of the instrument, controlled by a rotating knob at the base, and is applied to the anesthetized eye by the tip of a split prism device through which the cornea may be viewed with the slit lamp. The area of cornea flattened is 3.06 mm in diameter. Topical anesthetic and fluorescein are applied to the eye, and the tear film illuminated using the cobalt blue filter of the slit lamp. As the instrument is applied to the eye (Figure 4.8), the applanating head creates a circular tear film meniscus which may be viewed through the slit lamp. The prism in the applanating head splits the circular image into two semicircles, and the end point of the measurement is determined by adjusting the knob until the inside edges of the semicircles are just touching (Figure 4.9). The intraocular pressure is then determined by multiplying the reading on the scale on the knob by ten.

HAND-HELD TONOMETERS

The Perkins tonometer

The Perkins tonometer (Figure 4.10) is a batterypowered hand-held applanation tonometer which functions identically to the Goldmann tonometer. The mires are viewed directly through the instrument so a slit lamp is not necessary. It may be used in situations where the patient cannot sit at the slit lamp. The instrument is counterbalanced to allow it to be used in a variety of positions.

Figure 4.8 Measuring intraocular pressure with the Goldmann tonometer. The eye is anesthetized and the tear film stained with fluorescein. The instrument is then brought into the appropriate position on the slit lamp and the cobalt blue filter placed in the light path. The slit aperture should be opened widely. The tip of the instrument is gently placed against the cornea, and the image of the tear film is viewed through the biomicroscope. The knob is turned until the inner edges of the semicircles are just touching, indicating that 3.06 mm of corneal flattening has been attained, corresponding to the intraocular pressure. The reading on the knob is multiplied by ten to obtain the pressure reading.

The Mackay–Marg tonometer

The Mackay–Marg tonometer (Figure 4.11) was designed to measure intraocular pressure in eyes with scarred, irregular, or edematous corneas using a tip that is approximately half the diameter of that of the Goldmann tonometer. The movable tip protrudes from a surrounding foot plate and is supported by a spring connected to a transducer. The transducer senses the tension on the spring and translates that tension into a pressure reading. Basically, by advancing the tip on to the cornea, the bending pressure of the cornea (opposed by the intraocular pressure) is measured. Pressure readings are recorded on a moving paper strip.

Figure 4.10 The Perkins tonometer. This tonometer employs the same principles as the Goldmann tonometer. The prism is mounted on a counterbalanced arm and the change in force obtained by rotation of a spiral spring. The disc at the top of the instrument is placed against the patient’s forehead as a brace. The instrument can be used in any position and does not need to be held vertically.

Advances in electronics and miniaturization have led to the development of a self-contained, compact, hand-held applanation tonometer that works on the same principle as the Mackay–Marg tonometer.

NON-CONTACT TONOMETRY

The non-contact (or air-puff) tonometer (Figure 4.16) was originally developed to allow the measurement

Figure 4.11 The Mackay–Marg Tonometer. This is another applanation-type device in which the movement of the tip is sensed by a transducer contained in the hand piece. The bending pressure of the cornea, opposed by the intraocular pressure, is determined. The instrument is useful for measuring the intraocular pressure in scarred, irregular, and/or edematous corneas.

Figure 4.12 The pneumatic tonometer. The measuring device consists of a gas-filled chamber with a transducer capable of sensing the pressure in the chamber. As the device is applied to the eye, the counterpressure in the eye pushes back on the tip, raising the pressure on the gas until the end point is reached and recorded, either on a moving paper strip or displayed on a digital readout. This device may also be used for tonography.

Figure 4.13 The Tonopen™. This miniaturized electronic applanation tonometer is battery powered and can be used in any position. It is very useful for measuring intraocular pressure in patients who cannot sit at a slit lamp. It is also useful for measuring pressures in patients with irregular corneas, since the applanation area is much smaller than that of the Goldmann instrument.

Figure 4.14 Measurement of intraocular pressure with the Tonopen™. The eye is anesthetized and the instrument tip is covered with a latex sleeve. The examiner holds the upper lid open, rests the hand holding the instrument on the patient’s cheek, and gently and quickly touches the central cornea with the tip, moving the instrument rapidly on to and off the eye. A short beep will be emitted by the instrument when a measurement has been recorded, and the reading displayed when five successive measurements are obtained.

Figure 4.15 Intraocular pressure reading displayed on the Tonopen. The number indicates the average of the five readings in mmHg. A solid line will appear at the bottom of the printout corresponding to the percentage spread of the five readings (5%, 10%, 20%, or 20%).

of intraocular pressure by non-medical personnel since measurements may be obtained without the use of topical anesthetics. No part of the device touches the eye. The device uses a jet of air to applanate the cornea. Figure 4.17 shows the air jet and the sensors detect corneal flattening by means of reflected light. When activated, an air jet is blown at the cornea with increasing force over time. The cornea is illuminated with a collimated light beam which is reflected back to a detector, which determines when the reflected light reaches a maximum, corresponding to flattening of a corneal area 3.06 mm in diameter. The time required to reach this maximum reflection is directly related to the force of the air jet, counterbalanced by the intraocular pressure, and is translated into the intraocular pressure reading (Figure 4.18).

Figure 4.16 The non-contact tonometer. A patient is properly positioned for measurement of intraocular pressure with this instrument. The operator is seated to the left. No anesthetic is used. The eye to be measured is viewed on a CRT monitor, and, when the eye is properly aligned, the operator pushes a button which releases the air jet. Light sensors positioned on either side of the jet determine the degree of corneal flattening and when the end point has been reached. The patients usually react to the air jet with a slight startled response, but the measurement is not uncomfortable.

Figure 4.17 The ‘business’ end of a non-contact tonometer.

The air jet is in the middle, with the two sensors on either side. As the air jet increases in force, the sensors determine when the end point of corneal flattening has been reached. The time required to reach this end point is directly related to the force opposing the flattening, i.e. the intraocular pressure.

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INTRODUCTION

Collectively, primary angle closure and open angle glaucoma continue to be the leading causes of irreversible blindness afflicting at least 60 million people worldwide with over 7 million legally blind. Gonioscopy, the evaluation of the angle of the anterior chamber of the eye, is the only method to consistently diagnose and accurately differentiate these glaucomas. The long-term goal of gonioscopy is to reduce visual disability through the systematic evaluation and management of the chamber angle.

Gonioscopy is an essential component of vision care but continues to be underutilized. Recent studies of initial office visits for glaucoma in the United States found gonioscopy documented in less than half of all cases while evaluation of the disc was noted in 94%. Gonioscopy at an early stage of glaucomatous disease separates narrow from open angles with the resultant appropriate laser therapy and prevention of visual disability.

WHY PERFORM GONIOSCOPY?

Gonioscopy is underutilized because the function and appearance of the angle is typically inferred from the view of the anterior chamber as seen at the slit lamp. In addition, the reserve of the chamber angle is immense and may hide disease for years. Thus, even though the anterior chamber is always evaluated during routine slit lamp biomicroscopy, the chamber angle, with its vital outflow system and diagnostic clues, is hidden from ordinary view due to total internal reflection of light rays. Many examiners use the van Herick test to estimate depth of the peripheral anterior chamber. It is a screening tool designed to estimate depth of the iridocorneal angle but not a substitute for gonioscopy.

The intraocular pressure (IOP) is normal, and the chamber looks quiet. Why perform gonioscopy? (Table 5.1). It is good medical care to initially document the appearance of the chamber angle, just as it is to record the baseline appearance of the optic nerve for future reference.

The insurmountable problem of total internal reflection of light at the air–cornea–aqueous boundary is overcome with gonioscopy. With appropriate training, gonioscopy is easily integrated into a busy office schedule with minimal effort. This facilitates the differentiation of normal from abnormal angle structures and allows the early detection of ophthalmic disease. One gonioprism will not suffice for all occasions; the best gonioprism to rapidly screen the chamber angle in the clinic is not the best for laser therapy of the chamber angle or the best for an examination under anesthesia (Figure 5.1). This is due to the variable size, contour, contact area, mirror size and handle of the multitude of goniolenses and gonioprisms introduced in the next section.

PRINCIPLES OF GONIOSCOPY

The most important gonioscopic principle is to understand why total internal reflection of light occurs in the anterior compartment of the eye. Conquering this optical problem allows a more thorough appreciation of angle optics. Light rays emanating from the iris undergo four possible optical properties as they emerge from the cornea:

(1) emerge bent; (2) emerge unbent or parallel;

(3) follow the boundary; or (4) reflect internally. The observer does not see light rays from the chamber angle because of the last property, total internal reflection. A basic understanding of geometric optics is useful. Figure 5.2 is a good example of a patient with iris defects that involve angle structures that are clearly appreciated only through gonioscopy.

Without gonioscopy it is impossible to know how serious the angle pathology is.

The two most common clinical methods of viewing the chamber angle include direct and indirect gonioscopy (Figure 5.3). A Koeppe goniolens represents the prototype direct view of the angle and is most commonly used in the operating room. This is an excellent method to view the angle during an examination under anesthesia. A hand held slit lamp magnifies the view, creating a beautiful stereoscopic view of angle structures. The angle structures are viewed directly through the goniolens as seen in Figure 5.2c. Indirect gonioscopy is a rapid and efficient office-based approach to indirectly

view the chamber angle as seen in Figure 5.2e. The light is reflected to the examiner from angle structures 180 away from the mirror. This is an important distinction because angle structures are directly viewed through a goniolens and indirectly seen through a gonioprism.

THE NORMAL ANGLE

An understanding of the immense variability of the normal angle is imperative in order to recognize the abnormal angle. Each angle is unique and changes as the patient ages, further complicating