Section 6-A: What to expect
An eye examination to assess for glaucoma is not very different from the standard check-up visit conducted by an eye doctor. In fact, many elements of a glaucoma evaluation are routinely conducted at all visits to an eye doctor. These glaucoma vital signs are listed in Table 6-1 and are discussed in detail below. The elements of this examination are painless and well-tolerated by most patients.
|Element of the exam||Purpose|
|Visual acuity test (reading the eye chart)||Assess central vision|
|Intraocular pressure (IOP) measurement||Test for pressure within the eye|
|Gonioscopy||Assess drainage angle (open or closed)|
|Visual field test||Assess peripheral vision|
|Optic nerve exam (for cupping)||Assess optic nerve for damage|
|Imaging Study (color photos, computerized optic nerve analyzers)||Assess health of optic nerve|
Some parts of a glaucoma examination are done at every visit, while other parts are evaluated less frequently. Visual acuity, intraocular pressure (IOP), and the optic nerve appearance are assessed at every visit, while the drainage angle and peripheral vision are measured at regular intervals (typically once a year or so).
Color photographs are taken of the optic nerve using a camera that is focused through the pupil towards the back of the eye. These pictures document the appearance of the optic nerve at a particular timepoint. The photos are used by the eye doctor to help recognize progressive damage to the nerve (increased cupping) by allowing a comparison of the current optic nerve appearance to a prior photograph.
Specialized imaging devices such as an Optical Coherence Tomograph (OCT), Heidelberg Retinal Tomograph (HRT), or a scanning laser polarimeter (GDx) may also be used to help assess the health of the optic nerve. These instruments take images of the optic nerve and retina similar to a photographic camera. However, the images captured by these devices are used to quantify the amount of cupping and thickness of the fibers that make up the optic nerve.
Section 6-B: Intraocular pressure and corneal pachymetry
The normal intraocular pressure (IOP) is between 10 to 21 mmHg. Any intraocular pressure lower than 5 mmHg is considered abnormally low and is called hypotony. An intraocular pressure greater than 21 mmHg is considered abnormally high and is often referred to as ocular hypertension. High intraocular pressure does not necessarily mean that you have glaucoma. It simply increases the risk for the development of glaucoma. The higher the IOP, the greater the risk of developing glaucoma (see Chapter 5, Glaucoma Risk factors).
Intraocular pressure is not constant and can vary throughout the day. A diurnal variation is IOP fluctuation during the day, while a nocturnal variation is fluctuation of IOP during the night. Sometimes when the eye doctor is suspicious that there is a great fluctuation of IOP during the day, he may recommend measurement of diurnal IOPs. It is not uncommon to see intraocular pressures slightly higher in the morning than in the afternoon or evening. However, this is not necessarily true for all patients. While there is small diurnal fluctuation of IOP even in normal subjects, the glaucoma patient tends to have higher IOP and greater fluctuation of IOP throughout the day compared to normals.
There are a number of ways to measure the intraocular pressure (the measurement of IOP is referred to as tonometry). The most common measurement technique is the Goldmann applanation tonometry. With an anesthetic eyedrop, this device measures the IOP by putting a biprism plastic tip against the cornea and applanating (or flattening) the cornea. The intraocular pressure is read by dialing in an appropriate amount of force to flatten the surface of the cornea. The Goldmann applanation technique is based on the principle that the force required to flatten a certain defined area of the cornea (which has a curved surface) is proportional to the IOP. (Figure 6-1)
Figure 6-1. Checking intraocular pressure (IOP) using a Goldmann applanation tip at the slit lamp
Besides the Goldmann applanation tonometry, there are other devices that can measure the intraocular pressure. Other commonly used tonometers are Tonopen (Reichert, Buffalo, NY) and Perkins (Clement Clarke Inc., Columbus, OH) applanation tonometers; both of them are hand-held and portable (Figure 6-2 and 6-3). These devices come in handy when the patient is unable to easily place his chin at the slit lamp device, as in children or elderly in a wheelchair. Another commonly used device is a non-contact applanation tonometer (commonly referred to as “air puff” tonometer). It is based on a similar principle as the Goldmann applanation tonometry, except that it uses an air puff to flatten the cornea, rather than a direct contact with the tonometer tip. There are do-it-yourself devices that are available for self-measurement of intraocular pressure at home by the patient. The reliability of the home tonometer is considered to be less than the ones commonly used in the doctor’s office.
Figure 6-2. Tonopen XL. A hand-held digital device to measure the intraocular pressure. It is useful in patients who are not able to get into the slit lamp for Goldmann applanation tonometry.
Figure 6-3: Perkins tonometer. It is hand-held device used to measure the intraocular pressure. It is often used in infants or elderly in a wheelchair. It works on a similar principle as the Goldmann applanation tonometer.
As we have discussed in Chapter 4, accurate intraocular pressure measurement depends on the thickness of the cornea. Corneal thickness is measured using a pachymeter (Figure 6-4) and the normal corneal thickness is in the range of 0.53 – 0.55 mm. After an anesthetic eye drop is applied, corneal thickness is measured by gently touching the smooth tip of the pachymeter probe to the surface of the cornea (Figure 6-5). Intraocular measurements are underestimated in patients with thin corneas. In other words, with thin corneas the true IOP is higher than what is measured using an applanation tonometer. Conversely, in the thicker cornea the true IOP is lower than what is measured by applanation tonometry. If you have corneal thickness less than 0.50 or greater than 0.60 mm, the applanation tonometry will significantly underestimate or overestimate the intraocular pressure readings respectively.
Figure 6-4. Pachymeter. A pachymeter (Pocket Pachymeter, Quantel Medical, Bozeman, MT) is an ultrasound device that measures cornea thickness by determining the time it takes for a sound wave to reflect off the inner surface of the cornea.
Figure 6-5. Pachymetry. Corneal thickness is measured by administering an anesthetic drop to the eye and then gently placing the pachymeter probe against the outer surface of the cornea.
Intraocular pressure reading is performed using a topical anesthesia; it typically takes less than 15 seconds to measure the IOP in each eye in a cooperative patient. It is painless procedure when performed properly. A complete IOP assessment requires both the applanation tonometry as well as the corneal thickness measurement (or pachymetry).
Section 6-C: The drainage angle
Examination of the drainage angle is referred to as gonioscopy. The drainage angle is examined to determine if it is open or closed. To visualize the angle, an anesthetic eye-drop is applied to the eye and a special contact lens (gonioscopy lens, Figure 6-6) is placed against the cornea (Figure 6-7). This lens allows an examiner to see into the drainage angle using a slit lamp biomicroscope. This examination is usually painless; however, the anesthetic drop may cause a minor burning sensation that lasts a few seconds.
Figure 6-6. A Posner Gonioscopy contact lens. The smooth surface of this contact lens is placed against the cornea to allow the examiner to see the drainage angle.
Figure 6-7. Gonioscopy. An examiner gently places the gonioscopy lens (arrow) against the cornea and examines the eye with a slit lamp biomicroscope.
The goal of gonioscopy is to visualize the structures of the drainage angle including the trabecular meshwork (see Chapter 3). Fluid exits the eye through the drainage angle by passing through the trabecular meshwork. If the trabecular meshwork can be seen the drainage angle is said to be open. If the drainage angle is obstructed and the trabecular meshwork cannot be visualized, the angle is said to be closed.
Section 6-D: Optic nerve examination
One of the hallmarks of glaucoma is the optic nerve damage, which is characterized by cupping of the optic nerve. Even a normal optic nerve has a small amount of cupping. However, the patients with glaucoma tend to have larger cupping than normal subjects. As discussed in Chapter 1, the cup-to-disc ratio of normal subjects is typically around 0.2 to 0.3 (Figure 6-8). The cup-to-disc ratio is often measured both in the vertical and horizontal position to estimate the amount of cupping and amount of optic nerve damage (Figure 6-9). The cup size is simply the area of the optic nerve that is not occupied by the optic nerve fibers (an empty space). However, with glaucoma, there is progressive loss of optic nerve fibers, and consequent increase in the cup size of the optic nerve. When an eye doctor says there is a cup-to-disc ratio close to 1.0, this refers to almost complete cupping and an advanced damaged optic nerve from glaucoma (Figure 6-10). Conversely, if the cup-to-disc ratio is 0.3 or less, then this refers to a relatively healthy looking optic nerve. While there is no one cup-to-disc ratio that separates normal from glaucoma, the cup-to-disc ratio greater than 0.6 or 0.7 is suspicious of glaucoma and often requires further testing to rule out glaucoma. As glaucoma progresses, the cup-to-disc ratio enlarges (as more optic nerve fiber dies off), and the patient may start to develop peripheral vision loss. A small fraction of glaucoma patients, if detected late or inadequately treated, may become blind in one or both eyes with a complete loss of optic nerve fibers.
Figure 6-8. Normal optic nerve - 0.3 C/D ratio
Figure 6-9. The optic nerve is divided into tenths and the cup is compared to the entire optic nerve (optic disc) to obtain the cup-to-disc ratio. This C/D ratio here is 0.4.
Figure 6-10. Glaucomatous optic nerve – 0.9 C/D ratio
Figure 6-11. Optic nerve hemorrhage. There is a flame-shaped hemorrhage of the optic nerve located at the arrow at 5 o’clock.
In glaucoma the position of the blood vessels within the optic nerve can shift with progressive cupping that can be an important feature of glaucoma progression. Other important optic nerve findings include hemorrhages or bleeding around the optic nerve (Figure 6-11). The optic nerve bleeding is especially common in normal tension glaucoma. It often indicates an ongoing damage to the optic nerve and inadequate control of glaucoma.
Optic nerve can be viewed using a slit lamp with an appropriate lens by an eye doctor. However, in order to objectively document the status of the optic nerve one has to take stereo photographs of the optic nerve, which can be used as a baseline for comparison in the future. Therefore, it is important to take an optic nerve photographs at initial exam and periodically afterwards. The stereo photographs of the optic nerve are taken with a fundus camera (a camera designed to take pictures of the retina and optic nerve of an eye) in a doctor’s office. There are different models of fundus camera that can take digital pictures (Figure 6-12).
Figure 6-12. A Stereo Disc Camera. (3-Dx, Nidek Co. Ltd, Fremont, CA). A fundus camera such as this is used to take stereo optic nerve photographs of glaucoma patients. It has a digital camera-back which is connected to a computer and hard disc storage device (computer not shown).
Figure 6-13. Heidelberg Retina Tomograph 2 (HRT2). It uses a scanning laser to map out the topography (surface contour) of the optic nerve. The images are then analyzed using a computer to assess the amount of cupping and glaucoma optic nerve damage.
Figure 6-14. Stratus Ocular Coherence Tomography (OCT). Stratus Ocular Coherence Tomography (OCT). It uses a coherent laser source to scan a cross-sectional picture of the retina and optic nerve. A computer analysis of the cross-sectional picture allows it to measure the thickness of the nerve fiber layer, which correlates with the amount of cupping and glaucoma optic nerve damage.
A number of computerized optic nerve imaging devices has been introduced commercially to aid the eye doctor in documenting the optic nerve damage. These devices include HRT2 (Heidelberg Retina Tomograph 2, by Heidelberg Engineering, Inc. Vista, CA, Figure 6-13) Stratus OCT (Ocular coherence tomography by Carl Zeiss Meditec, Dublin, CA, Figure 6-14), and GDx (or scanning laser polarimeter by Carl Zeiss Meditec, Dublin, CA). These sophisticated instruments provide an accurate map of the optic nerve and quantitative analysis of the optic nerve cupping. In addition, devices such as Stratus OCT and GDx provide the thickness of the nerve fiber layer around the optic nerve, which is related to the amount of cupping. In general, the greater amount of cupping (or the larger the cup-to-disc ratio), the thinner the nerve fiber layer. This is because as you lose more optic nerve fibers, the optic nerve cup gets larger and the nerve fiber layer becomes thinner. Your eye doctor may utilize one or more of these computerized optic analyzers (as well as optic nerve photographs) to evaluate the amount of optic nerve damage in glaucoma to diagnose and follow progression of glaucoma over time. Therefore, it is useful to obtain optic nerve photographs and/or computerized optic nerve imaging on a periodic, on-going basis. Undoubtedly, there will be improved or new optic nerve imaging devices in the future that may further enhance our ability to diagnose and follow patients with glaucoma.
Section 6-E: Peripheral vision (visual field testing)
Visual fields measure both central and peripheral vision. Central vision (or visual acuity is used for fine-detail tasks such as reading, recognizing faces, and watching television. Peripheral vision is more important for navigating through obstacles in the environment. Early vision loss from glaucoma generally affects peripheral vision. The early peripheral vision loss is not commonly noticed by patients because central vision is usually spared in the early stages. Therefore, it is important to assess the visual fields to detect glaucoma early.
The systematic measurement of visual fields is referred to as perimetry. Patients keep one eye fixed on a target directly forward, while the other eye is covered. Next a test object is presented to the test eye at various positions. Patients signal when they see these objects, allowing their visual field to be mapped. Areas in which visual stimuli are not perceived are plotted to indicate the location of visual field defects (blindspot or scotoma).
In glaucoma patients, visual fields are assessed using special devices (perimeters) that allow a systematic mapping of blindspots. Patients are placed in front of a bowl-shaped screen and are instructed to indicate when they see test lights that are projected onto various positions on the bowl screen. Some perimeters are computerized and project lights at fixed positions on the screen (Humphrey’s field analyzer, Carl Zeiss Meditec, Thornwood, NY, Figure 6-15), while other perimeters are manual and project moving lights on the screen (Goldmann perimeter, Haag Streit AG, Mason, Ohio Figure 6-16). Patients indicate that they see a target by pressing a button with a finger. The output from a Humphrey’s field analyzer is a computer generated plot of the central 24 degrees (Figure 6-17), while the output of the Goldmann perimeter is a manual plotted diagram (Figure 6-18).
Figure 6-15. Humphrey’s field analyzer. The Humphrey’s field analyzer is a commonly used automated perimeter. A computer projects test lights of varying brightness at different positions on the target screen. The visual field is determined by the patient’s ability to detect the presence of the test lights.
Figure 6-16. Goldmann perimeter. An examiner uses the Goldmann perimeter to present the patient with moving test lights of varying size and brightness are used to map the visual field.
Figure 6-17. Humphrey’s field analyzer visual field reports. These plots represent visual field graphically. The center of the field is at the intersection of the axes. Shading represents areas of vision loss that is proportional to the darkness of the shading. A. Normal visual field (left eye). There is a normal blindspot where the optic nerve enters the eye shown by the arrow. B. Glaucomatous visual field (right eye). There is moderate vision loss in the superior aspect of the visual field in an arching pattern (arcuate defect) that extends from the normal blindspot (indicated with an arrow). C. Glaucomatous visual field left eye. There is significant vision loss in the inferior and nasal (towards the nose) aspect of the visual field (nasal step) as well as an early inferior arcuate defect.
Figure 6-18. Goldmann perimeter visual field reports. The center of the visual field is represented by the intersection of the lines on the grid. Patients are presented with targets of increasing size and brightness. The areas in which these targets can be seen are encircled with colored markings to depict the visual field. Focal areas in which the targets cannot be seen are indicated with color shading using the same color scheme. A. Normal visual field. A normal Goldmann visual field (right eye) consists of concentric circular markings. The circular markings (called isopters) indicate that larger, brighter targets can be seen farther into the periphery (blue isopter) than the smaller, dimmer targets (red isopter). The normal blindspot is shown with an arrow. B. Glaucomatous visual field. There is an arc-shaped loss of superior visual field (arcuate defect) that extends from the normal blindspot (indicated with an arrow). This is a typical pattern of vision loss in glaucoma that spares central vision (marked with “OK”).
Visual field testing is painless, but requires a high degree of concentration throughout the test. For accurate field measurements, it is important for patients to keep their eye pointed straight ahead at a fixation target while the test lights are presented at various positions on the screen. Visual field testing may take from 5 to 20 minutes per eye depending on type of perimeter and the degree of visual field loss.
Although perimetry provides reliable measurements of visual fields, some variations in the visual fields can occur that are NOT associated with glaucoma. Consequently, it is often necessary to repeat examinations to confirm the presence of a visual field defect or its progression.
Chapter 6. References
Allingham RR, Damji K, Freedman S, Moroi S, Shafranov G. Ch 2: Intraocular Pressure and Tonometry. In: Shield’s Textbook of Glaucoma. 5th Ed. Lippincott Williams and Wilkins, Philadelphia, p36-58, 2005.
Alward, WLM. Ch 5: Optic Nerve Head Evaluation. In Glaucoma: The Requisites in Ophthalmology, Mosby, St. Louis, p. 46-55, 2000.
Alward, WLM. Color Atlas of Gonioscopy, Mosby, St. Louis, 1994.
Brandt, JD. Corneal Thickness in Glaucoma Screening, Diagnosis, and Management. Current Opinion in Ophthalmology, 15: 85-89.