Glaucoma vs. glaucoma suspect

Normal intraocular pressure is 10–20 mmHg and should be measured at different times of day as there is a circadian rhythm. Pressure 28 mmHg or more is usually treated regardless of other findings. Treat pressures of 20–27 mmHg when there is loss of vision, a family history of glaucoma, damage to the optic nerve as evidenced by disk pallor with increased cupping and thinning of the ganglion nerve fiber thickness (Figs 334–343). Patients with pressures of 20–27 mmHg without other suspicious findings of glaucoma are called glaucoma suspects. They are followed with more frequent visits than usual, with monitoring of eye pressure, visual fields, and optic nerve changes. When treatment is started, pressures are usually kept below 20 mmHg, which most often prevents loss in vision. However, some patients may lose vision even when pressures are kept in the high teens. These eyes require further lowering of pressure to the low teens and such patients have the condition referred to as low‐pressure or normal‐tension glaucoma. It is present up to 90% of Asians with glaucoma and in less than 50% of glaucoma patients worldwide.

Several instruments can be used to indirectly measure intraocular pressure by indenting the cornea, as follows.

1. A Goldmann applanation tonometer (Fig. 323) is the most accurate instrument for this purpose. It is used in conjunction with a slit lamp, and requires the use of anesthetic drops and fluorescein dye.
   
2. The Schiötz tonometer and Tono‐Pen are portable instruments (Fig. 324) that also indent the anesthetized cornea and are used for bedside measurements.
   
3. The air‐puff tonometer tests the pressure by blowing a puff of air at the eye. It is used by technicians since it does not require eye drops or corneal contact, but is more uncomfortable and slightly less accurate.

With all three instruments, the tonometric pressure reading is only an estimate of the real pressure. A thick cornea requires extra force to indent and, therefore, gives a falsely elevated reading, and the opposite is true with thin corneas. To better approximate the real pressure—especially in glaucoma suspects where exactitude is important—an ultrasonic pachymeter is used to measure the central corneal thickness. A conversion factor for corneal thickness then adjusts the tonometric reading upward with thin corneas or downward with thick corneas (Fig. 325). With scarred distorted corneas or uncooperative patients, finger‐tip assessment may yield a gross evaluation.

The iridocorneal angle

Most aqueous leaves the eye by entering the trabecular meshwork (Fig. 326) which is the tan to dark brown band at the angle between the cornea and iris. It then exits the eye after entering the canal of Schlemm, which is a 360° circular tube leading into the scleral and episcleral venous plexus. The angle between the iris and the cornea is normally 15–45° and can be estimated with a slit lamp (Figs 328 and 329), but a goniolens (Figs 330 and 331) is more accurate. In open‐angle glaucoma, the trabecular meshwork is obstructed, whereas in narrow‐angle glaucoma, the space between the iris and cornea is too narrow, so aqueous cannot reach the trabecular meshwork. A narrow angle at risk of closing is graded 0–2 (see Fig. 333). Angles of grade 3 or 4 are considered wide open with no chance of closing.

The optic disk (optic papilla)

The disk is the circular junction where the ganglion cell axons exit the eye, pick up a myelin sheath, and become the optic nerve (Figs 334–337, 339, and 474). The lamina cribrosa is the perforated continuation of the scleral wall of the eye that allows passage of the retina ganglion cell axons and the central retinal artery and veins to exit the globe (Fig. 335). A central depression within it forms the optic cup that is usually less than one‐third the disk diameter, although larger cups can be normal (Fig. 340).

Signs of nerve fiber damage

As pressure damages the nerve:

1. cup/disk ratio increases (Fig. 340),
   
2. cup becomes more excavated and often unequal in the two eyes,
   
3. vessels shift nasally (Fig. 340D),
   
4. disk margin loses capillaries and turns pale; flame hemorrhages on the disk (Fig. 340C) are associated with more rapid loss of central visual field, as opposed to the usual peripheral loss in glaucoma,
   
5. diffuse loss of retinal nerve fiber layer (Figs 337 and 343).

The optic disk changes can be followed by accurate drawings, photographs, or OCT or GDx testing (Figs 338, 339, 341, and 343). Retinal nerve fiber layer thickness is usually measured around the optic disk (less often the macula) with OCT or GDx. It is most useful in detecting early stages of glaucoma before visual field loss becomes evident. A 5 μm progressive loss of thickness between tests is significant. (A red blood cell has a diameter of 7 μm.)

Visual field defects pathognomonic of glaucoma (Fig. 344)

1. Bjerrum’s scotoma extends nasally from the blind spot in an arc.
   
2. Island defects could enlarge into a Bjerrum’s scotoma.
   
3. Constricted fields occur before loss of central vision.
   
4. Ronne’s nasal step is loss of peripheral nasal field above or below the horizontal.

The diagnosis and treatment of open‐angle glaucoma should initially be made before visual field loss based on eye pressure, optic nerve findings, nerve fiber layer thickness, and family history. If one waits for visual field loss, 20% of the nerve fiber layer may have already been lost.