| CHAPTER | 11 | The Glaucoma |
▃Aqueous Humor Dynamics (PY10.17)
Aqueous humor is a clear fluid occupying both anterior and posterior chambers of the eye. It is continuously secreted and drained out of the eye. IOP (intraocular pressure) is determined by the rate of aqueous secretion and rate of its outflow. The study of glaucoma deals primarily with the consequences of elevated IOP. Therefore, aqueous humor dynamics is necessary to understand the pathophysiological mechanism of glaucoma (Fig. 11.1). Two main structures related to aqueous humor dynamics are ciliary body, which is the site of aqueous secretion, and limbal region, which is the site of aqueous outflow.
Fig. 11.1 Stepwise construction of structures involved in aqueous humor dynamics. (1) Limbus is the transition zone between cornea and sclera on the inner side of which is an indentation called scleral sulcus. Scleral sulcus has a sharp posterior margin called scleral spur. (2) Trabecular meshwork bridges the scleral sulcus and converts it into Schlemm canal which is connected by intrascleral channels to episcleral veins. A ridge is created at the insertion of meshwork into the peripheral cornea known as Schwalbe line. Thus, the main route of aqueous outflow is formed comprising trabecular meshwork, Schlemm canal, intrascleral channels, and episcleral veins. (3) The ciliary body attaches to scleral spur creating a potential space between itself and sclera called supraciliary space/suprachoroidal space. The ciliary processes are the site of aqueous production at the inner and anterior portion of ciliary body. (4) The iris is inserted into the ciliary body which is visible between the scleral spur and the root of iris. The lens is suspended by the suspensory ligaments from the ciliary body. The angle formed by the iris and cornea is called the angle of anterior chamber.
■Applied Anatomy (AN41.2)
Ciliary Body
The ciliary processes are the actual site of aqueous production. Each ciliary process is lined by outer pigmented epithelial layer and inner nonpigmented epithelial layer which contain mitochondria, Na+-K+ ATPase, and carbonic anhydrase. Ciliary processes are supplied by branches from major arterial circle which end in capillary network. Capillaries of ciliary processes are fenestrated. The fenestrated capillaries allow easy passage of fluid and macromolecules but tight junctions between adjacent nonpigmented epithelial cells together with nonfenestrated iris vessels constitute the blood–aqueous barrier (Fig. 11.2a).
To reach posterior chamber, constituents of aqueous must traverse capillary wall, stroma of ciliary processes, and both epithelia.
Limbal Region
Limbus is the transition zone between cornea and sclera with an indentation on its inner surface called scleral sulcus. Scleral sulcus has a sharp posterior margin called scleral spur and a sloping anterior wall that extends to peripheral cornea. Scleral sulcus is bridged by a sieve-like structure called trabecular meshwork which converts the sulcus into a tube called Schlemm canal. Where the trabecular meshwork inserts into peripheral cornea, a ridge is formed called Schwalbe line. Schlemm canal is connected by intrascleral channels to the episcleral veins (Fig. 11.2b).
Fig. 11.2 (a) Ciliary process. (b) Limbal region.
Iris inserts into anterior side of ciliary body leaving a part of ciliary body between root of iris and the scleral spur.
Iris separates aqueous compartment into posterior chamber and anterior chamber.
Angle of anterior chamber is a peripheral recess formed by:
•Root of iris and part of ciliary body—posteriorly.
•Scleral spur and trabecular meshwork—anteriorly.
Longitudinal fibers of ciliary muscle attach at scleral spur which insert into suprachoroidal lamina (fibers connecting choroid and sclera) as far back as equator.
■Applied Physiology (PY10.17)
It involves aqueous formation and aqueous outflow.
Aqueous Formation
Aqueous is formed in ciliary processes by two processes, namely, passive secretion (by ultrafiltration and diffusion) and active secretion (by nonpigmented epithelium) (Flowchart 11.1).
Flowchart 11.1 Algorithm depicting aqueous formation.
Passive Secretion
It constitutes ultrafiltration and diffusion. Ultrafiltration from capillaries of ciliary processes depends on capillary hydrostatic pressure, plasma oncotic pressure, and level of IOP.
Lipid-soluble substances that easily penetrate cell membranes readily move by diffusion.
Active Secretion
Certain substances are actively transported (secreted) across blood–aqueous barrier formed by inner nonpigmented epithelium which contains mitochondria, Na+-K+ ATPase and carbonic anhydrase. Substances that are actively transported include Na+, K+, Cl−, HCO3–, ascorbic acid, and amino acids.
Control of Aqueous Formation
Aqueous formation is subject to endogenous influences. Humoral or neurohumoral mechanisms influence the rate of aqueous formation. Ciliary epithelium is not innervated but ciliary body vessels have a dense adrenergic innervation. Catecholamines released from sympathetic nerve endings diffuse to adrenergic receptors on ciliary epithelium and aqueous humor secretion is regulated. The fluctuation in the rate of aqueous formation accounts for diurnal variations in IOP.
Aqueous Outflow
Aqueous formed by ciliary body is secreted into posterior chamber which circulates into anterior chamber via pupil and is drained out by two routes:
•Trabecular (conventional) route.
•Uveoscleral (unconventional) route.
Trabecular Outflow
It accounts for 75 to 90% of aqueous outflow.
Route
Aqueous escapes through drainage channels at the angle of anterior chamber (Flowchart 11.2).
Flowchart 11.2 Main route of aqueous outflow.
It is a pressure sensitive route so that increasing the pressure head will increase outflow. The spaces in trabecular meshwork sheets progressively decrease in size moving outwards. The extracellular spaces contain mucopolysaccharide, collagen, and elastic components which offer the greatest resistance to the aqueous outflow. Schlemm canal lies circumferentially in scleral sulcus, i.e., it is a 360-degree channel lined by endothelium. Endothelium of its inner wall contains giant vacuoles while the outer wall of the canal contains openings of large collector channels (intrascleral channels). There are two systems (direct and indirect systems) of intrascleral channels connecting Schlemm canal to episcleral veins:
Direct system: Intrascleral vessels (aqueous veins) drain directly into episcleral veins.
Indirect system: Intrascleral vessels form an intrascleral plexus before draining into episcleral veins.
Mechanism of Aqueous Outflow
The mechanism of outflow across inner wall endothelium of Schlemm canal is partially understood. The most accepted theory is vacuolation theory. Initially, the vacuole is formed by the infolding of basal surface of endothelial cell. A progressive enlargement of this infolding leads to the formation of macrovacuolar structure which eventually opens on the luminal aspect of endothelium. Thus, temporary vacuolar transcellular channel are formed which drains the bulk of aqueous humor down the pressure gradient. Finally, the basal infolding is occluded and the cell returns to nonvacuolated stage.
Uveoscleral Outflow
It is an unconventional outflow accounting for remaining 10 to 25% of aqueous outflow. It is a pressure-independent route. Flowchart 11.3 describes the route of aqueous outflow through uveoscleral route.
Flowchart 11.3 Uveoscleral route of aqueous outflow.
Effect of Cholinergic Pathway on Aqueous Outflow
Parasympathetic fibers originate from Edinger–Westphal nucleus to innervate ciliary muscle (Flowchart 11.4).
Flowchart 11.4 Effect of parasympathetic stimulation on aqueous outflow.
▃Aqueous Humor
The aqueous humor is a transparent fluid similar to plasma but the blood–aqueous barrier is responsible for differences in chemical composition between the plasma and the aqueous humor. Properties of aqueous humor are listed in Table 11.1.
Table 11.1 Physiological properties of aqueous humor | |
Features | Properties |
•Refractive index •Functions •Viscosity •Density •Volume •Osmolarity •pH •Composition | •1.336, slightly lower than that of cornea. •It nourishes avascular structures of the eye (cornea and lens), maintains proper IOP and also provides transparent medium as a part of optical system of eye. •1.025–1.040 relative to water. •It is slightly greater than that of water. Specific gravity of aqueous is 1.002–1.004. •It is 0.25 mL in anterior chamber and 0.06 mL in posterior chamber. •Aqueous is slightly hyperosmotic to plasma. •It is 7.2 (alkaline) but acidic relative to plasma. •Water constitutes 99.9% of normal aqueous. •Two most striking characteristics of aqueous compared to plasma are: ◊Marked excess of ascorbate: 15 times greater than that of arterial plasma. ◊Marked deficit of protein: 0.02% in aqueous and 7% in plasma. Blood–aqueous barrier normally limits the total protein content of aqueous. A/G ratio is several times higher in aqueous as compared to that of plasma due to exclusion of heavy globulins from aqueous by blood–aqueous barrier. •Cornea and lens take up glucose from aqueous, so there is slight deficit of glucose in aqueous relative to plasma. •Lactic acid is released by cornea and lens into aqueous, so there is slight excess of lactic acid in aqueous relative to plasma. •Other constituents are amino acids, norepinephrine, HCO3−, Cl−, Na+, K+, oxygen in dissolved state, urea, nitric oxide, sodium hyaluronate, and tissue plasminogen activator. |
▃Intraocular Pressure
Intraocular pressure (IOP, the pressure inside eyeball) is determined by the rate of aqueous formation and the rate of its outflow. Normally, there is a balance between formation and drainage of aqueous. Normal range of IOP is 11 to 21 mm Hg with a mean of 16 mm Hg, and 21 mm Hg is considered the upper limit of normal IOP.
■Factors Affecting IOP
IOP is dependent on ocular, genetic, systemic and certain other factors.
Ocular Factors
IOP is a function of aqueous formation and its drainage. The changes in IOP are caused either by alterations in aqueous formation or alterations in aqueous outflow.
Alterations in Aqueous Formation
Aqueous formation may increase or decrease. Normal rate of aqueous formation is 2.4 ± 0.6 µL/min approx. No condition of excess aqueous formation has been observed.
Increased Aqueous Formation
It depends upon the following factors:
•Capillary hydrostatic pressure: Rise in capillary hydrostatic pressure causes rise in IOP and vice versa.
•Osmotic pressure of blood: Hypotonicity induces rise in IOP as in water drinking test and hypertonicity induces a fall in IOP.
•Permeability of capillaries: Flowchart 11.5 explains the mechanism of rise in IOP due to increased aqueous formation.
Flowchart 11.5 Effect of iridocyclitis on intraocular pressure (IOP).
Decreased Aqueous Formation
It results due to the following factors:
•Drugs: For example, β-blockers, carbonic anhydrase inhibitors (CAIs), and sympathomimetics. Sympathomimetics cause vasoconstriction leading to reduced ultrafiltration and decreased aqueous formation.
•Detachment of ciliary body.
•Retinal detachment.
•Cyclocryotherapy: It is a cyclodestructive procedure.
Alterations in Aqueous Outflow
Aqueous outflow may increase or decrease.
Increased outflow: Parasympathomimetics (e.g., pilocarpine) cause ciliary muscle contraction which results in traction on scleral spur because of its attachment. IOP decreases due to increased outflow.
Decreased outflow (increased resistance): Aqueous flows from posterior chamber via pupil into anterior chamber and mostly drained through trabecular route. Obstruction to aqueous flow may occur either at pupil or angle of anterior chamber where it may be pre-trabecular, trabecular, or post-trabecular (Fig. 11.3).
Fig. 11.3 Sites of increased resistance to aqueous outflow. Abbreviation: IOP, intraocular pressure.
Genetic Factors
IOP is under hereditary influence so that first-degree relatives of patients with primary open-angle glaucoma (POAG) have higher IOP.
Systemic Factors
Diabetes mellitus and systemic hypertension are the two most common systemic diseases that have been implicated as risk factors for POAG.
Myotonic dystrophy results in reduced aqueous production and increased outflow by uveoscleral route from atrophy of ciliary muscle. So, IOP is markedly low in myotonic dystrophy.
Other Factors
Age—IOP generally increases with age. Thus, increasing age is a major risk factor for glaucoma.
General anesthesia is usually associated with reduction in IOP but ketamine elevate IOP.
Succinyl choline causes extraocular muscle contraction and intraocular vasodilatation resulting in transient increase in IOP.
Corticosteroids may cause IOP elevation.
Exertional influences: Valsalva maneuver elevates episcleral venous pressure (EVP) and IOP. Prolonged exercise lowers the IOP.
■Diurnal Variation of IOP
IOP is subjected to cyclic fluctuations throughout the day. Normally, diurnal variation up to 5 mm Hg occurs. Fluctuations in IOP of >8 mm Hg are considered to be pathologic even though the reading may fall within normal limits (21 mm Hg).
IOP is maximum in morning hours between 8 a.m. and 12 p.m. and lower in afternoon and evening. So, a single normal reading, particularly if taken during late afternoon, is of no value and may be misleading (Fig. 11.4).
Fig. 11.4 Diurnal variation of IOP. Abbreviation: IOP, intraocular pressure.
Mechanism of diurnal IOP variation has been related to the diurnal variation of plasma cortisol.
■Postural Variation of IOP
IOP increases when changing from sitting to supine position.
Total body inversion (whole body head down tilt) leads to a further increase in IOP (greater in glaucomatous eyes) and appears to be related to the elevated EVP. Thus, obtaining clinical history on the type of exercise (yoga and inversion iIt consists of the followingn particular) may be relevant for the patients with glaucoma.
▃Classification of Glaucoma (OP6.6, 6.7)
Glaucoma is a group of condition associated with characteristic optic neuropathy and visual field defects. Glaucoma is frequently but not invariably associated with raised IOP. Normal range of IOP is 11 to 21 mm Hg. If IOP is more than 21 mm Hg (up to 30 mm Hg) but without any detectable glaucomatous damage, the condition is termed as “ocular hypertension.” If IOP is less than 21 mm Hg and characteristic optic disc changes and visual field defects of glaucoma occur with normal or low IOP, the condition is termed as normal tension glaucoma (NTG), also referred to as “low-tension glaucoma.” Thus, raised IOP is the most important risk factor for development of glaucoma, but not always. Glaucoma may be developmental or acquired.
Based upon etiology, glaucoma may be primary or secondary glaucoma.
Primary glaucoma: In primary glaucoma, elevation of IOP is not associated with other ocular or systemic disorder. It is typically bilateral and probably has a genetic basis.
Secondary glaucoma: It is secondary to ocular or systemic disorder. It may be unilateral or bilateral. It may be developmental and may have a genetic basis or may be acquired.
Based upon mechanism, glaucoma may be open-angle glaucoma or angle-closure glaucoma.
With an emphasis upon etiology and mechanism, glaucoma can be classified as follows:
•Primary glaucoma: It may be:
◊Open-angle glaucoma: With raised IOP, it is known as primary open-angle glaucoma (POAG).
◊With normal IOP, it is known as normal-tension glaucoma (NTG).
◊Angle-closure glaucoma: Primary angle-closure glaucoma (PACG).
•Secondary glaucoma: It may be:
◊Secondary open-angle glaucoma.
◊Secondary angle-closure glaucoma.
•Developmental glaucoma.
▃Clinical Examination of Glaucoma (OP6.6, 6.7)
The clinical examination of glaucoma is vital to make the diagnosis, which includes:
•Tonometry to record accurate IOP.
•Gonioscopy to identify angle pathology.
•Optic nerve head (ONH) examination.
•Visual field examination by perimetry.
•Retinal nerve fiber layer (RNFL) analysis by OCT.
■Measurement of IOP: Tonometry and Tonography
Tonometry is essential for the diagnosis of glaucoma and also for monitoring of antiglaucoma medications.
Assessment of IOP can be done digitally or by an instrument called “Tonometer.”
Digital assessment of IOP (digital tonometry) is a qualitative method. The patient is asked to look down and the two fingers (tips of index finger) are placed a short distance from one another just above the superior tarsal plate. IOP is assessed in the same manner by testing the fluctuations.
If fluctuations are feeble or absent, IOP is high and if the globe gives a feeling of water bag, IOP is low.
Assessment of IOP by tonometer works behind the principle that the force necessary to deform a globe is directly related to the pressure within that globe.
According to the shape of deformation, tonometers are of two basic types:
•Indentation tonometer.
•Applanation tonometer.
Indentation Tonometry
It is done by Schiotz tonometer. The shape of deformation with this type of tonometer is a truncated cone. It is based on the principle that a plunger will indent a soft eye more than a hard eye.
Schiotz Tonometer
It consists of the following structures:
•Foot plate which rests on the cornea.
•Weighted plunger, which moves freely within a shaft, that ends in foot plate. A 5.5 g weight is permanently fixed to plunger, which can be increased 7.5 and 10 g by adding additional weights. Plunger indents the cornea.
•A bent lever with short arm which rests on upper end of plunger and a long arm acting as a pointer needle move on a scale (Fig. 11.5). The amount of indentation of cornea by plunger is indicated by the movement of pointer needle on the scale and the reading is converted into mm Hg by using a special conversion table. Generally, using the standardized plunger weight of 5.5 g, the normal eyes give scale readings of 5 to 8 units and high-pressure eyes read <4 units.
Fig. 11.5 Parts of Schiotz tonometer. Source: Intraocular Pressure and Tonometry. In: Morrison J, Pollack I, ed. Glaucoma: Science and Practice. 1st Edition. Thieme; 2002.
Method of Schiotz Tonometry
Following are the steps of Schiotz tonometry:
•Sterilize foot plate and lower end of plunger with acetone, ether.
•Ask the patient to lie down in supine position (Fig. 11.6).
•Cornea is anesthetized with 4% xylocaine or 0.5% proparacaine.
•Patient is asked to look at ceiling and lids are gently retracted.
•Foot plate of tonometer is placed over corneal surface.
•Scale reading is noted and converted into mm Hg with conversion table.
Fig. 11.6 Schiotz tonometry.
Advantages of Schiotz tonometer are that it is cheap, easy to use, convenient to carry, does not require a slit lamp, and may be used in operation room.
There are potential sources of error with indentation tonometry due to ocular (scleral) rigidity, accommodation, and contraction of extraocular muscles.
Ocular infection or discharge, corneal abrasion, marked nystagmus, corneal scarring, and blepharospasm serve as the probable contraindications of indentation tonometry.
Potential Sources of Error with Indentation Tonometry
•Errors due to ocular rigidity: The indentation is dependent on IOP and the distensibility of the ocular walls (ocular rigidity). Conversion table is based on an “average” coefficient of ocular rigidity. Difference in ocular rigidity among different eyes gives false IOP measurements.
In low-ocular (scleral) rigidity, small force of indentation is required giving the falsely low IOP reading while in high ocular (scleral) rigidity large force of indentation is required and the falsely high IOP is measured. Thus, IOP measured by Schiotz tonometer is inaccurate because of variations in ocular rigidity.
•Errors due to accommodation: Practically, as soon as tonometer is put on cornea there is a tendency of patient to look at the tonometer and accommodation comes into play. The use of miotics also causes accommodation. Accommodation causes contraction of ciliary muscle and pull on trabecular meshwork. It increases the facility of aqueous outflow and results in falsely low IOP reading.
•Errors due to contraction of extraocular muscles (EOM): Contraction of EOM causes false impression of high IOP. So, because of many potential sources of error, Schiotz tonometry largely has been replaced by applanation tonometry.
Applanation Tonometry
It is more accurate than indentation tonometry. It assesses the force required to flatten (or applanate) the standard area of cornea, disturbing relatively little aqueous. Goldmann applanation tonometer (AT) is most popular and used with slit lamp. When it flattens (applanates) the area having a diameter of 3.06 mm, the factor of ocular rigidity is eliminated and a little amount of aqueous is displaced. Thus, AT does not influence the IOP measurement and records IOP more accurately than Schiotz tonometer. It is based on Imbert–Fick principle (Fig. 11.7), which states that
Fig. 11.7 Imbert–Fick principle. F, force required to flatten the surface of sphere; IOP, intraocular pressure.
Grams of force applied to flattened 3.06 mm diameter of cornea multiplied by 10 is directly converted to mm Hg.
The shape of deformation with AT is simple flattening.
Applanation Tonometer
Goldman AT is a very accurate variable force tonometer. It is mounted on a slit lamp. It consists of a plastic biprism attached by a rod to a housing which contains a coil spring and series of levers that are used to adjust the force of biprism against the cornea. Biprism on contact with cornea creates two half circles, one above and one below the horizontal midline. These semicircles are easy to view with a cobalt blue light after fluorescein application in conjunctival sac (Fig. 11.8).
Fig. 11.8 (a) Goldman applanation tonometer. (b) The Goldmann applanation tonometer produces a tear film meniscus when the tip contacts the cornea. (c) The biprism splits the view of the circular meniscus into semicircles, which are aligned at their inner edges once the tip flattens a corneal area with diameter of 3.06 mm as shown in (b). Source (b, c) (a, b): Intraocular Pressure and Tonometry. In: Morrison J, Pollack I, ed. Glaucoma: Science and Practice. 1st Edition. Thieme; 2002. Abbreviation: IOP, intraocular pressure.
Technique of Using Goldman Applanation Tonometer
•Patient is positioned at slit lamp after anesthetizing cornea with topical preparation.
•Fluorescein strip is applied in conjunctional sac to stain tear film.
•Cobalt blue light from slit lamp is projected obliquely at prism.
•Biprism is advanced until it touches the apex of cornea.
•When biprism touches the cornea, two fluorescent semicircles will be seen. These semicircles represent the fluorescent-stained tear film touching the upper and lower outer halves of prism.
•Force against cornea is adjusted by rotating adjustment knob (dial) until inner margins of semicircles overlap. Inner margins of semicircle overlap when 3.06 mm of cornea is applanated.
•IOP is determined by reading on dial × 10.
Types of Applanation Tonometer
There are two types of AT, namely constant force AT and variable force AT. In constant force AT, a constant force (weight) is applied to the cornea and IOP is estimated by measuring the diameter of flattened area of the cornea, e.g., Maklakov AT. Variable force AT measures the force that is required to flatten a standard area (3.06 mm) of corneal surface. For example, Goldmann AT (slit-lamp mounted) and Perkin AT (handheld tonometer).
Sources of Error in Applanation Tonometry
•Amount of fluorescein: Appropriate amount of fluorescein is important because width of semicircles influences the reading. Excessive fluorescein causes thick (wide) semicircles with small radius and falsely high IOP estimation while insufficient fluorescein causes thin semicircles with large radius and falsely low IOP estimation.
Improper vertical alignment (one semicircle larger than the other) will also lead to a falsely high IOP estimation (Fig. 11.9).
•Central corneal thickness (CCT): Calculations of IOP in Goldmann applanation tonometry assume that average CCT is 530 to 545 µm. Deviations from this value are a source of errors.
Thick corneas due to corneal edema are associated with falsely low IOP readings.
Thick corneas secondary to other processes (i.e., from increased collagen fibrils) give a falsely high IOP reading.
Thin corneas usually give falsely low IOP readings. In refractive surgery (LASIK), cornea becomes thinner and the measured IOP is low.
Fig. 11.9 Amount of fluorescein and semicircles during applanation tonometry. (a) Too thin semicircles. (b) Normal semicircles. (c) Too thick semicircles.
Other Tonometers
Perkins tonometer: It is a handheld applanation tonometer and uses Goldmann prism. It does not require slit lamp, so can be used in anesthetized patients or reclining patients (Fig. 11.10).
Fig. 11.10 Perkins tonometer.
Tono-Pen: It is a digital, handheld, portable contact tonometer. It can measure IOP through a soft contact lens as well as in the eyes with scarred or irregular cornea.
Pneumotonometer: It is an electronic tonometer. The cornea is indented by flow of gas against a flexible diaphragm covering the sensing nozzle. Sensor measures air pressure and converts to a record of IOP on a paper strip.
Noncontact tonometer (NCT): It is based on the principle of applanation but, instead of using a prism, puff of air is used to flatten the cornea. The time required to flatten the cornea relates directly to the level of IOP. It may be portable (handheld pulsair NCT) or nonportable. Advantages of using NCT are:
•It does not require topical anesthesia.
•It is useful for screening by nonophthalmologists.
•There is no risk of infection spread or of corneal abrasion.
Dynamic contour tonometer (DCT): It is a nonapplanating, slit-lamp mounted, contact tonometer and offers IOP measurements independent of corneal biomechanics such as CCT (central corneal thickness). Biomechanical changes induced by LASIK do not affect IOP measurements using DCT. It is based on contour matching. DCT has a contoured tonometer tip surface which contains an electronic pressure sensor. Pressure sensor generates an electric signal, the voltage of which is digitized by an analogue-to-digital converter and pressure in mm is displayed on the digital panel. It aims to match the contour of cornea and, therefore, corneal pathology does not interfere in recording of IOP.
Disease Transmission During Tonometry with Contact Tonometers
There is risk of transmission of infections such as:
•Adenovirus keratoconjunctivitis.
•Herpes simplex virus type I.
•Hepatitis B virus.
•Human immune deficiency virus (HIV)— AIDS.
•Bacterial conjunctivitis.
To reduce the risk of cross-infection, avoid tonometry in individuals with overt infection.
Disinfection of Tonometers
Sterilization of tonometers is essential. Wipe the tonometer tip with 3% hydrogen peroxide or 70% isopropyl alcohol and allow it to dry. It destroys HIV, adenovirus, and hepatitis virus. Other chemical disinfectants are:
•1:1,000 merthiolate.
•1:10 sodium hypochlorite.
•Ethylene oxide.
Tonography
It is a form of tonometry in which IOP is measured continuously over the course of 4 minutes with electric Schiotz tonometer placed on cornea. The measurement is recorded graphically as a continuous tracing on a paper strip.
Aim of tonography is to estimate the ease with which aqueous leaves the eye. This estimate is called facility of aqueous outflow (C). C value may be influenced by certain factors like IOP, increase in EVP, and variable ocular rigidity.
The weight of tonometer acts as a force and aqueous is expressed from the eye resulting in lowering of IOP.
■Gonioscopy
Gonioscopy is the technique to visualize the angle of anterior chamber. The angle of anterior chamber cannot be visualized directly with slit lamp because of total internal reflection (TIR). For TIR, the angle must be greater than the critical angle which is 46.5 degree for cornea and the essential condition for TIR is that the light ray must pass from denser to rarer medium (Fig. 11.11). Problem of TIR at cornea–air interface is solved with a goniolens (a denser medium) placed in front of cornea. Now the light ray passes from rarer to denser medium.
Fig. 11.11 Total internal reflection (TIR) from corneal surface.
Thus, goniolens eliminates TIR and angle of anterior chamber can be visualized. The contact lenses used for gonioscopy are called goniolenses. There are two types of goniolenses: direct and indirect types. Direct goniolenses provide direct view of angle, for example, Koeppe lens and Barkan lens. In indirect goniolenses, mirrors are used, so light rays are reflected by mirror in goniolens and provide an image of the opposite angle, e.g., Goldmann single mirror, Goldmann 3-mirror lens, and Zeiss 4-mirror lens (Fig. 11.12).
Fig. 11.12 Types of goniolenses. (a) Koeppe lens (a 50D concave lens). (b) Goldmann single-mirror lens. (c) Goldmann three-mirror lens. (d) Zeiss lens.
Goldmann 3-mirror lens contains two mirrors for the examination of fundus and one for the examination of angle. Goldmann lenses require the use of viscous solution to couple this lens to cornea (Fig. 11.13).
Fig. 11.13 (a) Parts of Goldmann three-mirror lens. Posterior pole is visible through central lens (1). Equator is visible through 73-degree largest mirror (2). Ora serrata is visible through 67-degree mirror (3). Angle of anterior chamber is visible through 59-degree smallest mirror (4). (b) Central lens. (c) 73-degree mirror. (d) 67-degree mirror. (e) 59-degree mirror.
In Zeiss 4-mirror lens, each quadrant of angle is visualized with the opposite mirror. A coupling fluid is not required in Zeiss 4-mirror lens.
Goldmann 3-mirror lens has a contact surface diameter of 12 mm. This large posterior diameter stabilizes the globe and is therefore suitable for laser trabeculoplasty but prevents the luxury of indentation gonioscopy. Contact surface of Zeiss lens has a diameter of 9 mm which does not stabilize the globe and cannot be used for laser trabeculoplasty but is useful for indentation gonioscopy.
Indentation Gonioscopy (Pressure Gonioscopy)
It is carried out if the angle appears narrowed. In indentation gonioscopy, lens is pressed against cornea and aqueous is displaced into the angle of anterior chamber. The peripheral iris is forced posteriorly, tending to open the angle more widely (Flowchart 11.6).
Flowchart 11.6 Significance of indentation gonioscopy.
Thus, indentation gonioscopy allows easy distinction between appositional angle closure and synechial closure of angle.
Purposes of Gonioscopy
Diagnostic gonioscopy: It facilitates the identification of abnormal angle structures and estimation of the width of angle.
Surgical gonioscopy: It is done to visualize the angle during procedures such as laser trabeculoplasty and goniotomy.
Cleaning of Goniolenses
Goniolenses are a potential source of infection. These are disinfected in the same way as tonometer heads.
Gonioscopic Identification of Angle Structures
Starting posteriorly at the root of iris and moving anteriorly toward cornea, following structures can be identified gonioscopically in a normal angle (Fig. 11.14):
Fig. 11.14 Structures identified on gonioscopy. Source: History. In: Singh K, Smiddy W, Lee A, ed. Ophthalmology Review: A Case- Study Approach. 2nd Edition. Thieme; 2018.
•Ciliary body band (anteromedial surface of ciliary body): It stands out as a gray or dark brown band.
•Scleral spur: It is usually seen as a prominent white band between ciliary body and trabecular meshwork and represents posterior lip of scleral sulcus. It is a very important landmark because in laser trabeculoplasty application of burns posterior to scleral spur will result in uveitis with early rise in IOP and formation of peripheral anterior synechiae (PAS).
•Trabecular meshwork: It is faintly pigmented.
•Schwalbe line: It is seen as a glistening white line.
Grading of Angle Width
Gonioscopically, angle is graded according to the visibility of various angle structures. Following systems have been suggested to grade the angle width:
•Shaffer system.
•Scheie system.
•Spaeth system.
Shaffer grading system is the most commonly used system. It describes:
•Angle width in degree (angle between iris and trabecular meshwork).
•Anatomical structures visible.
•Type of angle.
•Clinical interpretation.
The system assigns a numerical grade (4–0) to each angle (Table 11.2 and Fig. 11.15).
Table 11.2 Shaffer grading system of angle width | ||||
Grade | Angle width | Structures visible | Type of angle | Clinical interpretation |
IV | 35–45 degree | Ciliary body Scleral spur Trabecular meshwork Schwalbe’s line | Wide open angle | Closure is impossible. |
III | 20–35 degree | Scleral spur Trabecular meshwork Schwalbe’s line | Open angle | Closure is impossible. |
II | 10–20 degree | Trabecular meshwork Schwalbe’s line | Moderately narrow angle | Angle closure is possible but unlikely. |
I | 0–10 degree | Schwalbe’s line | Very narrow angle | High risk of angle closure. |
0 | 0 | None of the structure is visible (Iridocorneal contact) | Closed angle | Indentation gonioscopy with Zeiss goniolens is necessary to differentiate “appositional” from “synechial” closure. |
Fig. 11.15 Diagrammatic grading of angle width.
■Optic Nerve Head (ONH) Analysis
ONH or intraocular part of optic nerve extends from the retinal surface to the posterior scleral surface. The term ONH is generally preferred over optic disc because latter suggests a flat structure without depth. However, the terms “disc” and “papilla” are frequently used for the portion of ONH that is clinically visible by ophthalmoscope.
Divisions of ONH
Optic cup is a central, depressed, pallor area of disc or ONH which represents the volume with partial or complete absence of axons. Its bottom is formed by lamina cribrosa. Neuroretinal rim (NRR) is the tissue between outer edge of cup and the disc margin. It represents the location of the bulk of axons. It has an orange-red color because of associated capillaries (Fig. 11.16).
Fig. 11.16 Right normal optic nerve head (ONH).
Order of the width of NRR: inferior rim > superior rim > nasal rim > temporal rim (remembered by the acronym ‘ISN T’).
As inferior NRR is broadest followed by superior, nasal, and temporal rims, so cup is horizontally oval in most of normal eyes. Thus, a vertical cup should be considered suspicious.
Border between optic cup and NRR is determined by contour and not by pallor. Pallor is the area of optic disc lacking small blood vessels or the maximum area of color contrast. Cupping, the border of optic cup, is determined by kinking (bending) of blood vessels as they cross the optic disc.
Diameter of optic cup is commonly expressed as a fraction of disc diameter both in vertical and horizontal meridian and is known as cup-disc ratio (CDR).
Normal vertical CDR = 0.3. CDR is genetically determined. A difference in CDR between two eyes of >0.2 should be regarded with suspicion until glaucoma has been excluded.
Normally, optic cup is horizontal oval, so in normal eyes, horizontal CDR > vertical CDR. In glaucoma, vertical CDR increases faster than horizontal (because of large fenestrations in superior and inferior portions), so, in glaucoma, horizontal CDR < vertical CDR. This is important for diagnosis of glaucoma.
An enlarged cup may represent a normal physiological variant. An estimation of cup size alone is therefore of limited value in diagnosis of early glaucoma unless it is found to be increasing.
Blood vessels enter the disc centrally and then course nasally and follow the edge of cup. Central retinal artery is usually nasal to the vein.
ONH may be divided into four portions:
•Surface nerve fiber layer (NFL).
•Prelaminar region.
•Lamina cribrosa region: Lamina cribrosa consists of fenestrated sheets of scleral connective tissue through which the nerve fibers pass. Fenestrations in superior and inferior portions are large with thin connective tissue and glial cell support is not well. Therefore, initial damage occurs superiorly and inferiorly at ONH to produce characteristic arcuate defects.
•Retrolaminar region.
Changes in Neuroretinal Rim (NRR)
The disc changes in glaucoma range from focal loss of neural rim tissue with notching of NRR to diffuse rim damage with concentric enlargement of cup. Disc changes are typically progressive and asymmetric. Order of involvement in localized tissue loss—Localized loss of neural rim tissue begins usually in inferotemporal region of ONH and to a lesser extent in superotemporal region in early stages. As the glaucomatous process continues, temporal neural rim is typically involved after the vertical poles, with the nasal rim being the last to be affected.
Change in Optic Cup
Following changes are seen in optic cup due to glaucomatous changes:
•There occurs vertical enlargement of cup (vertical oval shape of cup) due to notching of NRR at lower and upper poles. So vertical CDR > horizontal CDR (in normal eyes, horizontal CDR > vertical CDR).
•Increase in CDR (normal CDR ≤ 0.3).
•Asymmetry of CDR of > 0.2 between the two eyes.
•Deepening of cup (excavation). There is increase in depth of cup leading to exposure of underlying lamina. The fenestrae of lamina cribrosa become visible due to loss of nerve fibers, referred to as lamellar dot sign (as fenestrae of lamina cribrosa have a dotlike appearance on ophthalmoscopy).
•Pallor-cup discrepancy: Initial enlargement may lead to a diffuse, shallow cupping with sloping margins and extending up to disc margins with retention of central pale cup, referred to as saucerization of cup. Thus, area of pallor appears smaller than area of cupping and may be an early sign of glaucoma (Fig. 11.17).
Fig. 11.17 Pictorial representation of the progression of glaucomatous cupping.
Vascular Changes
Following vascular changes are seen due to glaucomatous changes:
•Bayonetting of vessels: If progressive changes of glaucoma are not arrested, advanced glaucomatous cupping with loss of all neural rim tissue occurs. The vessels dive sharply backwards and then ascend up along the steep wall under the overhanging edge of the cup and bend again sharply to emerge at the disc margin (that resemble bayonet of a rifle), i.e., double angulation of blood vessels occur and is referred to as Bayonetting of vessels. This total cupping has also been called “bean pot cupping” because cross-sectional view reveals extreme posterior displacement of lamina cribrosa and undermining of disc margin.
•Baring of circumlinear blood vessels: In many normal ONH, one or two vessels may curve to outline a portion of physiologic cup and run along superior and inferior margins of cup (circumlinear vessels). With enlargement of cup, these circumlinear vessels may be “bared” from the margins of cup and come to lie on the floor of the optic cup. Baring of circumlinear vessels is an early sign of rim thinning and thus diagnostic of glaucoma.
•Disc hemorrhages (splinter hemorrhages): These are flame-shaped, usually near the margin of ONH. They typically cross the disc margin and extend on to the NFL. Their most common location is inferotemporal region.
They are more common in NTG than in patients with POAG and in glaucoma patients with diabetes, hypertension, and use of aspirin. They precede RNFL defects and visual field defects in glaucoma and are a sign of progressive disease.
■Visual Field Analysis in Glaucoma
Glaucoma causes damage to ganglion cell axons at ONH which results in loss of retinal nerve fiber bundles. Therefore, glaucoma causes mostly nerve fiber bundle defects which may be diffuse, localized, or both. Retina is divided into temporal and nasal halves by a vertical line at fovea. Retina is also divided into superior and inferior halves by a horizontal meridian (raphe) that passes from fovea to temporal periphery. Fibers do not cross the horizontal meridian (Fig. 11.18). Important points for consideration in analysing visual field are:
Fig. 11.18 Divisions of right visual field. Abbreviations: BS, blind spot; F, foveola of retina.
•Fibers from macula pass straight to ONH forming papillomacular bundle.
•Fibers arising temporal to macular follow an arcuate path around papillomacular bundle to reach ONH.
•Fibers from nasal retina also pass straight to ONH (Fig. 11.19).
Fig. 11.19 Normal anatomy of retinal nerve fiber layer (RNFL).
Nerve Fiber Bundle (NFB) Field Defects in Glaucoma
The nature of nerve fiber bundle defects relates to the anatomy of RNFL and depends on the location of damaged nerve fibers. Therefore, nerve fiber bundle defects are of three main types: papillomacular bundle defects, arcuate NFB defects and nasal NFB defects. Characteristic features of nerve fiber bundle field defects are:
•NFB visual field defects respect the horizontal meridian especially in nasal portion of visual field corresponding to temporal retina.
•NFB defects have a tendency to be found in Bjerrum area, which is between 10 and 20 degrees from fixation.
•Typically, there is an abrupt change in sensitivity across the horizontal midline.
Papillomacular Defects
Papillomacular fibers are resistant to glaucomatous damage. Therefore, a central island of visual field is retained even in advanced glaucoma. A defect in this bundle of nerve fibers results in one of the following (Fig. 11.20):
Fig. 11.20 Papillomacular defects, Abbreviation: B.S., blind spot.
•Central scotoma: A defect involving central fixation.
•Centrocecal scotoma: A central scotoma connected to the blind spot.
•Paracentral scotoma: These are the small isolated visual field defects between 2 and 10 degrees. Initially, these scotomas are relative but eventually become denser and larger forming an absolute defect in the center surrounded by a relative scotoma.
Arcuate NFB Defects
Arcuate fibers are most sensitive to glaucomatous damage. Usually lower fibers are affected earlier than upper fibers in glaucoma. Because superior and inferior parts of lamina cribrosa contain larger pores and thinner connective tissue, superotemporal and inferotemporal parts of ONH are most vulnerable. Therefore, arcuate fibers are susceptible to damage and arcuate defects tend to occur first. These fibers do not cross horizontal raphe. Defect of these bundles may cause one of the following (Fig. 11.21):
Fig. 11.21 Arcuate nerve fiber bundle (NFB) defects. (a) Arcuate (Bjerrum) scotoma. (b) Seidel scotoma. (c) Double arcuate (ring) scotoma with nasal step. (d) Temporal sector defect.
•Arcuate scotoma: Arcuate scotoma starts from blind spot to reach the horizontal raphe, arching around fixation. These never cross horizontal raphe and may be superior or inferior arcuate scotoma.
•Double arcuate (Ring) scotoma: When arcuate scotomas both above and below the horizontal meridian are present, they join to form a double arcuate or ring scotoma.
•Seidel scotoma: Occasionally, the paracentral scotoma (early arcuate defect) may connect with blind spot to form a sickle-shaped scotoma, known as seidel scotoma, with concavity toward fixation point.
•Roenne nasal step: It is caused by an asymmetry in the rate of nerve fiber loss above and below the horizontal meridian in the nasal field which results in unequal contraction of peripheral isopters. A steplike defect is created where the nerve fibers meet along horizontal meridian.
Differential diagnosis of arcuate scotomas
Although arcuate scotoma is usually associated with glaucoma, it is not pathognomonic because it can be found in other conditions too (especially when field and disc do not seem to correlate) such as chorioretinal lesions (e.g., juxtapapillary retinochoroiditis, retinal artery occlusions); ONH lesions (e.g., Drusen of ONH, papillitis, colobomas) and anterior optic nerve lesions (e.g., pituitary adenoma, optico chiasmatic arachnoiditis).
Nasal NFB Defects
Fibers from nasal retina course in a straight fashion to ONH. A defect in this bundle results in a wedge-shaped temporal scotoma arising from blind spot and does not necessarily respect the temporal horizontal meridian which extends from foveola to temporal retinal periphery.
Other Visual Field Defects in Glaucoma
There are many other causes of these types of VF defects. These are:
•Generalized loss of retinal sensitivity and generalized constriction of visual field.
◊Enlargement of blind spot.
◊Baring of blind spot.
Generalized Constriction of Visual Field
The diffuse reduction in peripheral visual fields along with double arcuate scotoma results in tubular vision (only central vision remains). Causes of tubular vision are glaucoma, retinitis pigmentosa, and central retinal artery occlusion with sparing of cilioretinal artery.
Enlargement of Blind Spot
Enlargement of blind spot, due to depression of peripapillary retinal sensitivity, is also considered to be an early glaucomatous field change. However, it may be seen with other optic nerve or retinal disorder. So, it is not pathognomonic sign of glaucoma.
Baring of Blind Spot
Because the reduced sensitivity of peripapillary retina is greater in upper and lower poles, kinetic perimetry with very small test object may show a vertical elongation of blind spot and localized constriction of central visual field to exclude the blind spot (baring of blind spot). It is not specific for early detection of glaucoma as it may also be found in lens opacities, miosis, or aging process (Fig. 11.22).
Fig. 11.22 Baring of blind spot.
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