Glaucoma

9 Glaucoma

ANATOMY/PHYSIOLOGY

Anatomy/physiology

Ciliary body (CB)

6-mm-wide structure located between the scleral spur anteriorly and the ora serrata posteriorly; composed of the pars plicata (anterior 2 mm with ciliary processes) and the pars plana (posterior 4 mm, flat)

Pars plicata consists of

Ciliary muscle: longitudinal fibers (insert into scleral spur and affect outflow facility), circular fibers (anterior inner fibers oriented parallel to limbus and affect accommodation), and radial fibers (connect longitudinal and circular fibers)

Ciliary vessels: major arterial circle of the iris (in CB near iris root) formed by anastomosis of branches of anterior and long posterior ciliary arteries

Ciliary processes: 70 finger-like projections composed of pigmented and nonpigmented epithelial cell bilayer, capillaries, and stroma

Functions

Suspends and alters shape of lens: zonular fibers that originate between the ciliary processes of the pars plicata, attach to the crystalline lens, and suspend it. Helmholtz’s theory of accommodation explains the changes in lens shape and thus refractive power with contraction and relaxation of the ciliary muscle. Contraction of the longitudinal fibers pulls the lens forward, shallowing the anterior chamber. Contraction of the circular fibers relaxes the zonules making the lens more spherical with greater focusing power. Relaxation of circular fibers (or cycloplegia) tightens the zonules, stretching the lens and making it thinner with less focusing power.

Produces aqueous humor: production of aqueous is primarily by active secretion (also diffusion and ultrafiltration); both an Na⁺/K⁺ pump and carbonic anhydrase are involved. β-blockers act through adenylcyclase to inhibit the Na⁺/K⁺ pump; glucose enters by passive diffusion.

Rate = 2–3 µL/min, AC volume = 250 µL, posterior chamber volume = 60 µL (AC volume turnover is approximately 1%/min). Rate measured by fluorophotometry (direct optical measurement of decreasing fluorescein concentration) or tonography (indirect calculation from outflow measurement). Diurnal production of aqueous (may be related to cortisol levels) decreases with sleep (45%), age (2%/decade; counterbalanced by the decreased outflow with age), inflammation, surgery, trauma, and drugs

AQUEOUS COMPOSITION: slightly hypertonic and acidic (pH 7.2) compared with plasma; 15× more ascorbate than plasma; lower protein (0.02% vs 7% in plasma); lower calcium and phosphorus (50% of the level in plasma); chloride and bicarbonate vary (from 25% above or below plasma levels); sodium, potassium, magnesium, iron, zinc, and copper levels similar to those in plasma

FUNCTIONS OF AQUEOUS: maintains intraocular pressure, provides metabolic substrates (glucose, oxygen, electrolytes) to the cornea and lens, and removes metabolic waste (lactate, pyruvate, carbon dioxide)

Affects aqueous outflow: contraction of ciliary muscle causes traction on the trabecular meshwork, increasing outflow

Synthesizes acid mucopolysaccharide component of vitreous: occurs in nonpigmented epithelial cells of pars plana and enters vitreous body at its base

Maintains blood–aqueous barrier: within the ciliary processes, plasma enters from thin fenestrated endothelium of capillary core → passes through stroma → 2 layers of epithelium with apposing apical surfaces (which forms the ciliary epithelial bilayer):

OUTER PIGMENTED LAYER: continuous with the RPE (basal lamina continuous with Bruch’s membrane), contains zonula occludens

INNER NONPIGMENTED LAYER: equivalent to the sensory retina (basal lamina continuous with ILM), site of active secretion

Both layers have basal basement membranes with their apices facing each other. During passage from the bloodstream to the posterior chamber, a molecule must pass through capillary basement membrane, pigmented epithelium basement membrane, pigmented epithelium, nonpigmented epithelium, nonpigmented epithelium basement membrane

Cryodestruction and inflammation cause loss of barrier function (tight junctions open), resulting in flare (proteins in aqueous); atropine reduces flare by closing tight junctions

Outflow Pathways

Trabecular meshwork (traditional pathway)

represents major aqueous drainage system; uveoscleral meshwork → corneoscleral meshwork → juxtacanalicular connective tissue → Schlemm’s canal → collector channels → aqueous veins → episcleral and conjunctival veins → anterior ciliary and superior ophthalmic veins → cavernous sinus

The pore size of the meshwork decreases towards Schlemm’s canal:

Uveal meshwork: collagenous core surrounded by endothelial cells; pore size up to 70 µm; linked to ciliary muscle

Corneoscleral meshwork: sheet-like beams insert into scleral spur; pore size up to 30 µm

Juxtacanalicular tissue: links corneoscleral trabeculae with Schlemm’s endothelium; pore size = 4–7 µm; site of greatest aqueous outflow resistance

Canal of Schlemm is lined by a single layer of endothelial cells (mesothelial cells) and connects to the venous system by 30 collector channels

Uveoscleral outflow (15–20% of total outflow)

aqueous passes through face of ciliary body in the angle, enters the ciliary muscle and suprachoroidal space, and is drained by veins in the ciliary body, choroid, and sclera

Cyclodialysis cleft increases aqueous outflow through the uveoscleral pathway (without reducing aqueous production); cycloplegics and prostaglandin analogues increase uveoscleral outflow; miotics decrease uveoscleral outflow and increase trabecular meshwork outflow

Angle Structures

Visible only by gonioscopy because of total internal reflection at the air/cornea interface (Figure 9-1)

Figure 9-1 Composite drawing of the microscopic and gonioscopic anatomy.

(From Becker B, Shaffer RN: Diagnosis and Therapy of the Glaucomas, St Louis, Mosby, 1965.)

Schwalbe’s line

peripheral/posterior termination of Descemet’s membrane, which corresponds to apex of corneal light wedge (optical cross section of the cornea with narrow slit beam reveals 2 linear reflections, 1 from external and 1 from internal corneal surfaces, which meet at Schwalbe’s line)

Trabecular meshwork (TM)

anterior nonpigmented portion appears as a clear white band; posterior pigmented portion has variable pigmentation (usually darkest inferiorly). Increased pigmentation of trabecular meshwork occurs with pseudoexfoliation syndrome (Sampaolesi’s line), pigment dispersion syndrome, uveitis, melanoma, trauma, surgery, hyphema, darkly pigmented individuals, and increasing age

Schlemm’s canal

usually not visible or only faintly visible as a light gray band at the level of posterior TM; elevated venous pressure or pressure from the edge of the gonioscopy lens may cause blood to reflux, making Schlemm’s canal visible as a faint red band

DDx of blood in Schlemm’s canal

elevated episcleral venous pressure, oculodermal melanocytosis (nevus of Ota), neurofibromatosis, congenital ectropion uveae, hypotony, secondary to gonioscopy

Scleral spur (SS)

narrow white band that corresponds to the site of insertion of longitudinal fibers of ciliary muscle to sclera

Ciliary body (CB)

pigmented band that represents the anterior face of the ciliary body; iris processes may be seen as lacy projections crossing this band but not the scleral spur (occur in 33% of population)

Angle Abnormalities

Peripheral anterior synechia (PAS)

any pigmented structure that crosses scleral spur

Etiology: angle-closure, uveitis, neovascularization, flat anterior chamber, ICE syndrome, ciliary body tumors, mesodermal dysgenesis

Normal vessels

radial iris vessels, portions of arterial circle of CB, and rarely, vertical vessels deep in CB; do not branch or cross the scleral spur; present in 7% of patients with blue irides and 10% with brown

Abnormal vessels

fine, often branch, no orientation, cross-scleral spur

DDx: neovascularization (rubeosis iridis), iris neoplasm, Fuchs’ heterochromic iridocyclitis (sparse, faint, and delicate; bleed easily on decompression of AC)

Angle recession

tear between longitudinal and circular fibers of the ciliary muscle. Because longitudinal fibers are still attached to the scleral spur, miotics still work, but because they decrease uveoscleral outflow, IOP may actually increase. Breaks in posterior TM result in scarring and a nonfunctional TM; aqueous drains primarily through uveoscleral outflow; 60–90% of patients with traumatic hyphemas have angle recession; 5% of eyes with angle recession will develop glaucoma

Gonioscopic findings: widened ciliary body band, increased visibility of scleral spur, torn iris processes, sclera visible through disrupted ciliary body tissue, marked variation in CB width in different quadrants of same eye

Cyclodialysis cleft

separation of ciliary body from scleral spur; often from trauma. Results in direct communication between AC and suprachoroidal space causing hypotony; spontaneous closure may occur (unlikely after 6 weeks) with marked IOP rise; shortly thereafter, the TM should begin to function normally again

Gonioscopic findings: cleft at junction of scleral spur and CB band

Treatment: cycloplegic to relax ciliary body in an attempt to close cleft (avoid pilocarpine, which may open the cleft through ciliary muscle traction); laser (argon induces inflammation to close the cleft; spot size = 50–100 µm, duration = 0.1–0.2 s; power = 0.5–1.0 watts to uvea and 1–3 watts to sclera); cryotherapy; suture CB to sclera (direct cyclopexy); intravitreal air bubble for a superior cleft; YAG laser can be used to open a closed cleft

Iridodialysis

tear/disinsertion of iris root. If large or symptomatic, consider surgical repair with mattress sutures

Optic Nerve (Figure 9-2)

Approximately 1.2 million axons; cell bodies of ganglion cells are located in the ganglion cell layer

Figure 9-2 Vascular supply and anatomy of the anterior optic nerve.

(From Hart WM Jr: In Podos SM, Yanoff M [eds]: Textbook of Ophthalmology, vol 6, London, Mosby, 1994.)

4 layers of optic nerve head based on blood supply:

Nerve fiber: supplied by branches of central retinal artery

Prelaminar: supplied by capillaries of the short posterior ciliary arteries

Laminar (lamina cribrosa): supplied by dense plexus from short posterior ciliary arteries

Retrolaminar: supplied by both ciliary (via recurrent pial vessels) and retinal (via centripetal branches from pial region) circulations

Optic nerve blood flow is influenced by mean blood pressure, IOP, blood viscosity, blood vessel caliber, and blood vessel length

Testing

Intraocular Pressure

Goldmann equation

IOP = F/C + EVP relates 3 factors important in determination of IOP

F = rate of aqueous formation = 2–3 µL/min

C = facility of outflow = 0.28 µL/min/mmHg; <0.20 is abnormal; decreases with age, increases with medication; measured by tonography

EVP = episcleral venous pressure = 8–12 mmHg; increases with venous obstruction or A-V shunt; measured by manometry

IOP = 8–21 mmHg is considered normal; average = 16 +/−2.5 mmHg; distribution is not Gaussian and is skewed to higher IOPs

IOP is influenced by age (may increase with age), genetics, race (higher in African Americans), season (higher in winter, lower in summer), blood pressure, obesity, exercise (lower after exercise), Valsalva, time of day (diurnal variation [2–6 mm/day]; peak in morning), posture (higher when lying down vs sitting up), various hormones, and drugs. Also ocular factors: refractive error (higher in myopes) and eyelid closure

Tonometry

IOP measurement can be performed with a variety of devices (tonometers)

Indentation

Schiøtz tonometer: known weight indents cornea and displaces a volume of fluid within the eye; amount of indentation estimates pressure; falsely low readings occur with a very elastic eye (low scleral rigidity as with high myopia, buphthalmos, retinal detachment, treatment with cholinesterase inhibitors, thyroid disease, and previous ocular surgery) or a compressible intraocular gas (e.g. SF₆ and C₃F₈); falsely high readings occur with scleral rigidity and hyperopia

Applanation

based on the Imbert-Fick principle: P = F/A (for an ideal thin-walled sphere, pressure inside sphere equals force necessary to flatten its surface divided by the area of flattening). The eye is not an ideal sphere: the cornea resists flattening, and capillary action of the tear meniscus pulls the tonometer to the eye. However, these 2 forces cancel each other when the applanated diameter is 3.06 mm

Goldmann tonometer: biprism attached to a spring; fluorescein semicircles align when area applanated has a diameter = 3.06 mm; unaffected by ocular rigidity; error can occur with squeezing, Valsalva, irregular or edematous cornea, corneal thickness (thick corneas overestimate IOP [∼5 mmHg per 70 µm], and thin corneas underestimate IOP [∼5 mmHg per 70 µm]), amount of fluorescein, external pressure on or restriction of globe, and astigmatism >1.5 D (must align red mark on tip with axis of MINUS cylinder)

Perkins tonometer: portable form of Goldmann device

Mackay-Marg tonometer: applanates small area; good for corneal scars and edema. Examples of this style tonometer include the tonopen and the pneumotonometer

TONOPEN: probe indents cornea, and microprocessor calculates IOP and reliability

PNEUMOTONOMETER: central sensing device is controlled by air pressure

Noncontact

Air puff tonometer: noncontact device; time required for air jet to flatten cornea is proportional to IOP; varies with cardiac cycle

Gonioscopy

Classification systems

Grade I = wide open (CB visible)

Grade II = SS visible; CB not visible

Grade III = only anterior TM visible

Grade IV = closed angle (TM not visible)

Schaffer: opposite of Scheie (Grade 0 is closed; Grade IV is wide open) (Figure 9-3)

Grade I = 10% open

Grade II = 20% open

Grade III = 30% open

Grade IV = 40% open

Figure 9-3 Shaffer’s angle-grading system.

(From Fran M, Smith J, Doyle W: Clinical examination of glaucoma. In Yanoff M, Duker JS [eds]: Ophthalmology, 2nd edn. St Louis, Mosby, 2004.)

Example: (A)B15 r, 1+ (appositionally closed 15° angle that opens to TM with indentation, regular iris configuration, and mildly pigmented posterior TM)

Visual Fields

Perimetry measures the ‘island of vision’ or topographic representation of differential light sensitivity. Peak = fovea; depression = blind spot; extent = 60° nasally, 60° superiorly, 70–75° inferiorly, and 100–110° temporally

Central field tests points only within a 30° radius of fixation

Humphrey (static)

Test distance = 0.33 m; background illumination = 31.5 apostilbs (asb); stimulus size = III; stimulus duration = 0.2 s; various programs (i.e. central 30°, 24°, 10°, neuro fields, ptosis fields, etc.)

Reliability indices:

FIXATION LOSS: patient responds when a target is displayed in blind spot. There is also a gaze tracking printout at the bottom of the page that shows the deviation of fixation during each stimulus presentation

FALSE-POSITIVE: patient responds when there is no stimulus (nervous or trigger-happy; causes white areas)

FALSE-NEGATIVE: patient fails to respond to a superthreshold stimulus at a location that was previously responded to (indicates loss of attention or fatigue; causes cloverleaf pattern)

Global indices:

MEAN DEVIATION (MD): average departure of each test point from the age-adjusted normal value. This represents the overall deviation (mean elevation or depression) of the visual field from the normal reference field

PATTERN STANDARD DEVIATION (PSD): standard deviation of the differences between the threshold and expected values for each test point. This represents the change in shape of the field from the expected shape for a normal field

SHORT-TERM FLUCTUATION (SF): variability in responses when the same 10 points are retested; measure of consistency

CORRECTED PATTERN STANDARD DEVIATION (CPSD): PSD adjusted for patient reliability (correcting for SF)

Patient vision ≤20/80 will cause a scotoma to appear larger and deeper

Pupil <3 mm will cause reduction in total deviation

Tangent screen (usually kinetic)

test distance is 1m, test object may vary in size and color; tests only central field. Magnifies scotoma and is of low cost; however, poor reproducibility and lack of standardization

VF defect

a scotoma is an area of partial or complete blindness

Corresponds to a defect that is 3° wide and 6 decibels (dB) deep; also 1 point on Humphrey testing that is depressed >10 dB or at least 2 points that are depressed at least 5 Db

Typical localized glaucomatous scotomas: (Figure 9.5)

PARACENTRAL: within central 10°

ARCUATE (Bjerrum): isolated, nasal step of Rönne and Seidel (connected to blind spot)

TEMPORAL WEDGE

Glaucomatous scotomas do not respect the vertical meridian (vs neurologic VF defects, which do)

VF should correlate with optic nerve appearance; otherwise, consider refractive error, level of vision, media opacities, pupil size, and other causes of VF defects (tilted ON head, ON head drusen, retinal lesions, etc.)

Figure 9-5 Composite diagram depicting different types of field defects.

(From Bajandas FJ, Kline LB: Neuro-Ophthalmology Review Manual, 3rd edn, Thorofare, NJ, Slack, 2004.)

Optic Nerve Head (ONH) Analyzers

Various digital and video cameras that capture ONH image; computer then calculates cup area in an attempt to objectively quantify ONH appearance (Table 9-1)

TABLE 9-1 Summary of Techniques for Retinal Nerve Fiber Layer Analysis

Confocal scanning laser ophthalmoscopy (CSLO; Heidelberg retinal tomograph [HRT]; TopSS)

low-power laser produces digital 3D picture of ON head by integrating coronal scans of increasing tissue depth; indirectly measures nerve fiber layer (NFL) thickness (Figures 9-6, 9-7)

Figure 9-6 Confocal scanning laser ophthalmoscopy.

(Adapted from Schuman JS, Noeker RJ: Imaging of the optic nerve head and nerve fiber layer in glaucoma. Ophthalmol Clin North Am 8:259–279, 1995.)

Figure 9-7 Confocal scanning laser ophthalmoscopy printed report.

(From Zangwill L, de Souza K, Weinrob RN: Confocal scanning laser ophthalmoscopy to detect glaucomatous optic neuropathy. In Shuman JS [ed]: Imaging in Glaucoma. Thorofare, NJ, Slack, 1997.)

Optical coherence tomography (OCT)

measures optical backscattering of light to produce high-resolution, cross-sectional image of the NFL (Figure 9-8)

Figure 9-8 Optical coherence tomography.

(From Pedut-Kloizman TP, Schuman JS: Disc analysis. In Yanoff M, Duker JS [eds]: Ophthalmology, London, Mosby, 1999.)

Scanning laser polarimetry (SLP; Nerve Fiber Analyzer, GDx)

uses a confocal scanning laser ophthalmoscope with an integrated polarimeter to detect changes in light polarization from axons to measure the NFL thickness; quantitative analysis of NFL thickness to detect early glaucomatous damage (Figure 9-9)

Figure 9-9 Printout from the GDx software of the Nerve Fiber Analyzer; normal eye.

(From Chopin NT: Retinal nerve fiber layer analysis. In Yanoff M, Duker JS [eds]: Ophthalmology, London, Mosby, 1999.)

Optic nerve blood flow measurement

color Doppler imaging and laser Doppler flowmetry (Figure 9-10)

Figure 9-10 Color Doppler imaging of the ophthalmic artery.

(From O’Brien C, Harris A: Optic nerve blood flow measurement. In Yanoff M, Duker JS [eds]: Ophthalmology, London, Mosby, 1999.)

Pathology

Glaucoma

Dropout of ganglion cells, replacement of NFL with dense gliotic tissue and some glial cell nuclei; partial preservation of inner nuclear layer with loss of Müller’s and amacrine cells (normal 8–9 cells high; in glaucoma, 4–5 cells high); earliest histologic changes occur at level of lamina cribrosa; advanced cases may show backward bowing of lamina or ‘beanpot’ appearance (Figures 9-11, 9-12)

Figure 9-11 POAG demonstrating cupping of the optic nerve head.

(From Yanoff M, Fine BS: Ocular Pathology, 5th edn. St Louis, Mosby, 2002.)

Figure 9-12 POAG demonstrating atrophy of the inner retinal layers. A, low power, B, higher magnification.

(From Yanoff M, Fine BS: Ocular Pathology, 5th edn, St Louis, Mosby, 2002.)

Schnabel’s Cavernous Optic Atrophy

Histologic finding in eyes with increased IOP or atherosclerosis and normal IOP; hyaluronic acid infiltration of nerve from vitreous due to imbalance between perfusion pressure in the posterior ciliary arteries and IOP; also occurs in eyes with ischemic optic neuropathy

Tags: Review of Ophthalmology Expert Consult

Jun 4, 2016 | Posted by admin in OPHTHALMOLOGY | Comments Off

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