Anterior Segment Assessment.pdf

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ANTERIOR SEGMENT ASSESSMENT

Overview:

This course provides an overview of assessment of the anterior segment. Section I covers

the anatomy of anterior segment, introduction to slit lamp biomicroscopy. Common

disorders of the anterior segment are discussed in the second section (Section II). The use

of various imaging techniques to quantify the anterior segment disorders will be covered

in Section III.

Disclaimer: This course is not intended to market any instruments.

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SECTION I

Anatomy of the anterior segment:

The external demarcation of the anterior segment lies at the limbus and extends till the

anterior hyaloid. Functionally, the anterior segment begins at the tear film and ends at the

posterior capsule of the lens. As can be seen from the figure 1.1 the anterior segment

comprises of the lids, conjunctiva, sclera, cornea, the anterior chamber, iris, posterior

chamber and the crystalline lens.

Figure 1.1: Anatomy of the anterior segment

Slit lamp biomicroscopy:

Slit lamp biomicroscope is a binocular optical microscope that typically has a

illumination system and a viewing system (Figure 1.2). A beam of light is projected on to

the structure that is to be examined and the structure is viewed through a series of

magnifying lenses. The anatomic structures are accentuated when the slit of light is

directed at a particular angle.

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Figure 1.2: Slit lamp Biomicroscope

Using various accessories (Figure 1.3) and filters along with the slit lamp enables better

assessment of the anterior segment.

Figure 1.3: Slit lamp accessories

Slit lamp biomicroscopy is a scientific way of assessing the health of the ocular structures

using the slit lamp, both quantitatively and qualitatively.

Pictorial representation of various illumination types are given in figure 1.4 (a-d)

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Figure 1.4a: Diffuse: Full slit height and width, direct illumination

Figure 1.4b: Focal illumination: Optic section-Slit height: Full; Width:

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Figure 1.4d: Sclerotic scatter: Parallelepiped focused at the temporal limbus

The normal anterior segment:

The following section briefly explains the normal appearance of the structures in the

anterior segment and the technique of assessing these structures with slit lamp.

Lids: The lids are best assessed under diffuse illumination. The lids are further divided

into three parts: The lashes, lid margin and the puncta.

The lashes are more numerous in the upper lid than the lower lid. Normally, the lashes

are pigmented and are distributed with uniform density throughout the lids. Lid margin is

the junction between the skin of the lids and the palpebral conjunctiva. The lid margin is

a lubricated structure and contains various glands in addition to the lashes. A normal lid

margin is regularly thick and follows the structure of the globe. The puncta are small

openings present in the nasal aspect of both the upper and lower lids. The puncta are

small openings and the normal size of the punctum is 0.2mm. The punctum is well

apposed to the ocular surface.

Tear Film: The tear film consists three layers namely the lipid, aqueous and mucin

layers. The tear film spreads over the entire ocular surface. A normal tear film appears

clear on the ocular surface with a width of approximately 1mm at the surface and mm at

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the lid margins. For assessment of tear quality diffuse illumination is used and for

assessing the thickness of the tear layer an optic section is used.

Sclera: is composed of collagen fibres arranged haphazardly. The sclera contains

numerous blood vessels. The normal color of sclera is whitish to yellowish white. The

sclera is covered by a layer of transparent structure called the episclera. Sclera is

examined with an optic section under direct illumination.

Conjunctiva: The conjunctiva is the outermost membrane between the tear film and

sclera. The conjunctiva is a transparent membrane with numerous fine blood vessels. The

interspace between the conjunctival membrane and the sclera regular, uniform and is

usually devoid of fluid. The conjunctiva ends anteriorly at the limbus and posteriorly

extends as tenons capsule. Conjunctiva can be assessed using both diffuse illumination

and optic section.

Cornea: The cornea is the convex transparent structure starting after the conjunctiva. The

cornea is a five layered structure: Epitheium, bowmans membrane, stroma, descemets

membrane and the endothelium. It is made of collagen fibers that are arranged in a

regular fashion. The cornea is devoid of any blood vessels.

The epithelium is lubricated by the tearfilm. It is best assessed with direct diffuse

illumination. In a parallelepiped section, the stroma represents the larger middle section.

The stroma appears regularly transparent. The endothelium is a monolayered structure.

Edothelium is best assessed with specular reflection. The bowmans and the descemets

membranes are not usually visible with a conventional slit lamp.

Anterior Chamber: Anterior chamber is the structure in front of the iris till the posterior

surface of the cornea. The aqueous humor is secreted in the posterior segment, flows

through the pupil and is circulated in the anterior chamber. The aqueous humor is a clear

fluid that circulates in the anterior chamber. Anterior chamber is assessed with an optic

section.

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The depth of the anterior chamber is the space between the corneal endothelium and the

anterior iris. The anterior chamber depth (ACD) is deeper at the center and shallow

peripherally. The normal anterior chamber depth in the periphery is equal to at least half

the thickness of a normal cornea. The region where the iris meets the corneo-scleral

junction is called the anterior chamber angle. Anterior chamber angle is assessed using

gonioscope. A gonioscopic view of the anterior chamber looks as given in figure 1.6.

Figure 1.6

The angle is said to be opens or closed based on the furthest structure seen through a

gonioscope (Schaffers grading). The grading of anterior chamber angle is given in table

1.1

Table 1.1 Gonioscopic grading of anterior chamber angle

Structure seen Angle Grade

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Iris 0 Closed

Schwalbes line 10o 1, Narrow

Anterior trabecular

meshwork 20o 2, Occludable

Scleral spur 30o 3, Open

Ciliary body band 40o 4, Widely open

IRIS: The pigmented diaphragm in front of the crystalline lens is called the iris. The iris

is a uniformly pigmented muscle that has cryptic appearance in the slit lamp. The iris is

the thicker at he pupillary margin compared to the center.

CRYSTALLINE LENS: The crystalline lens is a multilayered, biconvex structure with a

denser central nucleus surrounded by cortex on the anterior and the posterior sides. The

lens fibers are made of proteins. These proteins render the lens translucent and increases

in opaqueness with age.

Summary:

The key for successful slit lamp biomicroscopy are:

Using appropriate illumination and magnification (Table 1.2) Proper angling of the illumination and viewing systems Following a systematic approach

Table 1.2 Slit lamp illumination

Illumination Structures

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Diffuse External overall view, lid, lashes, conjunctiva, cornea

Parallelepiped Cornea, meniscus, iris, lens

Optical Section Angle estimation, corneal layers, lenticular layers

Conical Beam Anterior chamber (cells)

Retroillumination Transillumination of the iris, lenticular opacities

Specular Reflection Tear Layer, endothelium

Sclerotic scatter Corneal scars, central edema

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SECTION II

Common disorders of the anterior segment

The objective of the following section is to list out the most common conditions and

disorders of the anterior segment seen in our population. The conditions are dealt with

respect to each anatomical structure.

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Table 2.1: Lids

Condition Image Slit lamp Sign

Meibomitis: Inflammation

and obstruction of the

meibomian glands

Small oil globules capping the

meibomian gland orifices.

Oily and foamy tear film

Blepharitis: Eyelid

inflammation eye infection

or dry eyes

Hyperaemia, telangiectasia, hard

and brittle scales at the bases

Entropion: Inward folding

of eyelids

Inturned eye lashes, associated

with corneal ulceration

Ectropion: Outward folding

of eyelid

Abnormal lid globe apposition,

corneal exposure, tearing

Trichiasis: Misdirected

eyelashes

Traumatic punctate epithelial

erosions, corneal ulceration and

pannus

Phthiriasis palpebrarum:

Infestation of the lashes by

crab louse nit

Lice and nits gripping to the

roots of the lashes at the base of

the cilia.

Madarosis:

Decrease in number or complete loss of lashes

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Poliosis:

Whitening of the lashes and eyebrows

Chalazion: Cyst in the

eyelid that is caused by

inflammation of a blocked

meibomian gland

Non tender and painless swelling

on the eyelid asociated with

blepharitis

Stye: Acute staphylococcal

abscess of a lash follicle

Tender inflamed swelling on the

lid margin, pointing anteriorly

through the skin

Lid edema: Due to allergic

reaction or infection

associated with itching,

redness or pain

Severely swollen lid, tenderness,

redness or pain.

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Table 2.2: Conjunctiva and Sclera


Follicles: Characterized by

hyperplasia of lymphoid tissue

within the stroma

Multiple, discrete, slightly

elevated lesions, encircled by

tiny blood vessel

Papillae: Composed of hyperplastic

conjunctival epithelium and a

diffuse infiltrate of chronic

inflammatory cells.

A fine mosaic-like pattern of

elevated polygonal projections

with central blood vessels

Conjunctivitis: Acute inflammation

due to an allergic reaction or an

infection

Red eyes (Difffuse congestion),

Dilated conjunctival vessels,

Puffy eyelids, Tearing (watery

eyes), Stringy eye discharge

Subconjunctival hemorrhage:

Beeding underneath the conjunctiva

Bright red or dark red patch on

the sclera

Pterygium: Benign fibrovascular

growth of the conjunctiva

Elevated, superficial, external

fibrovascular mass over the

perilimbal conjunctiva to corneal

surface Pinguecula: Non-cancerous growth

of conjunctiva

Yellowish white deposit on the

conjunctiva adjacent to the

limbus

Scleritis/Episcleritis: Inflammation

of the sclera associated with

infection, chemical injuries, or

autoimmune diseases

Red or purplish sclera and

conjunctiva

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Table 2.3: Tear film


Tear film height

A thin strip of tear fluid with

concave outer surfaces at the

upper and lower lid margins

Tear film debris: Associated

with blepharitis or a

dysfunctional meibomian gland

Dark specks in the tear film of

the eye moving quickly with a

blink

Tear film break up: Faster tear

film break up in dry eyes

The tear film stained with

observed under cobalt-blue

filtered light. Time elapsed

between last blink and

appearance of the first break is

TBUT

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Table 2.4: Cornea


Ectasia: Thinning and steepening of cornea

Keratoconus: Inferior

corneal steepening and

thinning, with pigment line

Munsons sign, Fleishers ring,

ectasia

Pellucid Marginal

Degeneration: Thinning

bellow the area of

steepening

Ectasia, Fleishers ring

Terriens Marginal

Degeneration: Thinning of

peripheral cornea

Mostly superior ectasia

Keratoglobus: Uniform

thinning and steepening

Global ectasia

Other Corneal Conditions

Arcus: Lipid deposition in

stroma, usually seen in

adults

Diffuse, white band along limbal

margin

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Dendrite: Typically seen in

viral keratitis

Branched opacities at stromal

level

Guttata: Loss of endothelial

cells

Warts/Shadow like gaps in

endothelium

Ulcer: Wound resulting

from infection or injury

Appearing as

sore/opening/erosion

Opacity/Scar: Translucent

regions in cornea

Can be minimally translucent

(grey) to completely opaque

(white)

Infiltrate: Focal areas of

active inflammation

Granular opacities in stroma

Bullous Keratopathy:

Secondary to compromised

endothelial functions

Corneal edema with epithelial

bullae

Band Keratopathy:

Deposition of calcium in

anterior Bowmans

membrane

Calcification with sharp margins

in band shape from limbus

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Punctate Keratitis:

Granular, swollen epithelial

cells

Stained epithelial cells with

fluorescein

Corneal Dystrophy

Lattice Dystrophy: Spidery

branching lines

Ropy lines extending from

limbus. Little/No haze. Best seen

in retroillumination

Macular dystrophy: Poorly

delineated spots causing

haze

Superficial to deep opacities

involving limbus. Not well

demarcated

Granular Dystrophy: Well

confined dots, does not

involve limbus

Sharply demarcated white

deposit like lesions

Fuchs Endothelial

Dystrophy: Associated with

vision loss, more in women

associated with open angle

glaucoma

Guttata, haze, edema, bullae in

later stages

Congenital Hereditary

Endothelial Dystrophy:

Focal or generalized

absence of stromal

endothelium

Edema, ground glass appearance

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Corneal surgeries

Penetrating Keratoplasty:

Full thickness replacement of

opacified cormea

Lamellar Keratoplasy:

Descemet Stripping

Endothelial Keratoplasty

Anterior Lamellar

Keratoplasty

Replacement of selected corneal

layers

LASIK:

Flap of corneal tissue over region

of ablated stroma

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Table 2.5: Anterior Segment


Flare:

Flare

Cells or particles moving in

anterior chamber

Hypopyon: Seen in

inflammatory conditions, or

as response to infection

Hypopyon

Sedimentation of collection

of cells in anterior chamber

Hyphema: Seen post trauma

Hyphema

Blood in anterior chamber

Shallow anterior chamber:

Narrow space between

cornea and iris

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Table 2.6: Iris


Coloboma: Commonly

associated with coloboma

of other ocular structures

Ectopic pupil-Iris coloboma

Absence of iris tissue,

typically called as key hole

appearance

Aniridia: Commonly

associated with subluxated

lens

Aniridia

Absence of iris

Synechiae: Anterior

synechiae is a risk factor for

angle occlusion, posterior

synechiae associated with

inflammatory conditions

like uveitis

AS

PS

Adherence of iris to cornea

(Anterior synechiae-AS) or

lens (Posterior synechiae-

PS)

Iridectomy: Induced

surgically or by laser.

Usually as treatment for

narrow angle glaucoma or

during cataract surgery

Patent PI

Hole in iris. A patent

peripheral iridectomy

passes light with

retroillumination

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Atrophy: Degeneration of

iris muscle

Iris atrophy

retroilluminated

Appears white on direct

illumination, transmits light

on retro illumination

Iris Nevus: Pigmentation in

iris. Indicative of tumor if

increases in size

Iris nevus

Appears as patch of excess

pigmentation

Rubeosis iridis: Seen

commonly in diabetes,

neovascular glaucoma

Rubeosis Iridis

Blood vessels in iris

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Table 2.7: Crystalline Lens


Cataract: Clouding and opacification of the crystalline lens

Sutural Cataract: Mostly

congenital and non-

progressive

Sutural Cataract

Opacity in the shape of

anterior or posterior Y suture

Sub-capsular Cataract: Most

common type of cataract.

Associated with steroid use

Posterior sub-capsular cataract

Yellowening of the lens in

the posterior or anterior sub

capsular region

Cortical Cataract: Opacities

located in cortical layer

Spokes in Cortical

cataract

Appears as water clefts/

vacuoles in early stages

Spoke-like or wedge-shaped

peripheral opacities in

advanced stages

Nuclear Cataract (Sclerosis):

Shows myopic shift in

refraction

Nuclear cataract

Age related gradual

opacification of lens nucleus:

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Brunescent Cataract: A type

of nuclear cataract

Brunescent cataract

Nucleus appears brown to

black

Total Cataract: Completely

opacified crystalline lens

Total cataract

Appears whitish through

pupil (Leucocoria)

Other disorders of lens

Lenticonus: Protrusion of

lens at the center caused by

thin capsule. Can be

associated with keratoconus,

polar cataract and retinal

abnormalities.

Anterior Lenticonus

Posterior Lenticonus

Appears as sudden increase

in central lenticular curvature

in the anterior or posterior

surface

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Dislocation: Absence lens

from patellar fossa

Dislocated cataractous

lens

Lens seen in anterior or

posterior chamber

Subluxation: Partial

dislocation of lens. May be

associated with collagen

tissue disorders Superior subluxation

Lens partially absent from

visual axis. Lens edge and

zonus visible

Pseudophakia: Artificial lens,

usually as a substitute for

crystalline lens after cataract

extraction Pseudophakia

Artificial lens in place of

crystalline lens

Posterior Capsular

Opacification (PCO):

Opacified posterior capsule

post cataract surgery PCO with yag opening

Haze beyond pseudophakic

lens

Aphakia: Absence of

crystalline lens

Aphakia

Void in pupillary zone

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Summary: Diagnosis involves associating symptoms with signs Right diagnosis leads to right management Consider differential diagnoses for common slitlamp signs

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SECTION III

Anterior segment imaging and diagnostics:

We have seen in previous sections that Slit lamp plays an important role in disease

diagnosis. However, it is to be understood that the slit lamp is useful to detect a

disease in its manifest stage. In many of the anterior segment disorders diagnosing the

disease at the sub-clinical stage would help in better management modalities and

thereby provides a better prognosis. Also, slit lamp biomicroscopy is more a

subjective technique and quantification of the disease stage. Thus, the role of a

diagnostic instrument becomes critical. A diagnostic instrument is important for the

following reasons:

Screening: Diagnosis of subclinical disease Diagnosis: Confirmation of the clinical diagnosis Progression: Quantitative improvement/worsening in follow up

When it comes to anterior segment, the disorders of the anterior segment can be

broadly classified into Diseases of the Anterior Chamber and Corneal Diseases.

The following section contains detailed description of the diagnostics specific to each

of the conditions.

Advanced imaging for the anterior chamber assessment

The anterior chamber, the region between the iris and posterior border of cornea, is

imaged by techniques that enhance the tomographic property of these structures. Such

diagnostics are

Ultrasound Biomicroscopy Anterior Segment Optical Coherence Tomography Scheimpflug technique

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Unlike the anterior chamber diagnostics, the corneal diagnostics not only quantify the

volume of the cornea but also quantify the corneal shape. These include

Corneal topography Corneal tomography

o Anterior Segment Optical Coherence Tomography

o Scheimpflug technique

Pachymetry o Ultrasound and non contact techniques

Specular microscopy

Ultrasound Biomicroscopy (UBM)

Ultrasound biomicroscopy (Figure 3.1) is an imaging technique that uses high frequency

ultrasound to produce images of the eye a high, near microscopic resolution of the

structures. Though UBM is primarily a diagnostic tool for glaucoma, it can be used for a

comprehensive anterior segment assessment.

Principle: The ultrasound principle involves passing a sound wave through the tissue and

the delay in reflection and amount of absorption helps in imaging the tissue. A transducer

produces waves of 50 MHz frequency. At this frequency the tissue penetration is 4-5mm

and the resolution is approximately 50 microns.

Procedure: The procedure is done with patient lying in supine position (Figure 3.2).

After instilling the topical anesthetic a 20mm eye cup filled methyl cellulose solution is

placed between the lids. The transducer probe is placed close to the corneal surface

perpendicular to the structure of interest. The probe is moved radially to visualize the

structures. In vivo, cross-sectional or transverse images can then be obtained detailing the

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cornea, iris, ciliary body, anterior chamber angle, and peripheral sclera to demonstrate

structural relationships.

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Figure 3.1 Ultrasound Biomicroscope Figure 3.2 Ultrasound Biomicroscopy

Quantitative assessment: Table 3.1 given below has the list of quantifiers that helps in

quantification of the anterior chamber.

Table 3.1: Anterior Segment Quantifiers

Parameter Description

Angle Opening Distance (AOD) Distance between trabecular meshwork and iris at 500 m anterior to scleral spur

TrabecularIris angle (TIA) Angle of angle recess

TrabecularCiliary process distance (TCPD)

Distance between trabecular meshwork and ciliary process at 500 m anterior to scleral spur

Iris thickness (IT)

Iris thickness at: 500 m anterior to scleral spur, at 2 mm from iris root, maximum iris thickness near pupillary edge

IrisCiliary process distance (ICPD)

Distance between iris and ciliary process along TCPD line

IrisZonule distance (IZD) Distance between iris and zonule along TCPD line

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IrisLens contact distance (ILCD)

Contact distance between iris and lens

IrisLens angle Angle between iris and lens near pupillary edge

Normal Anterior segment and UBM:

The structures that can be visualized using the UBM include the cornea, Schwalbe's line,

sclera and scleral spur, anterior chamber, iris, anterior lens capsule, posterior chamber,

and ciliary body (Figure 3.3). Morphologic relationships among the anterior segment

structures alter in response to a variety of physiologic stimuli (ie, accommodative targets

and light); therefore, maintaining a constant testing environment is critical for cross-

sectional and longitudinal comparison.

Figure3.3: Normal Angle: Cornea (C), Sclera (S), Anterior Chamber (AC), Posterior

chamber (PC), Iris (I), Ciliary body (CB), Lens capsule (LC), Lens (L) and Scleral spur

(black arrow)

In the normal eye, the iris has a roughly planar configuration with slight anterior bowing,

and the anterior chamber angle is wide and clear. The scleral spur is the key landmark to

interpret UBM images in terms of the morphologic status of the anterior chamber angle

and is the key for analyzing angle pathology. The scleral spur is located at the junction of

the trabecular meshwork and the interface line between the sclera and ciliary body.

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Glaucoma and UBM:

Angle-closure: Angle closure is caused by iris apposition to the trabecular meshwork. It

can be caused change in sizes, positions of anterior segment structures or by abnormal

forces in the posterior segment that can cause pupillary block and plateau iris

configuration. Differentiating the affected sites is the important in deciding the mode of

treatment.

Occludable angle: Narrow angles closing on provocative environment like dim

illumination.

Figure 3.4a: Narrow angle, Figure 3.4b: Closed angle (in dim illumination)

Pupillary block: Pupillary block is caused by the iridolenticular contact resisting

aqueous flow from the posterior to the anterior chamber and anterior bowing of iris.

Figure 3.5: Pupillary block (indicated by arrows)

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Plateau iris: A plateau iris configuration can be caused by large/anteriorly positioned

ciliary body or short iris root.

Figure 3.6: Plateau iris: T sign (best observed by indentation technique)

Open-angle glaucoma: Pigment dispersion syndrome is a type of open angle glaucoma

caused by mechanical friction of posterior iris surface on zonules causing reverse

pupillary block. UBM typically shows concave iris and increased iridolenticular contact.

Other open angle types may not be typically seen in a UBM.

Figure 3.7: Pigment dispersion syndrome

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Abnormalities of iris and ciliary body and UBM: UBM is helpful in differentiating solid

and cystic lesions of the iris and ciliary body in addition to quantifying the size.

Figure 3.8a: Iris cyst; Figure 3.8b: Ciliary body cyst

Lens and UBM:

The intactness of the zonules, the optic and haptic locations of an intraocular lens can be

assessed accurately by UBM.

Figure 3.9: Haptic location in IOL (Arrow)

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Anterior Segment Optical Coherence Tomography(ASOCT)

The anterior segment OCT (Figure 3.10) is non-contact, non-invasive imaging technique

that acquires and analyzes cross-sectional tomograms of the anterior eye segment

(cornea, anterior chamber, iris and the central portion of the lens) in vivo.

Figure 3.10: Anterior Segment OCT

Principle: It works on low-coherence interferometry to obtain high-resolution images.

Low-coherence interferometry involves measuring the interference between the reference

and the reflected beams of infrared light. This wavelength, limits the penetration depth to

the anterior segment. Multiple A Scans are reconstructed to form a B-Scan like image.

Procedure: Appropriate anterior segment protocol is selected and the patient fixates at

the fixation target. After aligning the instrument at the X, Y and Z axis the instrument

acquires tomographic images of the anterior segment on click of joystick. It is important

to review the scan for reliability.

Glaucoma and ASOCT: For assessing the eye for glaucoma high resolution and quadrant

scan protocols of the ASOCT is selected. The anterior segment metrics can be

quantitatively assessed and an indirect estimate for risk of glaucoma can be obtained

using ASOCT.

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The quantitative parameters (figure 3.11) that help in diagnosis of glaucoma are the

anterior chamber depth (ACD), the angle to angle distance (ATA) and the anterior

chamber angle (Figure 3.12). The anterior segment parameters can be compared on

subsequent visits which help in analysis of progression.

Figure 3.11: Horizontal line indicates ATA and vertical line indicates ACD

Figure 3.12: Anterior chamber angle

Structural changes in the anterior chamber like iris cyst (Figure 3.13) and the patency of

the peripheral iridectomy (Figure 3.14) can also be assessed with ASOCT.

Figure 3.13: Iris cyst

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Figure 3.14 Peripheral Iridectomy

Cornea and ASOCT: Tomography of various corneal disorders can be assessed with

ASOCT. The high resolution corneal image and pachymetry protocol is chosen.

Following are the disease groups that can be assessed effectively using the ASOCT.

Corneal ectasia: The differential pachymetry map (Figure 3.15) provides a quantitative estimation of the zone of thinning. However, the tracing of the

corneal contour should also be considered to check for reliability.

Figure 3.15: Pachymetry - ASOCT

Post refractive/lamellar surgery: The ASOCT has a flap tool that helps in assessing the thickness of the LASIK flap or partial lamellar surgeries. The flap

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tool can be placed at various points in cornea and the flap/lamellar thickness,

bed/host tissue thickness, pachymetry at the point and the location can be

obtained (Figure 3.16).

Figure 3.16: Flap tool analysis

Corneal haze/scar: A hazy cornea appears as hyper-reflective zone in ASOCT. The depth and extent of the scar can be measured using a caliper (Figure 3.17

a&b)

Figure 3.17 a: Corneal haze

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Figure 3.17b: Center Corneal Scar-Hyperreflective in OCT color

Lens and ASOCT: The anterior segment OCT can be used to find the location of the

haptics and optics like the UBM. ASOCT can also be used to find the tilt of the intra

ocular lenses (Figure 3.18). While, the live image of the posterior lens capsule can be the

same cannot be acquired hence, ASOCT is not useful in assessing the posterior lens

surface.

Figure 3.18: Tilted IOL

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Scheimpflug technique

Like the ASOCT, scheimpflug technique is a non invasive technique to measure and

image the anterior segment in vivo. The resolution of a scheimpflug technique is lower

compared to the ASOCT. Oculus Pentacam (Figure 3.19) is a diagnostic based on

scheimpflug technique.

Figure 3.19 Pentacam

Principle: The scheimpflug uses two rotating camera to image the anterior segment in

three planes. These images cut at one point and are reconstructed to obtain a three

dimensional image of greater depth of focus.

Procedure: Appropriate scan protocol is chosen and the patient is instructed to fixate at

the red dot (fixation target). On aligning the camera with the center of the cornea in the

three dimensional axes, the scan process starts automatically.

Glaucoma and Scheimpflug technique: For assessing the characteristics of the anterior

chamber, 50 scans are taken per second. In addition to ACD, anterior chamber angle,

angle to angle measurements are possible with pentacam, volume of the anterior chamber

(Figure 3.20) from the posterior corneal surface can be obtained. A decreased anterior

chamber volume is indicative of a shallow anterior chamber. It is also possible to obtain a

corrective factor for measured IOP based on corneal contour and thickness.

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Figure 3.20 Anterior chamber parameters

Cornea and Scheimpflug technique: Using scheimpflug technique a variety of corneal

parameters can be obtained. The most important are: Corneal topography, corneal height

data or elevations for the anterior and posterior corneal surface from a refernce sphere

(Figure 3.21a&b), pachymetry and B Scan like images for densitometric assessment.

Figure 3.21a: Anterior corneal elevation Figure 3.21b: Posterior corneal elevation

For corneal analysis, 25 images are acquired per second. Comparison of the acquired

images between various follow ups is also possible with this instrumentation. Various

corneal conditions that can be assessed with scheimpflug technique are

Corneal ectasia: Steepened corneal curvature, less pachymetry and high positive elevations (Figure 3.21) are features of corneal ectasia in Pentacam. Depending

on the type of ectasia, the relationship in location of the three parameters change.

Keratoconus has a thinning, steepening and increased elevation in the same zone.

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Whereas in PMD the thinning and increased elevation is noted below the region

of thinning.

Corneal haze/scar: In addition to measuring the depth and size of the scar, objective measurement of the density of the scar can be analyzed using this

technique (Figure 3.22).

Figure 3.22: Corneal Densitometry

Lens and Scheimpflug technique: Objective assessment of cataract density change with

subsequent visits is possible with Pentacam. Figure 3.23 shows densitometric analysis of

a cataractous lens.

Figure 3.23: Cataract densitometry

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Corneal topography

Corneal topography is the technique of imaging the corneal shape contour. This

technique is otherwise called as videokeratography or photokeratoscopy.

Principle: The widely used principle for imaging the corneal contour is the Placidos

principle. The cornea is treated as a reflective mirror and a series of concentric rings are

projected. The deviation in size between the projected image and the reflected image

helps in calculation of the corneal curvature at each point.

Procedure: The patient is instructed to fixate at the fixation target (green dot). The

instrument center and the center of the central mires are aligned and focused in the X, Y

and Z axes. On click of the joystick the CCD camera acquires the image for processing.

Qualitative topographic assessment: The color coding of the topography is an

important qualitative factor. A steeper zone is given by warm colors (reddish) and flatter

zones are given by cool colors (bluish). Also, quantitative parameters displayed in green

represent a normal range; yellow indicates suspect and red indicates abnormal values.

The shape of the placido-mires is also important qualitative factor. The mires in a

kertaoconic cornea are crowded in the paracentral zone (Figure 3.26b). In PMD the mires

are oval/egg shaped (Figure 3.26c). In post refractive surgery the mires are far spaced

(Figure 3.26e) and any corneal irregularity distorts the regularity of the mires also.

Quantitative topographic parameters:

Simulated Keratometry (SimK): Corneal curvature in central 3mm (Figure 3.24). Surface regularity index (SRI) and surface asymmetry index (SAI): Quantifiers of

local abnormalities in corneal shape contour (Figure 3.24).

Figure3.24 Indices

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Keratoconus screening: Based on parameters that quantify the asymmetry in corneal contour, the probability of the given topographic pattern to be keratoconic

is given (Figure 3.25)

Figure 3.25: Keratoconus screening

Some typical topographic patterns:

Figure 3.26a: Astigmatism: The mires are elongated along the axis of steepening. Shows

symmetric bow tie corresponding to the type of astigmatism.

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Figure 3.26b: Keratoconus: Shows asymmetric paracentral or infero temporal steepening

in early stages. Increased area of steepening noted with progression

Figure 3.26c: Pellucid Marginal degeneration: Typical PMD shows a Butterfly or Bird

Peck pattern of steepening

Figure 3.26d: Terriens marginal degeneration: Shows T shaped pattern of steepening

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Figure 3.26e: Post myopic refractive surgery: Amount of flattening corresponds to the

refractive error corrected; should always be interpreted with pre operative topographic

pattern. It is important to look for centartion and extent of ablation.

Slit Scanning

Corneal tomographic and topographic information can also be obtained with Orbscan

which works on the principle of slit scanning. In this method the anterior corneal

topography is obtained with a placidos principle and the tomographic information like

the pachymetry and elevations are simulated. Figure 3.27 shows a typical Orbscan report

that contains topographic and tomographic information.

Figure 3.27: Orbscan analysis

However, the anterior segment OCT and the scheimpflug technique are the reliable

methods of obtaining corneal tomographic information.

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Pachymetry

Corneal pachymetry is the technique of measuring corneal thickness (Figure 3.28).

Principle: Ultrasound pachymetry uses high-frequency sound waves of 1640m/s to

detect the epithelial and endothelial layers, both of which are highly reflective surfaces.

Knowing the velocity of sound in corneal tissue, the distance between the two reflecting

surfaces can be calculated by detecting the time lapse between reflected sound waves

from the 2 surfaces.

Procedure: The patient is comfortably seated and topical anaesthetic is instilled.

The probe tip is now placed perpendicular on the cornea (Figure 3.29).

Measurement is initiated on indentation. The measurement is repeated and the

average of the ten measurements is considered.

t

90o

t

90o

Figure 3.28: Ultrasound pachymeter Figure 3.29: Probe placed perpendicularly

Corneal thickness is an important criterion for assessing the risk of postoperative

corneal decompression and for determining the appropriate surgical approach.

Sequential corneal pachymetry is used to document the resolution of corneal

disease or surgery affecting corneal thickness. The conditions in which

pachymetry is indicated are:

Corneal ectasia: Ectatic cornea has reduced corneal thickness. However, the zone of thinnest pachy changes with each condition In keratoconus, there is

central or paracentral corneal thinning (Figure 3.30a) while keratoglobus has

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overall corneal thinning (Figure 3.30b). In conditions like Pellucid marginal

degeneration and Terreins marginal degeneration, inferior (Figure 3.30c) and

superior corneal thinning (Figure 3.30d) may be noticed respectively.

Figure 3.30a: Keratoconus Figure 3.30b: Keratoglobus

Figure 3.30c: PMD Figure 3.30d: TMD

Corneal dystrophies: Corneal dystrophies usually have increased corneal thickness corresponding to compromised endothelial function. In Fuchs

endothelial dystrophy associated with epithelial edema, the corneal thickness

measure is higher than in cases of Macular corneal dystrophy, wherein the

thickness is reduced.

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Corneal decompensation: In cases of Bullous keratopathy, resulting in decompensation of the cornea, the thickness is generally increased, as a result of

corneal edema.

Glaucoma and Pachymetry:

The intraocular pressure (IOP) measurements are highly influenced by corneal thickness.

IOP is overestimated in thicker cornea and actual IOP may be underestimated in patients

with low pachymetry. The measurement of the central corneal thickness and

correspondingly correcting the measured IOP value is an important step in managing a

patient with high IOP.

Corneal Thickness-Contact Vs Non-Contact:

The corneal thickness measured using a contact technique like ultrasound pachymetry is

usually lesser than that obtained with non contact pachymetry given by the scheimpflug

and ASOCT by 10 to 20 microns. It is therefore important that in follow-ups, the

thickness be assessed with techniques using similar principle.

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Specular microscopy

The corneal specular microscope is a reflected-light microscope that projects light onto

the cornea and images the light reflected from an optical interface of the corneal tissue,

most typically the interface between the corneal endothelium and the aqueous humor. A

normal corneal endothelium is a single layer of uniform hexagonal cells.

Principle: When the angle of incidence and the angle of reflection is equal, the incident

light is partially reflected onto the photomicroscope which captures the magnified image

of the endothelium. It is therefore, difficult to image the endothelium of an edematous

cornea which causes scattering of the reflected light.

Procedure: The patient is seated comfortably and is instructed to look at the green

fixation light. The region of cornea that is to me imaged is selected and the image is

captured after appropriate focusing. The acquired image is analyzed by clicking at the

center of 100 subsequent cells.

Qualitative Morphometric Analysis of Specular Images: Qualitative cellular analysis

identifies abnormal endothelial structures and grades the endothelium either according to

the number or size of the abnormal structures present or on the basis of an overall visual

assessment of endothelial appearance. Guttata is a gap between cells (Figure 3.31a),

polymegathic cells (Figure 3.31b) appear larger and pleomorphic cells are not

hexagonal(Figure 3.31c).

Figure 3.31a Guttata Figure 3.31b: Polymegathism Figure 3.31c: Pleomorphism

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Quantitative Morphometric Analysis of Specular Images: Cell size (cell area or cell

density along with standard deviation), coefficient of variation of mean cell area, percent

of hexagonal cells. The normal ranges of the above parameters in an adult are given in

table 3.2.

Table 3.2: Quantitative parameters of Specular rmicroscopy

Parameter Normal Value

Cell Density (sq mm) 1500-2000

Percent of hexagonal cells >60

Coefficient of variation

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Summary:

Diagnostics help in screening of sub-clinical disease, quantification and confirmation of the disease and for assessing progression in follow up

Variability of parameters to be considered while assessing progression Slit lamp biomicroscopy is better than a bad imaging

Section I_Anterior Segment AssessmentSection II_Anterior Segment AssessmentSection III_Anterior Segment Assessment

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