Anterior Segment Assessment.pdf
Transcript of 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
Condition Image Slit lamp Sign
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
Condition Image Slit lamp Sign
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
Condition Image Slit lamp Sign
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
Condition Image Slit lamp Sign
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
Condition Image Slit lamp Sign
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
Condition Image Slit lamp Sign
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