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17 Intraocular Pressure
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209
17.10 Bioresonator Applanation
Resonance Tonometer (ART)
Technology—Eklund et al. (2003) proposed a
new tonometer based on the resonance principle.
It estimates IOP by a simultaneous continuous
sampling of the force needed to applanate the
cornea and the corresponding area of contact
[41]. It is commercially available as the
Bioresonator ART (Medical sensors and
Instruments, BioResonator AB, Sweden) in manual and automatic versions. In the manual
(BioResonator ART manual) version, the sensor
is pushed manually toward the cornea, and in the
automatic version (BioResonator ART servo),
this motion is servo-controlled(with a small
motor that controls the sensor movement). The
instrument gives the median of multiple IOP
measurements and a quality score that denotes
the standard deviation of the recordings.
A sensor and a transducer continuously measure the contact force along with the area of contact. A cylindrical piezoelectric element in the
resonance sensor has a known resonance frequency. When it comes in contact with the cornea,
the acoustic impedance changes the resonance
frequency with a shift in the frequency proportional to the area of contact between the sensor
and the cornea. The IOP is calculated using a
complex formula that considers the slope of the
relationship between the force and frequency in a
specic frequency shift with respect to the applanation area interval between 4.3 and 11.0mm2
[41]. The entire apparatus is slit-lamp mounted
like in GAT, or can be handheld. It warrants all
usage precautions that apply to GAT; i.e., local
anesthesia must be applied before IOP measurements can be taken, and the probe must be thoroughly sterilized before use.
The advantages of this technique are as follows: (a) there is no need for the uorescein dye,
(b) contact time with the cornea is less than that
for GAT (hence, the chances of corneal abrasion
damage are reduced) [42], (c) it is portable, and
(d) it is self-calibrated.
The disadvantages of this technique are: (a)
the patient needs to remain still during measure-
ment, (b) IOP may be overestimated at higher
ranges and if the alignment is off-center [42,
43], (c) it needs adequate surface moisture on
the cornea to get good acoustic impedance
(therefore, it is difcult to use in people with dry
eye and ocular surface disorders) [42]. In addition, ART is also affected by CCT and other corneal biomechanical factors [43].
17.11 Conclusion
There are numerous commercially available
tonometers, each with advantages and disadvantages. However, the IOP measurements of
these are not interchangeable. Despite several
recent developments and promising early
reports, none of the alternative tonometers have
gained widespread use in routine clinical practice. The Goldmann applanation tonometer
remains the gold standard despite some limitations in its use in certain clinical settings.
Although an ideal tonometer does not exist, it is
preferable to get consistent and reliable readings every time with the same type of equipment for each patient to diagnose and treat
people with glaucoma.
Funding Hyderabad Eye Research Foundation,
Hyderabad, India
Disclosure Ramyashri S: None; Aparna Rao: None;
Sardar M Khan: None.
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Gonioscopy
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ArnavPanigrahi , ShikhaGupta ,
andVineyGupta
18
18.1 Introduction
Gonioscopy is necessary when testing for glaucoma. It enables the visualization and magnication of the iridocorneal angle and helps identify
its conguration, thereby aiding a proper diagnosis. The primary purpose of this test is to distinguish between angle closure and open-angle
glaucoma and help diagnose the various open and
closed angle varieties of secondary glaucoma by
identifying the characteristics specic to these
conditions. Currently, intraoperative gonioscopy
is increasingly being used in angle-based surgeries. Although a simple technique, it takes years of
practice to master the art of gonioscopy. Hence,
practicing this technique as frequently as possible is advisable.
18.2 The Optics ofGonioscopy
andLenses Used
Gonioscopy entails the visualization of angle
structures. The main problem with the direct
visualization of angle structures is that it is difcult to ascertain the position of the angle with
respect to the eye due to the sudden rarecation
of the refractive index when the light rays pass
A. Panigrahi · S. Gupta · V. Gupta (*)
Dr. Rajendra Prasad Centre for Ophthalmic Sciences,
AIIMS, New Delhi, India
from the cornea (μ=1.33) into the air (μ=1.0).
Since the critical angle at the cornea–air interface
is about 46°, only the rays that are incident on the
cornea at an angle of <46° can emerge from the
anterior chamber into the air, allowing the direct
visualization of the angle. This is usually not possible in eyes with normal anatomy (Fig.18.1a)
but might be possible in pathological conditions
like distorted corneal contour, keratoconus, and
keratoglobus, where the angle structures may be
directly visible without any aid. In normal scenarios, this problem can be circumvented in two
ways. One entails the use of a thick concavoconvex lens placed on the cornea, with a power
higher than the corneal dioptric power, to prevent
the phenomenon of total internal reection; this
allows most of the rays to pass out of the cornea
following refraction (Fig. 18.1b) (the principle
behind direct gonioscopy). The second entails
using a plano-concave lens with a coupling agent
to nullify the difference in refractive index
between the two media, hence avoiding total
internal reection such that the emergent light
can then be brought onto the central frame using
plane mirrors placed at a specic angle
(Fig. 18.1c) (the principle behind indirect
gonioscopy).
The commonly used gonioscopes for slit-lamp
angle visualization utilize indirect optics. The
prototype for such a lens is the Goldmann single
mirror gonioscope. It consists of a plano-concave
lens at the base, the corneal interface, and a single
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_18
213
214
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A. Panigrahi et al.
Fig. 18.1 The schematic diagram shows the transmission
of light rays originating from the anterior chamber angle,
with and without the aid of a goniolens. In the natural
state, all the light rays undergo total internal reection,
and none emerges from the eye (left). A direct (middle) or
a
Fig. 18.2 Indirect gonioprisms commonly used for angle evaluation: (a) the Goldmann 2-mirror lens; (b) Goldmann
4-mirror lens; (c) Latina selective laser trabeculoplasty lens; and (d) Posner gonioprism with a handle
b
plane mirror placed within the body of the gonioscope at 62°, which reects the emergent light
rays out of the cornea almost perpendicular to the
lens. The base is larger than the corneal diameter
(12 mm for the Goldmann-type lens). These
lenses require a coupling agent to ensure homogenous contact between the lens and the cornea,
thus accounting for the sudden change in curvature at the corneoscleral junction. These lenses
are considered indirect as the visualization of the
structures occurs after reection by a plane mirror. The placement of the mirror is such that the
image obtained is reversed but not crossed. For
example, the inferior angle can be visualized by
ensuring superior placement of the mirror, but the
image on the right side of the mirror is still the
right side of the concerned angle. Other lenses
with similar builds are the Goldmann 2-mirror
(Fig.18.2a), 3-mirror, 4-mirror lenses (Fig.18.2b)
and the Allen Thorpe goniolens. The Goldmann
3-mirror lens has a mirror placed at 59° for the
indirect (right) goniolens allows the light rays to exit the
eye, allowing the observer to visualize the angle structures. The green lines indicate the placement of the
mirrors
c
d
gonioscopic visualization of the opposite angle.
The other two mirrors, placed at 67° and 73°, are
for visualizing the equatorial and peripheral retina, respectively. The Allen Thorpe lens uses 4
prisms instead of plane mirrors. The central lens
in each of these variants can be used to evaluate
the posterior pole. This allows for complete glaucoma evaluation, simultaneously assessing the
disc and the angle. It can also be used to stabilize
the globe, hence ensuring a more accurate examination, especially in eyes with nystagmus. The
Latina selective laser trabeculoplasty (SLT)
gonio laser lens (Fig.18.2c) is a modication of
a single mirror Goldmann-type goniolens specially designed to deliver laser light during laser
trabeculoplasty. It consists of a single plane mirror placed at an angulation of 63° and utilizes
indirect optics to visualize angle structures. It has
a 1× magnication; hence the size of the beam
emitted by the laser machine and that visualized
by the observer is the same. It may also
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215
incorporate a special lid ange that prevents the
lens from slipping due to the resistance caused by
squeezing the patient’s eyelids. A special modication includes the ve high-contrast bars imaged
over the trabecular meshwork (TM). The bars are
spaced at 400μm, providing a guide for delivering 10 laser spots, ve corresponding to and ve
between the aforementioned bars. The bar can be
rotated 10 times to complete a full circle around
the angle, allowing for the accurately spaced
delivery of 100 laser spots to the TM.
A modication to conventional indirect gonioscopy involves the use of goniolens with a base
smaller than the cornea. This allows these lenses
to use the precorneal tear lm as a coupling agent
by excluding the steep curvature change at the
corneoscleral junction from its contact surface.
This obviates the need for external coupling
agents like methylcellulose. The smaller diameter of these lenses also allows for diagnostic and
therapeutic maneuvers during gonioscopy.
Pressing the lens on the central cornea displaces
the aqueous humor from the central anterior
chamber (AC) toward the periphery, pushing the
iris root posteriorly and opening up the angle
structures. This maneuver, called “dynamic gonioscopy” or “indentation gonioscopy,” helps differentiate between appositional and synechial
closure. This maneuver can also be therapeutic as
it helps decongest the AC by partially opening up
the angle during an acute angle closure (AAC)
attack. The prototype for such a lens is the Zeiss
4 mirror goniolens. It uses a plano-concave lens
of 9 mm diameter as its base and 4 mirrors
inclined at 64° as the primary reecting surface.
All 4 quadrants are visible simultaneously, with a
rotation of merely 11° allowing for the areas
between the adjacent mirrors to be viewed. The
Posner (Fig.18.2d) and Sussman lens are other
prototypes used for indentation gonioscopy. A
comparison between the various types of indirect
gonioprisms is given in Table18.1.
Direct gonioscopy utilizes a thick convex lens
placed on the cornea to nullify the rarecation of
the refractive index when the light emerges from
the cornea into the air. This prevents total internal
reection and allows the direct visualization of
the angle. These may come with or without handles. Since a single lens is used, and there is no
provision of plane mirrors to make the emergent
light perpendicular to the cornea, the patient’s
head must be tilted away from the observer to
allow the emergent rays to reach the observer’s
cornea. These lenses can be used for diagnostic
purposes but are more commonly used as surgical goniolens for the intraoperative visualization
of angle structures during procedures like goniotomy and gonioscopic-assisted transluminal trabeculotomy (GATT). Koeppe lens (Fig. 18.3a)
and Swan Jacob lens (Fig.18.3b) are prototypes
for direct goniolenses. Other examples are the
Barkan lens, Thorpe lens, and Mori lens. The
Mori upright surgical goniolens incorporates a
two-mirror design that redirects the oblique
emergent rays to the coaxial position perpendicular to the cornea, hence obviating the need for
tilting the patient’s head during surgery. This
allows for 360° visualization of angle structures
while the head is supine, without requiring intraoperative maneuvers like head tilting.
Table 18.1 Comparison of indirect gonio prisms
Lens type
Diameter of corneal contact 12mm 12mm 9mm
Overall diameter 15mm 18m 9mm
Rim size 1.5mm 3mm None
Mirror angulation 62° 59° 64°
Mirror height 17mm 12mm 12mm
Distance from central cornea 3mm 7mm 5mm
Radius of curvature 7.4mm 7.4mm 7.85mm
Coupling uid Required Required Not required
Dynamic gonioscopy Manipulation Manipulation Indentation
Goldmann
Single mirror
Goldmann
Three mirrors
Zeiss
Four mirrors
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a
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Fig. 18.3 The direct
goniolenses commonly
used in routine
glaucoma practice: (a)
Koeppe lenses of
varying sizes and (b) a
Swan Jacob lens with a
handle
A. Panigrahi et al.
b
18.3 Techniques Used
inGonioscopy
If indentation gonioscopy is performed, gently
pressing the goniolens against the cornea opens
up the angle structures, allowing one to differen-
18.3.1 Indirect Lenses
tiate between appositional and synechial closure.
Contrary to traditional belief, similar maneuvers
There is no dened standardized technique for
performing indirect gonioscopy. Here is a simple
method for performing gonioscopy using an indirect handheld gonioprism without a handle. The
readers are advised to experiment with different
gazes and gonioprisms to nd the technique most
suitable for them.
Explain the procedure thoroughly to the
patient and take verbal consent. Anesthetize the
eye with 0.5% proparacaine eye drops. Ask the
patient to sit upright at the slit lamp, with the
forehead pressed rmly against the headrest. If a
Goldmann-type lens is used, put 2–3 drops of
coupling agent (generally methylcellulose) on
the corneal surface of the lens. Indentation gonio-
are also possible with the Goldmann-type goniolenses, although the process is far more complicated. To open up the angle visible in the superior
mirror, which is the inferior angle, the patient is
asked to look superiorly, and gentle pressure is
applied on the superior rim of the contact surface
of the goniolens. This maneuver displaces the
uid in the superior angle inferiorly, thereby
opening the inferior angle if appositional closure
is present. This “opening up” is visualized in the
superior mirror. Hence, to open the angle seen in
one of the mirrors of the Goldmann goniolens,
the patient is asked to look toward the direction
of the mirror, and the pressure is applied on the
sclera adjacent to the mirror.
lenses do not require coupling agents for performing gonioscopy. Ask the patient to look
down and gently elevate the upper eyelid. Dock
18.3.2 Direct Lenses
the corneal surface of the gonioscope onto the
inferior fornix and rmly appose it against the
cornea to prevent the formation of air bubbles in
the interface, which impedes clear visualization
of angle structures. Rotation of the lens and intermittent pressure on the goniolens against the cornea also helps remove air bubbles if present. Ask
the patient to look straight ahead and direct an
oblique beam of 3–4mm in length at an angle of
30°–45° onto the mirror of interest. Move the slit
lamp back and forth until the angle structures are
visible, and rotate the lens to visualize the whole
angle in the primary position.
Direct goniolenses are primarily used for intraoperative gonioscopy. The patient’s head is tilted
away from the observer, and the globe is pushed
toward the nasal bridge (to visualize the nasal
angle), with the observer seated in a temporal
position. A surgical visco-dispersive ophthalmic
viscoelastic device (OVD), like HPMC 2%, is
placed on the contact surface of the surgical goniolens. The lens is held with a handle and gently
placed on the nasal limbus without any undue
pressure not to induce any corneal folds. This
allows clear visualization of the nasal angle, the
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217
site of most angle surgeries, without apparent
image distortion.
Direct goniolenses, like the Koeppe lens, may
also come without a handle. They are held at the
equator between the thumb and index nger of
the right hand for right eye examination, the left
hand for the left eye, and the head tilted toward
the angle being visualized. The space between
the lens and the eye is lled with a coupling
agent. The light source is provided either by the
operating microscope or a handheld slit lamp.
18.4 Structures Seen
onGonioscopy
Figure 18.4 shows an open angle in a normal eye,
where all the angle structures are visible. From
anterior to posterior, the angle structures seen are
Schwalbe’s line (SL), anterior trabecular meshwork (ATM), posterior trabecular meshwork
(PTM), scleral spur (SS), and ciliary body band
(CBB). The condensation of collagen bers
forms the SL and marks the termination of
Descemet’s membrane (DM). Identication of
the SL is important because the labeling of the
angle structures begins from the SL and continues posteriorly toward the CBB.Hence, identifying the SL is crucial, especially in cases with
featureless angles, where none of the angle structures are seen. In these cases, the SL usually pro-
Fig. 18.4 Goniophotograph of a normal wide open
angle. Structures seen, from anterior to posterior, are
Schwalbe’s line (SL), anterior trabecular meshwork
(ATM), posterior trabecular meshwork (PTM), scleral
spur (SS), and ciliary body band (CBB)
vides a landmark from which the point labeling
of the angle structures can begin. The identication of the SL, although difcult in these cases,
can be aided by the “corneal wedge” technique as
follows: direct a narrow beam of light from the
slit lamp obliquely on the cornea; the beam is
split into an outer part (corresponding to the corneal epithelium) and an inner part (corresponding
to the corneal endothelium); a fusion of both
these parts marks the termination of the DM,
which is the landmark for the SL.
The trabecular meshwork (TM) can be divided
into two parts: the anteriorly placed nonfunctional anterior TM (ATM) and the posteriorly placed functional TM (PTM). ATM forms
the structural scaffold and has almost no outow
function owing to the lack of Schlemm’s canal
(SC) behind it. It is lightly pigmented or nonpigmented in appearance. Its posterior extent
marks the site of an incision during ab interno
angle surgeries like goniotomy and GATT.The
PTM, on the other hand, forms the site of maximum resistance to aqueous outow. Due to the
SC behind it, it marks the site of conventional
outow, also known as pressure-dependent outow or trabecular outow. It also indicates the
incision site for inserting minimally invasive
glaucoma drainage devices, like iStent and
Hydrus implants. It is relatively easier to identify
due to its pigmented nature. This might not be the
case in pediatric eyes, where pigmentation is
sparse, and a featureless angle may also be a possibility. The PTM in the superior angle usually
exhibits less pigmentation than in the inferior
angle. Any change in the intensity of pigmentation, or its distribution, may provide a clue to
underlying pathologies. Figure 18.5a shows a
densely pigmented TM in a patient with pigmentary glaucoma.
Another important landmark to identify in the
angle is the SS.It is a ridge formed by the condensation of the collagen tissue of the sclera, and
it is the site of attachment of longitudinal bers
of the ciliary muscle. Identifying the SS is relatively easy; the white band is immediately posterior to the PTM. Identifying the SS is very
important, as it allows us to differentiate between
open and closed angles. Due to the attachment of
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Fig. 18.5 Goniophotograph of a patient with (a) pigment dispersion syndrome, showing a densely pigmented trabecu-
lar meshwork of the superior angle; (b) angle recession, showing an irregularly widened ciliary body band
A. Panigrahi et al.
the ciliary muscle, contraction due to lightinduced miosis pulls the SS posteriorly, which
may artefactually open the angle. Hence, a preliminary examination of the angle structures is
advised under low illumination to avoid the
abovementioned phenomenon and falsely label
the angle as open. Occasionally, the scleral spur
might be covered by ne projections of uveal tissue known as the iris processes. These do not
cross the SS, are found in about one-third of
healthy individuals, and are not associated with
an increased risk of glaucoma.
The CBB forms the posterior-most part of the
angle and lies adjacent to the iris root. It is wide
in eyes with high myopia, trauma, posteriorly
subluxated lens, pseudophakia, or aphakia. It is
narrow in hyperopic eyes and those with an anterior insertion of the iris. A unilateral abnormally
widened CBB, especially with the history and
other signs and symptoms of ocular trauma, with
or without an associated cyclodialysis cleft
(Fig. 18.5b), is usually pathognomonic of posttraumatic angle recession.
cal and commonly used. Scheie [1] used a system
based on the visibility of angle structures, allotted with a higher number of narrower angles
(range: I–IV). Emphasizing the complexity of
angle structures, Spaeth [2, 3] proposed a system
incorporating multiple parameters limited to the
angle. These include iris insertion (range A–E),
iris conguration, iris angulation (range 10°–40°)
(Fig. 18.6), and pigmentation of PTM (range
0–4). Various combinations of these parameters
give a more accurate representation of the angle
[4]. Becker limited his classication to 2 important angle parameters: the width of the trabecular
zone (the space between the SL and SS, represented by numbers) and the distance between the
SS and iris root (represented by letters). ShafferKanski has devised the most clinically relevant
and practically useful system. It is a modication
of the system originally developed by Shaffer [5]
(Table18.2). It incorporates two parameters: iris
angulation and visibility of structures. Combining
both parameters can help the clinician estimate
the risk of angle closure. The readers are directed
to consult additional sources for a detailed discussion of these nomenclature systems.
18.5 Identifying Occludability
It is important to document the ndings clearly
and concisely, which, even if elaborate, must
Numerous systems have been devised to grade
the occludability of the angle based on the number of structures seen on the gonioscopy. The
nomenclature systems currently in clinical use
have been developed by Scheie, Spaeth, Becker,
and Shaffer-Kanski; the latter is the most practi-
impart as much information as possible and be
easily understandable by fellow ophthalmolo-
gists. This helps avoid repeat examinations and
hence saves time in clinics. A simple example of
a diagrammatic representation of gonioscopy
ndings is given in Fig.18.7. Divide the eye into
TM + SS
SS/CB
18 Gonioscopy
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Fig. 18.6 Varying degrees of angle recess and occludability as classied according to the Spaeth system: the
left and middle panels show occludable angle recesses of
Table 18.2 Classication and clinical application of
anterior chamber angle
Grade Angle width Description Risk of closure
4 45°–35° Wide-open Impossible
3 35°–20° Wide-open Impossible
2 20° Narrow Possible
1 <10° Extremely
narrow
Slit Slit Narrow to slit Probable
0 0° Closed Closed
Probable
SS
10° and 20°, respectively. The right panel shows a wide
open angle with a 30° angle recess
four quadrants using two oblique lines intersecting at an angle of 90°. The four quadrants correspond to the part of the eye being examined, as
seen by the examiner. The deepest part of the
angle seen under the baseline conditions in each
quadrant is marked. If an indentation or “manipulation” gonioscopy is performed, the quadrants
are designated with doubled-ended arrows, and
the part seen after the maneuver mentioned above
is also marked. Additional information regarding
the iris insertion, conguration, and angle pigmentation can be added per the Spaeth nomenclature. Special details, like PAS, cyclodialysis,
and new vessels, can be drawn at their location to
give a complete diagrammatic representation of
the angle.
18.6 Look Beyond What YouSee
PAS
Fig. 18.7 Schematic representation of angle ndings in
the right eye of a patient. The double-headed arrows represent a dynamic gonioscopy being performed. The furthest structure seen on the baseline condition and maximal
illumination and manipulation are marked for each corresponding quadrant. Any special ndings, like inferotemporal peripheral anterior synechiae (PAS) at 8 clock hours
in this gure, are also drawn
SS/CB
Although gonioscopy seems simple, mastering it
requires optimal vigilance and years of practice.
Gonioscopic ndings, on their own, may be of
limited importance; but in conjecture with the
relevant history and associated anterior segment
ndings, may provide a lot of information. Let’s
consider this example:
Case report: A 42-year-old male with prolifera-
tive diabetic retinopathy in both eyes was
referred to the glaucoma clinic because of
high IOP in both eyes. Anterior segment evaluation showed the presence of neovasculariza-
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