Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_103_библиотеки_им_акад_М_И_Перельмана
.pdf
178
https://t.me/med1917
A. Kate and S. Basu
chamber using the van Herick grading
system can help screen patients with
angle closure disease [15].
(e) Iris: Focal illumination using a slit beam
of varying width helps evaluate the iris
pattern and color. Changes in the contour
may indicate a local mass lesion, which
will not be visible unless a thin slit is
used. Similarly, a very high magnication
is needed to rule out the presence of neovascularization around the pupillary area.
Retro-illumination can identify focal
areas of thinning of the iris tissue, which
will transmit the reected light.
(f) Lens examination uses an optical section
where the illumination angle is usually
greater than the one used for examining
the cornea. Similar to the examination of
the cornea, it is imperative to rst examine with a wider slit to detect the presence
of small alterations such as glaucomecken, early posterior subcapsular cataracts, etc. Retro-illumination is perhaps a
more sensitive technique for detecting
early focal lenticular changes; it must be
employed during routine lens examination with the pupil dilated.
(g) Anterior vitreous: Examination of the
anterior vitreous is similar to the examination of the anterior chamber. It is done
using direct illumination with a moderately wide slit beam. Unlike the anterior
chamber, the vitreous cavity is not
optically clear and has a strand-like
appearance that moves with the eyeball's
movement. Using a conical light beam
helps identify the presence of oating
cells, dispersed vitreous hemorrhage, etc.
14.7 Slit Lamps Accessories
This section will discuss different accessories.
When used, the utility of the slit lamp can extend
beyond the examination of the anterior segment
and the vitreous cavity.
1. Goldmann applanation tonometer: This
instrument is attached either on the side of the
slit lamp or superiorly and is swiveled into
place when required. It consists of a prismholding unit, a sensor arm, and a metal house
for weights [16, 17]. The prism-holding unit
is shaped as a truncated cone with a surface
area of 7.35mm2 and a diameter of 3.06mm
[16, 17]. It houses a doubling prism which
breaks the circular meniscus and needs
realignment to measure the intraocular pressure (IOP). The Goldmann applanation
tonometer is the current gold standard for
measuring IOP (see Chap. 17 for further
details).
2. Lenses: Several handheld contact and noncontact lenses can augment slit lamp examination. These include lenses for viewing
angles, such as the Goldman, Posner, and
Sussman lenses. Fundus evaluation with a slit
lamp biomicroscope requires a +90D or +78D
lens. The distance between the lens and the
eye is indirectly proportional to the lens
power; thus, a greater distance is required for
lenses with lower powers [18]. Ensuring
proper cleaning of these lenses is vital to prevent contamination and cross infections [19].
3. Photography and videography: This is done
using cameras mounted on the slit lamp or
with digital compact cameras [20]. The following adjustments are made in the slit lamp
to permit photo-videography:
(a) A beam splitter is required to divide the
light traversing back from the patient's
eye so that half of it travels to the examiner’s viewing system and the other half is
captured by the photography system.
Alternatively, a reex mirror system can
divert all the light to the system.
(b) A crosshair graticule is incorporated in
one of the eyepieces, which allows proper
centration of the area of photographic
interest.
(c) Alongside the slit beam, a diffuse light
source is also necessary during slit lamp
examinations. This additional light source
14 Slit Lamp Biomicroscopy
https://t.me/med1917
179
should have an intensity approximately
2–3 times dimmer than the slit beam. This
diffuse light provides a softer illumination that complements the focused slit
beam, aiding in the observation of subtle
details and enhancing the overall visibility of ocular structures.
(d) The camera should be mounted in a man-
ner that it does not interfere with the
optics, the observer's movement, or the
illumination arm.
(e) The slit lamp is also equipped with an
electronic ash feature, specically
designed with minimal exposure unaffected by eye movement. This feature
ensures that the ash duration is extremely
short, minimizing the risk of motion artifacts and allowing for precise and accurate imaging of the ocular structures.
(f) With the advent of smartphones, it may
now be possible to do away with these
elaborate systems, which are difcult to
maintain and are expensive. Various
adapters designed for smartphones enable
obtaining high-resolution photographs
conveniently during slit lamp examinations [21, 22].
4. Lasers: The integration of lasers into slit
lamps expands the scope of these instruments
from diagnostics to therapeutics. Discussing
the indications, techniques, and uses of slit
lamp lasers is beyond the scope of this chapter. Briey, these can be used for treating several conditions, including posterior capsule
opacication through capsulotomy, angle closure disease through iridotomy, macular
edema, and neovascularization through retinal
laser procedures, among others [23–26].
more convenient for clinicians to transport
and maneuver the slit lamp within clinical
settings [27, 28]. It is also advantageous in
operating rooms to examine patients under
anesthesia and people in the supine position
or immobile for various reasons. A handheld
slit lamp proves particularly valuable when
evaluating children who may not cooperate
with a standard slit lamp examination.
Additionally, its portability facilitates eye
examinations in mass eye screenings, such
as in school screenings [27].
(ii) Incorporation of software within the pho-
tography systems has resulted in “functional
slit lamp examination.” This technique
offers an in vivo method of assessing the
hemodynamics of the conjunctiva by providing the blood ow rate, velocity, vessel
size, etc. [29] These parameters currently
provide an insight into contact lens tting
and dry eye disease, and this may soon
expand to include other diseases as well.
(iii) Slit lamps have also been adapted to optical
coherence tomography (OCT) machines to
enable instant OCT imaging of different
ocular structures [30]. Quantitative assessment of the meibomian glands can also be
performed with slit lamp adaptors [31].
Similar integration of additional diagnostic
modalities, such as automated refractometry,
pachymetry, tomography, or optical biometry
into the slit lamp platform, holds the promise of
comprehensive evaluations within a compact
setup. The ability to conduct these investigations
with a single instrument permits a more seamless
workow and facilitates rapid decision-making
in clinical practice.
14.8 Recent Advances andFuture
Directions
There are several new developments in slit lamps.
Some of these are as follows:
(i) Compact and portable: By reducing the size
and weight of the instrument, it becomes
14.9 Conclusion
The usefulness of the slit lamp for obtaining highresolution images and adjustable illumination and
magnication has revolutionized the evaluation of
the anterior structures of the eye. It allows for
identifying subtle abnormalities, precisely measuring structures, and monitoring changes over
180
https://t.me/med1917
A. Kate and S. Basu
time. Incorporating handheld lenses further
enhance these capabilities, enabling detailed
viewing of angle structures and the posterior segment. This aids in identifying pathologies, contributing to better treatment decisions and
improved patient outcomes. With newer innovations, the instrument is becoming more compact,
allowing a comprehensive range of functionality.
Eventually, performing all basic diagnostic investigations with one multipurpose instrument may
become possible. By expanding its utility to
encompass areas beyond diagnostics, the slit lamp
continues to be a cornerstone instrument in ophthalmology, empowering practitioners to provide
comprehensive care and deliver improved ocular
health outcomes for their patients.
Acknowledgments The authors are grateful to Drs.
Simmy Chaudhary, Anasua Kapoor, Shilpa Tarani, and
Rashmi Deshmukh for sharing their clinical photographs
for publication.
Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
Disclosure None.
References
1. Yuan J, Jiang H, Mao X, et al. Slit-lamp photography and videography with high magnications. Eye
Contact Lens. 2015;41(6):391–7.
2. Mazurak M, Kusa J.Jan Evangelista Purkinje: a passion for discovery. Tex Heart Inst J. 2018;45(1):23–6.
3. Gellrich M.The slit lamp. 3rd ed. Berlin: Springer;
2013.
4. Muirhead JF.Adolph Barkan (1845-1935), European
ophthalmologist in San Francisco. JAMA Ophthalmol.
2014;132(3):346–9.
5. Ehinger B, Grzybowski A. Allvar Gullstrand
(1862–1930)—the gentleman with the lamp. Acta
Ophthalmol. 2011;89(8):701–8.
6. Gloor BRP. Hans Goldmann (1899-1991). Eur J
Ophthalmol. 2010;20(1):1–11.
7. Littmann H. A new slit lamp apparatus. Am J
Ophthalmol. 1950;33(12):1863–70.
8. Vogt A.Atlas of the slitlamp-microscopy of the living
eye. Surv Ophthalmol. 1962;7:316–21.
9. Sanderson J. Fundamentals of microscopy. Curr
Protoc Mouse Biol. 2020;10(2):e76.
10. Srinivas SP, Rao SK. Ocular surface staining: current concepts and techniques. Indian J Ophthalmol.
2023;71(4):1080–9.
11. Kaur K, Gurnani B. Slit-lamp biomicroscope. In:
StatPearls [Internet]. Treasure Island (FL): StatPearls
Publishing; 2023. [cited 2023 Jul 1]. http://www.ncbi.
nlm.nih.gov/books/NBK587440/.
12. Martin R.Cornea and anterior eye assessment with
slit lamp biomicroscopy, specular microscopy, confocal microscopy, and ultrasound biomicroscopy. Indian
J Ophthalmol. 2018;66(2):195–201.
13. Fritz B, Paschko E, Young W, et al. Comprehensive
compositional analysis of the slit lamp bacteriota.
Front Cell Infect Microbiol. 2021;11:745653.
14. Oshika T, Mori M, Araie M. A new approach to
the study of aqueous humor dynamics by measuring the intensity of Tyndall’s effect. J Fr Ophtalmol.
1990;13(10):471–80.
15. Baskaran M, Oen FTS, Chan YH, etal. Comparison
of the scanning peripheral anterior chamber depth
analyzer and the modied van Herick grading system
in the assessment of angle closure. Ophthalmology.
2007;114(3):501–6.
16. Zeppieri M, Gurnani B.Applanation tonometry. In:
StatPearls [Internet]. Treasure Island (FL): StatPearls
Publishing; 2023. [cited 2023 Jun 30]. http://www.
ncbi.nlm.nih.gov/books/NBK582132/.
17. Brusini P, Salvetat ML, Zeppieri M.How to measure
intraocular pressure: an updated review of various
tonometers. J Clin Med. 2021;10(17):3860.
18. Gellrich MM. The fundus slit lamp. Springerplus.
2015;4:56.
19. Sobolewska B, Buhl M, Liese J, Ziemssen F. Slit
lamps and lenses: a potential source of nosocomial
infections? Eye (Lond). 2018;32(6):1021–7.
20. Honavar S, Sen M.Beauty and the beam: Slit-lamp
photography essentials [Internet] [cited 2023 Jun 30].
https://ijoreports.in/article.asp?issn=2772- 3070;year
=2022;volume=2;issue=3;spage=643;epage=646;aul
ast=Honavar
21. Ludwig CA, Murthy SI, Pappuru RR, etal. A novel
smartphone ophthalmic imaging adapter: user
feasibility studies in Hyderabad, India. Indian J
Ophthalmol. 2016;64(3):191–200.
22. Muth DR, Blaser F, Foa N, etal. Smartphone slit lamp
imaging-usability and quality assessment. Diagnostics
(Basel). 2023;13(3):423.
23. Modena DAO, Miranda ACG, Grecco C, et al.
Efcacy and safety of ND:YAG 1064 nm lasers for
photoepilation: a systematic review. Lasers Med Sci.
2020;35(4):797–806.
24. He M, Jiang Y, Huang S, et al. Laser peripheral
iridotomy for the prevention of angle closure: a
single- Centre, randomised controlled trial. Lancet.
2019;393(10181):1609–18.
25. Saha BC, Kumari R, Sinha BP, et al. Lasers
in glaucoma: an overview. Int Ophthalmol.
2021;41(3):1111–28.
14 Slit Lamp Biomicroscopy
https://t.me/med1917
181
26. Yadav NK, Jayadev C, Rajendran A, Nagpal
M.Recent developments in retinal lasers and delivery
systems. Indian J Ophthalmol. 2014;62(1):50–4.
27. Hu S, Wu H, Luan X, et al. Portable handheld
slit- lamp based on a smartphone camera for
cataract screening. J Ophthalmol. 2020;2020:
1037689.
28. Yazu H, Shimizu E, Sato S, et al. Clinical observation of allergic conjunctival diseases with portable
and recordable slit-lamp device. Diagnostics (Basel).
2021;11(3):535.
29. Shu X, Wang J, Hu L. A review of functional
slit lamp biomicroscopy. Eye Vis (Lond). 2019;
6:15.
30. Mueller M, Schulz-Wackerbarth C, Steven P, et al.
Slit-lamp-adapted fourier-domain OCT for anterior and posterior segments: preliminary results and
comparison to time-domain OCT. Curr Eye Res.
2010;35(8):722–32.
31. Arita R, Itoh K, Maeda S, etal. A newly developed
noninvasive and mobile pen-shaped meibography system. Cornea. 2013;32(3):242–7.
Corneal Topography
https://t.me/med1917
15
RashmiDeshmukh andSayanBasu
15.1 Introduction
The cornea is the anterior transparent optical element of the eye. It plays a vital role in focusing
the rays of light on the retina. It contributes to
two-thirds of the total refractive power of the eye.
In Greek, “topos” means “place” and “graphien”
means “to write.” Topography relates to studying
a surface. Corneal topography was developed as
a technique to analyze the anterior surface of the
cornea. Traditionally, topography refers to a twodimensional representation of the corneal surface. In contrast, “tomos” means “section”;
corneal tomography is a three-dimensional representation of the cornea derived from the anterior
and the posterior surfaces of the cornea [1]. A
good understanding of the principles of corneal
topography and tomography and the ability to
interpret these images accurately is crucial to
diagnose corneal disorders such as keratoconus,
pellucid marginal degeneration, keratoglobus,
R. Deshmukh (*) · S. Basu
Shantilal Shanghvi Cornea Institute, Kallam Anji
Reddy Campus, L V Prasad Eye Institute,
Hyderabad, India
e-mail: sayanbasu@lvpei.org
regular or irregular astigmatism, or corneal ectasia following refractive surgery. With the recent
increase in the number of people opting for
refractive surgery, corneal topography has
become an indispensable tool for screening them
for their eligibility and determining the procedure of choice.
15.2 History
The rst measurements of the corneal shape were
reported by Scheiner in 1619. He used convex
mirrors of different curvatures next to the eye to
see the reections of window panes until he
found a mirror reecting an image of the same
size as the cornea [2]. Cuignet developed the rst
keratoscope to see the reected image from the
cornea, and the rst keratometer was developed
by Helmholtz in 1854. Using a keratometer made
it possible to measure the curvature of the central
3 mm of the cornea by assessing the distance
between two pairs of reected points. In 1882,
Placido developed a disc with alternating black
and white concentric rings with a convex lens at
the center of the disc to view the reected image.
This disc named after him (Placido disc) is still
widely used by reection-based topographers.
Using the Placido disc in keratometers made it
possible to analyze data from a large surface area
of the cornea. The following century did not see
much progress until the developments of contact
© 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_15
183
184
https://t.me/med1917
lens tting techniques made it necessary to get
information about the mid-peripheral curvature
of the cornea. Further developments in cataract
surgeries, keratoplasties, and corneal refractive
surgeries necessitated the measurements of the
curvature of a large area of the cornea.
Developments in videokeratoscopy made these
measurements possible. Further development of
the Scheimpug principle used the principle of
projection rather than reection to generate true
elevation data.
15.3 Principles andTechnology
15.3.1 Reection-Based Systems
Many topography systems use the principle of
reection-based imaging. The principle is to
direct the reected light from the corneal surface onto a two-dimensional image capture system. A reection-based topographer typically
has a Placido disc; it is illuminated and projected onto the cornea. The reected image from
the cornea is captured and analyzed to determine the shape of the cornea. Keratoscopes and
videokeratoscopes are examples of topographers based on this principle. The NIDEK-OPD
(Nidek Technologies, Gamagori, Japan) scan
combines a Placido disc with ray tracing aberrometry and measures anterior corneal curvature, refraction, and pupil size [3]. Most
Placido-based topographers use 22 rings with an
angular resolution of 2° and can be divided into
small and large cone topographers. Small cone
systems image more data points and are hence
more accurate, but it might be challenging to
image deep-set eyes with these systems [4].
There are some limitations to this technology.
The posterior cornea cannot be measured.
Anterior corneal measurements with the help of
reected mires might miss minor abnormalities.
A healthy ocular surface is a prerequisite for a
good image; hence, using this technology on people with ocular surface disorders is difcult.
Another major limitation is limited corneal coverage, as it obviates measurements from the paracentral and peripheral cornea (Fig.15.1).
R. Deshmukh and S. Basu
Fig. 15.1 Corneal topography using Placido disc technology shows poor peripheral cornea coverage
15.3.2 Slit Scanning System
Slit scanning topography combines the reection
of a Placido disc with the projection of a slit
beam, similar to a slit lamp biomicroscope [5].
The slit beam is moved across the cornea, getting
refracted at the anterior and posterior corneal surfaces. Ray tracing triangulation allows the mathematical analysis of the reected and refracted
rays of light to analyze the anterior and posterior
surfaces of the cornea. Simultaneous measurements of the anterior and posterior corneal surface enable global pachymetry measurement.
Orbscan I (Orbscan, Inc., Salt Lake City, UT,
USA) was the rst attempt at imaging the posterior corneal surface using a slit scanning system.
A Placido disc was incorporated in the Orbscan II
(Bausch & Lomb, Rochester, NY), and the latest
version, Orbscan IIz (Bausch & Lomb, Rochester,
NY), has an integrated Hartmann-Shack aberrometer [6].
15.3.3 Scheimpug Imaging
The Scheimpug principle describes the optical
imaging condition when the plane of an object is
not parallel to the lm of the camera; it has the
15 Corneal Topography
https://t.me/med1917
185
advantage of achieving a wide depth of focus. It
was described by Theodor Scheimpug, an
Austrian navy ofcer who noticed this phenomenon could correct the perspective distortion of
aerially acquired photographs. The Pentacam
(Oculus, Inc., Lynnwood, WA), Galilei (Ziemer,
Port, Switzerland), and Sirius (CSO, Italy) imaging devices use this principle for corneal tomography [7]. The devices use sub-pixel edge detectors
to precisely detect corneal and anterior chamber
edges. This technology allows the measurements
of anterior and posterior cornea, anterior chamber
angle and depth, and corneal and lenticular densitometry measurements (Fig.15.2) [8].
The Pentacam is the most popularly used
Schiempug system. It has a static camera in the
center to image the pupil center and control xation. A second rotating camera with a slit light
Fig. 15.2 A Scheimpug image showing the cornea,
anterior chamber, and lens imaging
source obtains slit images from 0° to 180°. The
analysis is then converted into detailed topography and thickness maps.
15.4 Color Coding Systems
The analyzed measurements represent keratometry, pachymetry, and elevation maps. These maps
show individual data points and are represented
using the Louisiana State University Color Coded
maps. In this scale, the warmer colors (orange
and red) represent steeper corneas, thinner areas,
and areas with a higher elevation than the reference surface. The cooler colors (violet and blue)
represent atter curvatures, thicker corneas, and
areas that are depressed compared to the reference surface. The greens and the yellows are seen
in normal corneas.
The color scale can further be absolute or normalized. An absolute scale has a xed color coding system with the same colors representing the
same dioptric steps for a particular instrument.
However, subtle changes are easily missed since
the dioptric steps are large. In contrast, in a normalized scale, the system identies the minimum
and maximum values for each map and automatically distributes the colors in a gradient manner.
Since the increment step assigned to each color is
smaller, this scale gives more topographic details
(Fig.15.3).
Fig. 15.3 Absolute (right) and normalized (left) scale of the same keratometry map
186
https://t.me/med1917
R. Deshmukh and S. Basu
15.5 Concept ofBest-Fit Sphere
The concept of the best-t sphere is a fundamental principle in corneal topography aimed at
understanding the baseline curvature of the cornea. The corneal surface is prolate, and its shape
is complex, varying across different regions. The
best-t sphere represents a smooth, symmetrical
sphere that minimizes the deviations or differences between the two when superimposed onto
the irregular corneal surface. It is a reference
point to analyze and quantify corneal curvature
changes accurately [9].
Elevation-based topography uses the best-t
sphere as a reference surface and plots the points
above or below the reference surface [10]. The
areas below the reference surface are represented
in cool colors, and the areas above the reference
surface are represented in warm colors.
15.6 Topography Maps
15.6.1 Keratometry Maps
Keratometry is the measurement of the corneal
radius of curvature. It is dened as the curvature
of the circle that touches the cornea at a given
point of interest. Keratometry maps essentially
provide details of the curvature of the cornea at a
given point. Consequently, these maps help
assess whether the cornea is regular or irregular
(as in corneal scars), steep or at and whether
there are any corneal ectatic disorders. Depending
on the method used for calculating the radius of
curvature, keratometry or curvature maps are displayed as either sagittal (axial) curvature maps or
tangential curvature maps. Normal keratometry
values range from 41D to 46D with a mean of
43D [11].
15.6.1.1 Axial Curvature Maps
In this type of map, the radius of curvature at a
given point on the corneal surface is measured
from a reference axis (optical axis). Due to this
common reference axis, small irregularities may
not be visible or “smoothened out” as they are
much smaller than the large corneal diameter.
These maps are calculated assuming that all the
rays of light striking the corneal surface are
refracted, forcing a focal point through the optical axis as a reference axis and assuming that the
center of rotation of the best-t sphere center lies
on the optical axis. It gives a good global view of
the cornea averaging out the values and reducing
noise. Axial curvature maps are particularly useful in classifying the corneal shape and determining the base curve of the contact lenses for tting.
However, details of minor irregularities may be
missed [12].
15.6.1.2 Tangential Maps
These maps more closely represent the actual
curvature of the cornea over the axial map. The
tangential radius of curvature is calculated at
each point with respect to its neighboring points
by tting the best-t sphere. It means that the tangents are projected at each point, and curvature is
measured along the tangent. This type of map
recognizes sharp power transitions more easily
than the axial map and is more sensitive to irregularities on the corneal surface. It is useful for
evaluating corneal shape, assessing refractive
surgery candidates, surgically induced changes,
and tting contact lenses [13].
15.6.2 Pachymetry Maps
These maps show the distribution of corneal
thickness at various points on the cornea. Normal
central corneal thickness ranges from 500 to
550 μm and gradually increases toward the
limbus. Most often, the thinnest point on the
cornea would be close to or almost coincide with
the corneal apex. Pachymetry maps are of
particular importance while screening for refractive surgery. A normal corneal thickness is a prerequisite for performing laser vision correction.
Displacement of the thinnest location from the
apex indicates ectasia, especially when accompanied by posterior elevation and keratometric
steepening at the corresponding points. A globally thin cornea is seen in the keratoglobus, while
a thick cornea might indicate endothelial
dysfunction.
15 Corneal Topography
https://t.me/med1917
187
15.6.3 Elevation Maps
The anterior and posterior elevation maps display
the areas in the anterior and posterior corneal surface above and below the reference surface. The
most used reference surface is the best-t sphere.
However, best-t ellipse and best-t toric ellipsoid can also be used. During screening for
refractive surgery, a best-t sphere to the central
8mm zone provides adequate data points to recognize subtle ectasias and astigmatism [14].
15.7 Interpretation ofMaps
A typical quad map consists of 4 maps: the keratometry map (usually axial keratometry),
pachymetry map, anterior elevation map, and
posterior elevation map. However, since the displays might differ between each machine, one
must read a map and look for the red ag signs
pointing toward ectasias.
15.7.1 Orbscan
A normal Orbscan quad map includes a keratometry map, pachymetry map, and anterior and posterior elevation maps with measurements
displayed at the center (Fig.15.4).
The Rousch and Efkarpides criteria give the
typical red ag signs on an Orbscan map.
15.7.1.1 Rousch Criteria [15]
(a) A thinnest pachymetry of <470μm.
(b) A difference of >100 μm from the thinnest
point to the values of the 7mm optic zone
implies a steep gradient of thinning from
mid-periphery to the thinnest point.
(c) The thinnest point on the cornea should cor-
respond with the highest point of elevation of
the posterior corneal surface. A posterior
high point should be >50μm above the bestt sphere on posterior elevation maps. A
best-t sphere of power >55D on the posterior prole is required.
Fig. 15.4 An Orbscan quad map with anterior elevation map (top left), posterior elevation map (top right), keratometry
map (bottom left), and pachymetry map (bottom right). The central box displays measurements of various parameters
188
https://t.me/med1917
R. Deshmukh and S. Basu
(d) A relative difference of >100 μm between
the highest and lowest point on the posterior
elevation map must be seen.
(e) Keratometric mean power map readings of
>46D and a lazy C on the axial power map
are suspect when the astigmatism shifts >20°
from a straight line.
(f) A change within the central 3mm optic zone
of the cornea of more than 3D from superior
to inferior can be correlated to the presence
of vertical coma (the commonest aberration
in keratoconus).
(g) Composite integrated information, which
includes the highest point on the posterior
elevation, coincides with the highest point on
the anterior elevation, the thinnest point on
pachymetry, and the point of steepest curvature on the power map, can be acquired.
Additionally, the Efkarpides criteria suggest
that ratio of the radii of the anterior and posterior
best-t spheres of the cornea should be >1.21. An
astigmatic discrepancy of >1.5D in the 3 mm
zone and a discrepancy of >2D in the 5mm zone
should be an alert sign.
15.7.2 Pentacam
A standard Pentacam map (Fig. 15.5) displays
certain numerical values on the left-hand side of
the map.
• Quality specication (QS): Species the
quality of the topographic capture and should
be displayed as “OK.” Otherwise, the
Pentacam software tends to extrapolate the
missing information leading to false readings
in the missed areas.
• K-readings: K1 indicates at keratometry,
K2 indicates steep keratometry. Consider at
K while treating myopia (should not be <34D
Fig. 15.5 Normal Pentacam display of a quad map with
axial keratometry map (top left), pachymetry map (bottom left), anterior elevation map (top right), and posterior
elevation map (bottom right). The rectangular display on
the left shows various parameters
Соседние файлы в папке Библиотека им академика М.И. Перельмана