INTRODUCTION
Conditions requiring investigations can be grouped as:
- Infectious keratitis
- Dry eye diseases
- Keratoplasty work-up
DIAGNOSIS IN INFECTIOUS KERATITIS
Corneal ulcer is an ocular emergency, which can lead to serious sight-threatening complications. Empirical treatment with broad spectrum antibiotic has to be started immediately after thorough clinical evaluation and after procuring the samples for microbiological evaluation. These microbiological investigations help in making accurate etiological diagnosis and in the modification of therapy in presence of non-improvement with the initial treatment.
Types of Samples
- Eyelid, corneal and conjunctival swabs
- Corneal scrapings
- Corneal biopsy
- Anterior chamber tap
Sample Collection Gadgets
The various sample collection gadgets, which are available and can be used are the platinum spatula, 26-gauge needle, Bard Parker blade no 57, hypodermic needle, surgical blade no 15 and calcium alginate swab.
Corneal Scraping
Corneal scraping is performed under topical anesthesia. The anesthetic agent, which is preferred is 0.5% proparacaine, as it is least bacteriostatic as compared to other anesthetic agents, such as tetracaine and xylocaine. General anesthesia and sedation may be required in children, uncooperative adults or mentally impaired patients. A lid speculum may be applied gently to separate the lids taking care not to cause undue pressure on the eyeball. Any mucous or debris on and around the ulcer is carefully cleaned with a sterile swab stick. After that the leading edges and base of the ulcer are scraped using a Kimura spatula or a Bard Parker knife. Multiple scrapings must be obtained to enhance the yield of the organisms. The material is gently transferred on to the glass slide. At least four slides are prepared. One for Gram staining, second for Giemsa staining, third for KOH wet preparation and fourth for viral antigen detection.
Transport of Sample
The single scraping sample obtained may either be transported in a liquid transport culture medium (indirect method) or inoculation of the multiple scrapes may be done directly onto the agar plates, which are preferred.
Corneal Biopsy
In certain cases of deep mycotic keratitis and intrastromal abscesses, a diagnostic superficial keratectomy or corneal biopsy may be necessary to harvest microbe-infested tissue to make an accurate microbiological diagnosis. The procedure is performed under an operating microscope under topical anesthesia. A dermatologic 2–3 mm trephine or a small Elliot microtrephine is advanced into the anterior corneal stroma, to incorporate both infected and clinically normal tissue. Subsequently a crescent blade or Bard Parker knife is used to undermine the tissue, which may then be cut with a surgical blade or microscissors.
Microbiological Investigations
Potassium Hydroxide Wet Mount Preparation
Ten percent KOH mount examined by conventional microscope is a useful test in helping identification of fungi and acanthamoeba. The test has high sensitivity (92%) and a high specificity (96%) and it can be performed in an outpatient area.4
Gram Staining
Gram's stain identifies the organism correctly in up to 75% of the cases caused by a single organism and in 37% cases of polymicrobial keratitis. Overall, Gram's stain is accurate in approximately 61% of cases of bacterial keratitis.
Giemsa Staining
This stain differentiates bacteria from fungi, and also identifies chlamydia inclusion bodies and cysts and trophozoites of acanthamoeba species.
Special Stains
Ziehl-Neelsen acid-fast staining procedure identifies mycobacteria, actinomyces and nocardia.
Fluorochromatic Stains
Fluorochromatic stains, such as acridine-orange and calcofluor-white require the use of an epifluorescence microscope to visualize the organisms and the cells.
Culture Specimens
The corneal scrapings are routinely inoculated onto blood agar plate, chocolate agar plate, Sabouraud's dextrose agar plate (if fungus is suspected) and anaerobic media (if anaerobes are suspected). Routine culture media for various organisms are shown in Table 1.1.
Serological Investigations
These techniques (e.g. polymerase chain reaction) detect whether deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) from a particular organism is present, but do not detect the viability of the organism. The advantages of polymerase chain reaction (PCR) include greater speed than culture methods (up to 4 hours) and the ability to analyze specimens far from where they are collected.
Role of Corneal Staining in Infectious Keratitis
Role of corneal staining is to determine the state of the corneal epithelium, the technique of corneal staining with a vital dye is employed, by which lesions often minute and invisible to the naked eye are dramatically accentuated in vivid colors. Three dyes are usually employed.
- Fluorescein is the most useful to delineate areas denuded of epithelium (abrasions, multiple erosions and ulcers), which stain a brilliant yellowish green, when examined with cobalt blue light.
- Rose bengal stains diseased and devitalized cells red.
- Alcian blue stains the mucus selectively and delineates excess mucus produced when there is a deficiency in tear formation.
DIAGNOSIS IN DRY EYE
Dry eye disease is a chronic inflammatory condition of the eye in which the precorneal film gets altered in function due to the dysfunction of tear volume or tear quality; alone or both leading to a complex symptomatology. The incidence varies from 8 to 30.5% in different countries. Various tests may be employed to help in the diagnosis of dry eye (Table 1.2).
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Tests of Tear Stability
Invasive Tear Break-up Time (Fluorescein Break-up Time)
This test requires observing the cornea using a slit-lamp biomicroscope, with a broad beam cobalt-blue light source set at, say, 10X magnification. To view the tear film, fluorescein dye is instilled. The patient is asked to refrain from blinking and in most cases within 60 seconds dark spots or streaks will form within the tear film. These discontinuities in fluorescence indicate breaks in the continuity of the tear film. The time elapsing between a complete blink and the appearance of the first ‘dark spot or streak’ is measured and taken to be the ‘break-up time’. Five successive measures are routinely taken and the mean value is calculated (Fig. 1.1).
Disadvantages
- There is a destabilizing effect of fluorescein on the tear film itself.
- The volume of fluorescein added is uncontrolled and relatively large in comparison with the natural tear reservoir.
- Contact with the ocular surface initiates some reflex lacrimation.
Noninvasive Tests of Tear Film Stability
The fundamental principle common to these techniques are based on the reflective properties of smooth and stable tear film. As the tear film distorts (as it thins), its ability to reflect, undistorted, a regular optical array or pattern diminishes. Noninvasive tests of tear stability are based on observing the quality and stability of the first Purkinje image.
Corneal topographers based on the Placido disc can also be used to assess tear stability.
A device not reliant on the first Purkinje image is the Keeler Tearscope, an instrument which provides a wide field, specularly reflected view of the anterior surface, using a diffuse hemispheric light source. By measuring the time between a blink and the appearance of the first discontinuity in the lipid layer, the noninvasive break-up time can be measured.
Invasive Test for Tear Volume
It estimates for the volume of tears covering the ocular surface range from 2.74+–2.0 ml to 7 ml. The bulk of this volume is made up of fluid secreted by the main (primary) and secondary lacrimal glands.
Schirmer Test
Schirmer-1 test
This test is carried out with Whartmann-41 paper 5 mm × 35 mm whose end is bent to adjust on the lid. It is placed between inner 2/3rd and outer 1/3rd lid for 5 minutes. The room should have controlled humidity and temperature. In original test, the eye was kept open. However it can be kept closed. The inferences drawn from wetting of filter paper are:
- If wetting < 3 mm = Very severe dry eyes
- If wetting 3–5 mm = Severe dry eyes
- If wetting 5–10 mm = Moderate dry eyes
- If wetting is 10 mm = Mild dry eyes
- If wetting is >10 mm = Normal
Schirmer-2 test
This test is carried out to note wetting of the filter paper after local anesthesia of conjunctival sac. This is carried out to note reading of basal tear function (Fig. 1.2).
Disadvantages
- Poor reproducibility.
- It is time consuming.
- It is irritating.
- It has poor diagnostic value especially when attempting to investigate the marginal dry eye.
- Type of paper and environmental factors affect the results.
Phenol Red Impregnated Cotton Thread Test
Cotton can soak up tear fluid by capillary action. The cotton thread is dyed with a pH sensitive phenol red, which changes from yellow-orange to red-orange on contact with tears. This is useful for quickly checking the length of wetted thread. The thread end is put in the lower fornix and readings are taken after two minutes (120 seconds). The wetting of thread is noted:
- Normal = >15 mm wetting (15–24 mm)
- Abnormal = <15 mm wetting: Aqueous deficiency.
The volume of tears taken up by the thread depends on the exact type of cotton and the duration of insertion.
The soft thin cotton is less irritating compared with the relatively stiffer Schirmer paper strip and more likely to infer basal tear volume.
Slit-lamp Fluorophotometry
Fluorophotometry is a laboratory-based system used to measure tear flow and turnover rates. A controlled measure of fluorescein is instilled in the eye and the fluorescence is gauged over time. The rate of decay in fluorescence indicates tear flow and turnover. By extrapolation, it is possible to predict the tear volume at the moment of fluorescein instillation.
Three microliter of 0.5% fluorescein solution was applied to cornea in untouched fashion. The ocular surface is washed after 10 minutes. After 20 minutes, corneal fluorescein is measured and converted into fluorescein concentration or is matched with standard fluorescein solution.
Interpretation
Grade-0 = | No superficial punctum corneal stain. |
Grade-1 = | No severe superficial punctate keratitis at the center of cornea. |
Grade-2 = | Mild superficial punctate keratitis at the center of cornea. |
Grade-3 = | Severe superficial punctate keratitis at the center of cornea. |
Fluorescein uptake: Normal = 22.4±16.9 ng/ml | |
Grade-1 = | 96.4±51.2 ng/ml |
Grade-2 = | 318.6±146.0 ng/ml |
Grade-3 = | 1479.1±671.9 ng/ml |
Noninvasive Test for Tear Volume
Tear Meniscus Height and Curvature
The tear meniscus is bound between the oculae surface, lid margin and the air. The surface exposed to air is concave and cylindrical. The distance from the lid margin to the boundary between the ocular surface and the edge of the tear rivulus is the tear meniscus height. It is claimed that 75 to 90% of the total fluid covering the ocular surface is contained within the upper and lower tear menisci. It follows that the height and curvature of either the lower or upper tear meniscus is proportional to the tear volume. In clinical practice tear meniscus height can be measured quickly and reliably at a magnification of 30 or more using a graduated eye piece (Table 1.3 and Fig. 1.3)
Ocular protection index
It is the tear break-up time in seconds divided by inter blink interval in seconds.
Inference. | Ocular protection index <1 = Patient at risk |
Ocular protection index >1 = Not at risk |
The inter blink time can only be accurately measured with a camera interfaced with a computer. Furthermore, measurement of tear meniscus height is a useful noninvasive technique for investigating not only the dry eye, but also the patient complaining of occasional epiphora. If the tear meniscus height is constantly high there may be a partial blockage of the nasolacrimal drainage system that requires treatment.
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Assessment of Tear Quality
Slit Lamp
- The slit lamp is ideal tool to investigate ocular surface cellular damage using vital stains such as fluorescein, lissamine green or rose bengal.
Fluorescein stain test: The conjunctiva and cornea is examined for staining under blue filter.
No staining | = Grade-0 |
1/3 | = Grade-1 |
2/3 | = Grade-2 |
3/3 | = Grade-3 |
Lissamine green stain or rose bengal test: It stains the cells, which are not covered by albumin or tears (Figs 1.4 and 1.5). A wet strip of filter paper dipped in Lissamine green stain is touched to the lower tarsal conjunctiva. The patients are asked to blink few times. The slit-lamp examination with 10x magnification using a yellow filter is carried out between 30 seconds to 2 minutes of the installation of dye and observation is noted. It is graded on the Oxford score. Each quadrant is graded from 0–4 and the total score is averaged.
- 0= No stain;
- 1= Mild staining dots;
- 2=Staining dots multiple;
- 3=Confined staining area;
- 4=Very big patches of staining.
If cornea is stained, it is further graded into 0–4 scores as in conjunctiva.
- Debris in the tears.
- Meibomian openings and oil droplets at the lid margins.
- Lashes for general state of hygiene, health and signs of inflammation.
- Contact lens, surface quality, movement and postlens debris.
Nonroutine Tests in Dry Eye
Meibomiometry
The meibomian gland can be observed by appropriate lid eversion and retro-illumination using a suitable light source. This is a useful way of recording the overall quality and gross morphology of the glands. The meibomian oil droplets seen at the orifices of the gland can be harvested using thin strips of grease-proof paper and either the oil could be assayed or the area of the meibomian impressions could be measured. The area of the impression is an indication of lipid volume.
Lactoplate Test
The lysozyme and lactoferrin are the dominant proteins secreted by the lacrimal glands. These proteins protect the ocular surface by virtue of their antibacterial properties. In lacrimal gland dysfunction, the concentration of tear proteins is reduced. The Lactoplate test is a simple test for lactoferrin content.
Refractometry
Refractometery could prove to be a rapid objective indicator of lacrimal function by indicating protein concentration. Lactoferrin concentration in normal tear sample averages at 1.64 (s.d. + 0.47) mg/ml and by exploration, it is estimated that tear refractive index reduces by 0.00095 units for 1 mg/ml fall in lactoferrin concentration.
Tear Ferning
When a tear sample is placed on a glass plate and allowed to dry out, the solid content of the sample precipitates forming an arborizing pattern. Dry eyes have tear ferning patterns different from normal because of reduced protein content. Monitoring tear ferning patterns could be useful in assessing the effects of treatment on dry eyes.
Osmolality
Osmolality can be determined by measuring the freezing point of minute (0.3 micro L) tear samples using a nanoliter 8osmometer. When lacrimal function is reduced, the osmolality of tear samples taken from the tear menisci increases from a normal level of <312 mOsm/kg to >320 mOsm/kg.
Laboratory Diagnosis
Impression Cytology
Impression cytology, a noninvasive or minimally invasive biopsy technique samples the superficial layers of the conjunctival and corneal epithelium. Impression cytology has become a useful research tool in both basic and clinical aspects for sampling ocular surface epithelium (Fig. 1.6).
Technique of Specimen Collection
Imprints from the surface of the bulbar and palpebral conjunctive are obtained using absorbent filter papers of different types. The widely accepted filter papers are those with pore size ranging from 0.025 µm and 0.45 µm, though 0.22 µm pore size renders the best results. They are pressed onto the ocular surface for 3–5 seconds with the aid of a solid rod and pealed off from the ocular surface. The back of the paper is marked before applying on to the surface for easy identification of the surface to be stained later.
Specimen Staining Technique
The commonly used stains for the routine histological staining of impression cytology specimens are the Papanicolaou or hematoxylin and periodic acid-Schiff (PAS) stains.
Applications of impression cytology
- Dry eye syndrome
- Ocular surface squamous neoplasia (OSSN)
- Vitamin A deficiency
- Diagnosis of limbal deficiency
- Detection of microorganism
- Monitoring effects of topical medication
- Other applications
Impression cytology technique has also demonstrated changes in the ocular surface in cases of chronic conjunctivitis or pterygium. In cases of systemic illnesses, such as diabetes, peripheral neuropathy, sick building syndrome, chronic renal failure, anorexia nervosa and radiation therapy, the utility of impression cytology has been reported in literature.
Corneal Aesthesiometer
It is concerned with the measuring of threshold of cornea to pressure stimuli with the help of a wire.
Nelson has done a cytological classification system of dry eye that is shown in Table 1.4.
DIAGNOSTIC MODALITIES IN KERATOPLASTY WORK-UP
Specular Microscopy
Optical Principles of Specular Microscopy
It is an epi-illuminated microscope that projects a slit of light onto the posterior corneal surface at nearly normal incidence. Most of this light is transmitted into aqueous humor. However, a small fraction of this light, 0.22% is reflected from the aqueous humor–endothelial cell interfaces back into cornea. The specularly reflected light is collected by the objective lens of photomicroscope and when the instrument is focused on posterior corneal surface, this light forms an image of corneal endothelium. The image may be viewed directly and photographed (Fig. 1.7).
According to Laing, if the beam is narrowed sufficiently four zones of reflection can be seen:
Zone 1 is the brightest region and is formed by the interfaces formed by the lens, coupling fluid and epithelium.
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Zone 2 is a larger region and represents light reflected from the stroma.
Zone 3 is the endothelial region.
Zone 4 represents light reflected from the aqueous humor.
Methods of Evaluation of Corneal Endothelium
Qualitative analysis: The normal specular micrograph should demonstrate a regular endothelia mosaic of hexagonal cells of approximately the same size. Cell boundaries should be well-defined.
Quantitative analysis: Quantitative analysis of a specular photomicrograph is the objective description of the attributes of a selected cluster of individual endothelial cells from a specular photomicrograph. Examples include cell density (measured as cells/mm2) mean cell area (measured as mm2/cell), and pleomorphism (usually measured as percentage of 3, 4, 5, 6, 7 or 8 sided cells).
Morphometric analysis: The variation in individual cell area (polymegathism) and cell shape, i.e. cells with a different number of sides (pleomorphism), may provide a better estimate of endothelial cell integrity and function.
Clinical Indications for Specular Microscopy
- Early diagnosis of Fuchs' endothelial dystrophy
- In certain eyes before cataract surgery
- Previous trauma
- Pseudoexfoliation
- Recurrent uveitis
- Corneal edema in contralateral eyes
- Clear graft with operable cataract
- Glaucomatous eye with cataract
- Subluxated lens or choosing the intraocular lens
- Posterior polymorphous dystrophy
- ICE syndrome, congenital glaucoma
- Use of various types and designs of intraocular lenses
- Effect of various irrigating solutions and intracameral products used during cataract on endothelium
- Different techniques of cataract surgery, related instruments and endothelial response
- Evaluation of donor endothelium
- Various refractive surgical procedures like LASIK, LASEK and their term effect over corneal endothelium
- Contact lenses and phakic intraocular lenses
Advantages
- Operator independent
- Noninvasive
- Simultaneous measurement of cell count.
Disadvantages
- Time consuming
- Less reproducible than ultrasonic and ultrasound biomicroscopic pachymetric measurements.
- Impractical for use in operation rooms.
- Clinical use is limited to corneas that are free of edema, scarring, deposits or opacities that may distort light transmission.
Eye Bank Specular Microscopy
Evaluation of corneal tissue can be performed on whole globes as well as corneas stored in tissue culture media. Cell counts required for transplant surgery should be at least 2,000–2,500 cells/mm2.
Eye bank specular microscope has a built-in high resolution camera that gives high quality images of donor cornea with a built-in cell analysis system. Instrument's XYZ and rocking platform mechanism makes tracking of the endothelial cell easy. It also has a built-in pachymeter.
Anterior Segment Optical Coherence Tomography and its Application in Corneal Diseases
Optical coherence tomography is a cross-sectional, three-dimensional and high resolution imaging modality that uses low coherence interferometry to achieve axial resolution in the range of 3–20 micrometers. It is a completely noninvasive technique. As it uses interferometry for depth resolution it can have a long working distance and a wide field of transverse scanning.
The primary limitation of optical coherence tomography imaging of the anterior segment is speed and penetration. The OCT systems used in commercial retinal scanner thus far have used 830 nm wavelength, with image acquisition time of 1 to 5 seconds. Recently ophthalmic optical coherence tomography system in 1310 nm wavelength with an acquisition time of 3.3 seconds have been introduced, which allowed deeper 10penetration. Cross-sectional imaging of the anterior chamber, including visualization of the angle is possible.
Applications in Cornea
- Laser assisted in-situ keratomileusis and other refractive surgeries by directly measuring corneal flap thickness intraoperatively, provide the comprehensive pachymetry map of the entire cornea.
- Anterior chamber width and other biometric parameters— Optical coherence tomography is a promising method for accurate anterior chamber depth owing to its high spatial resolution and can improve sulcus-supported phakic intraocular lens sizing also.
- Anterior segment optical coherence tomography can also be of use in delineating the anterior segment anatomy in case of opaque corneas, failed grafts, adherent leucomas and help in the surgical planning prior to keratoplasty (Fig. 1.8).
Advantages
- Noncontact
- Rapid acquisition during the pachymetry scan ensures an accurate and repeatable pachymetry map
- High resolution
- Measures through corneal opacity.
Confocal Microscopy
The confocal microscope exploits the pinhole effect. Minsky carefully placed two pinholes, the first before the condenser, which focuses the light rays into the tissue, and the second before the eyepiece or camera, which focuses the reflected light rays into an image. Computer technology permitted three-dimensional reconstructions of the images. Overall, this new technology improved lateral and axial resolution to 1–6 and 4–15 mm, respectively, and increased magnification up to 600 times.
Indications of Confocal Microscopy
- Amiodarone, amyloid, chloroquine, ciprofloxacin, gold and iron-induced keratopathy
- Epithelial abnormalities like in Thygeson keratitis, Meesmann corneal dystrophy and Salzmann's nodular degeneration
- Dry eyes
- Stromal dystrophies-lattice dystrophy, Reis-Bücklers dystrophy, granular dystrophy, Schnyder crystalline corneal dystrophy and cornea farinata
- Keratoconus
- Cornea guttata or Fuchs endothelial dystrophy
- Primary congenital glaucoma with megalocornea.
- Role in corneal infections to visualize fungus, acanthamoeba, bacteria microsporidium and adenovirus
- Role in refractive surgeries to examine the LASIK flap interface.
Advantages
- It is noninvasive
- It allows observation of corneal structure at high magnification; and it allows visualization of keratocytes and corneal nerve fibers
- It offers moderate to good repeatability.
Disadvantages
- Poor agreement between confocal microscopy and ultrasound pachymetry; the latter apparently overestimating corneal thickness
- Slower data acquisition
- Poor penetration in corneal opacity.
Corneal Topography
Corneal topography or computerized videokeratography evolved from the need to measure corneal curvature and topography more comprehensively and accurately than keratometry. The purpose of computer-assisted corneal topography is to provide both qualitative and quantitative information about the corneal surface.
Projection Device Systems
Three types of projection device systems are currently used to measure corneal topography, and they are categorized as Placido based, elevation based, slit scanning and interferometric. All of them are capable of measuring and analyzing more than 8,000 points on the corneal surface.
Scanning Slit Imaging
The Orbscan uses a scanning slit-beam and direct stereotriangulation to measure the anterior corneal surface. During the 1.5 second examination, the patient fixes on a light source, whose reflex is aligned with the instrument axis. A “tracking system” (software image registration) attempts to minimize the influence of involuntary eye movement during the 1.5 second examination.
11Formats for display of data: The interpretation of color contour maps is based on the recognition of the following regions:
- Hot colors: Red, orange and yellow (steep zones of the cornea)
- Intermediate colors: Green
- Cool colors: Blue (flatter zones of the cornea)
- Corneal power map (Axial/sagittal)
This is a 24 color representation of dioptric power at various points on the cornea. The radius of curvature is measured 360 times for each placido disc image, from center to vertex.
Numerical (Curvature) map: The corneal curvature of different areas on the cornea is shown in dioptric values.
Keratometric map: It depicts the two principal meridia (K1 and K2) of the cornea at 3 different zones, i.e. central 3 mm zone, intermediate 3 to 5 mm zone and the peripheral 5 to 7 mm zone.
Profile map: The profile map plots the steepest and flattest meridians of the cornea along with the difference in the two.
Tangential map: This gives a better geographical representation of the cornea.
Corneal topography in preoperative and postoperative cataract surgery through map is shown in Figures 1.9A and B.
Current Applications of Corneal Topography
- Screening patients before refractive surgery
- Diagnosis of irregular astigmatism and corneal warpage
- Diagnosis of early keratoconus.
- Evaluating postoperative changes in corneal shape after refractive surgery
- After cataract surgery, corneal topography has been used to understand the effect of cataract incision placement and size
- After penetrating keratoplasty
- An early application of corneal topography involved patients with high astigmatism who required relaxing incisions
- Surgical planning
- Corneal topography is used in planning astigmatic surgery. Symmetrical and asymmetrical bow tie pattern with against the rule and with the rule astigmatism can easily be picked up. Irregular astigmatism and asymmetric astigmatism can be identified.
Pachymetry
Pachymetry (Greek words: Pachos = thick + metry = to measure) is term used for the measurement of corneal thickness. It is an important indicator of health status of the cornea especially of corneal endothelial pump function. It also measures corneal rigidity and consequently has an impact on the accuracy of intraocular pressure measurement by applanation tonometry.
Corneal Thickness in Normal Eyes
The normal corneal thickness varies from central to peripheral limbus. It ranges from 0.7 to 0.9 mm at the limbus and varies between 0.49 mm and 0.56 mm at the center. The central corneal thickness (CCT) reading of 0.7 mm or more is indicative of endothelial decompensation. Peripheral corneal thickness is asymmetric so that temporal cornea is thinnest followed by the inferior cornea
Indications
- Glaucoma— for applying correction factor in actual intraocular pressure determination
- Congenital glaucoma— to assess the amount of corneal edema.
- Refractive surgeries— (a) preoperative screening and (b) treatment plan of keratorefractive procedures like LASIK, astigmatic keratotomy, and previously even prior to radial keratotomy
- Postoperative follow-up of keratoplasty patients to determine endothelial cell function and its recovery and to become alert to early graft decompensation
- Contact lens—to assess corneal edema and in orthokeratology
- Assessing the thinness of the cornea as in corneal disorders like Terrien's and Pellucid marginal degenerations, keratoconus, keratoglobus and post LASIK ectasia
- Other cases if corneal decompensation— for monitoring and evaluating corneal edema and endothelial function as in herpetic endothelitis.
Correction Factor
To get a correct IOP reading, many correction factors have been reported by various researchers. It is recommended that in chronic eye diseases like glaucoma and glaucoma suspects for every increase in central corneal thickness of 50 µm, the correction done is to decrease the recorded IOP by 2.5 mm Hg.
Techniques of Pachymetric Measurements
There are two types of pachymetric techniques:
Spot measurements: These technologies include traditional optical pachymetry, specular and confocal microscopy, ultrasound pachymetry, and optical low-coherence reflectometry.
Wide area mapping: These provide the capability to map a wide area of the cornea. Pachymetric mapping technologies include slit-scanning optical pachymetry and very high–frequency ultrasound imaging.
Methods of Measurements
- Ultrasonic techniques:
- Conventional ultrasonic pachymetry
- Ultrasound biomicroscopy
- Optical techniques:
- Manual optical pachymetry
- Specular microscopy
- Scanning slit technology
- Optical coherence tomography
- Optical low coherence interferometry
- Confocal microscopy
- Laser doppler interferometry
- Alternative measurements:
- Pentacam
- Pachycam
- Ocular response analyzer
Ultrasonic Pachymetry
This is the most commonly used method these days and is regarded as the gold standard.
Principle
The ultrasonic pachymetry measurements depend on the reflection of ultrasonic waves from the anterior and posterior corneal surfaces. It is the measurement of the time difference (transit time) between echoes of ultrasonic signal pulses from the transducer of the probe and the reflected signal from the front and back surface of the cornea to the transducer.
Corneal thickness is calculated by following simple formula: Corneal thickness = (Transit time × Propagation velocity) / 2 The sound velocity through normal cornea is taken as 1640 m/sec.
Disadvantages
- Contact method
- Accuracy is dependent on the perpendicularity of the probe's application to the cornea.
- Reproducibility relies on precise probe placement on the center of the cornea.
- Difficult to control the patients gaze during repeated measurements, so that the placement of the probe is difficult to reproduce. There is variable sound speed in wet and dry tissues.
- Low resolution
- Not accurate in edematous corneas.
Advantages
- Fast
- Simpler, therefore easier for paramedical staff to use
- Requires minimal observer judgment
- Portable
- Dry (no coupling medium required)
- Can be used intraoperatively.
Ultrasound Biomicroscopy
Ultrasound biomicroscopy is a high resolution ultrasound machine, which images the anterior segment of the eye. It has got a 12.5–50 MHz probe so that the depth of penetration is lesser (4 mm) than conventional ultrasound. It gives real-time images of anterior segment (Fig. 1.10).
Corneal thickness can be measured by the caliper incorporated in the machine or through the ultrasound biomicroscopy software after the acquisition of images.
Advantages
- Anterior segment examination (high resolution) can be carried out along with measurement of corneal thickness.
- Especially useful in cases where cornea is opaque.
- Various layers of cornea can be identified.
Disadvantages
- The main drawback of ultrasound biomicroscopy imaging is the inconvenient requirement of immersing the eye in a coupling fluid.
- Contact method
- Requirement for the patient to lie supine during the examination
- The device cannot be used intraoperatively
- Difficult to standardize.
Slit-Scanning Pachymetry
Principle
It measures anterior and posterior corneal elevations by comparing it to a best fit sphere. Pachymetry is done by calculating the difference between the elevation of anterior and posterior corneal surface.
Advantages
- It gives wide field pachymetry that is measurement across the entire cornea
- It also identifies the thinnest point in the cornea, both by value and location helping in the diagnosis of keratoconus.
- Corneal alignment is not required
- It can be used to calculate ablation depth and optical zones in corneal refractive surgeries.
Disadvantages
- It overestimates corneal thickness by 5%. The main drawback of Orbscan is the tendency to underestimate corneal thickness in Keratoconic, post-PRK and post-LASIK eyes
- This is not fast enough for pachymetric mapping because of motion artifacts in the measurements
- The Orbscan system shows decreased accuracy in measuring corneal thickness when clinically significant haze is present.
Pentacam
Pentacam analyses the complete anterior segment, corneal topography, the quantification of lens density, anterior chamber, angle measurements, and utility to monitor new therapeutic modalities like collagen crosslinking treatment for Keratoconus.
Principle
The Pentacam is also based on the true elevation measurement and images the anterior segment (cornea + lens) of the eye by a rotating Scheimpflug camera measurement, which supplies pictures in three dimensions.
Advantages
- Noninvasive, noncontact
- Even minute eye movements are captured and corrected simultaneously
- It gives precise representation and repeatability
- The high quality of the Scheimpflug image allows preoperative and postoperative monitoring as in the case of an intraocular contact lens.
Disadvantages
- It underestimates the corneal thickness in comparison to ultrasonic pachymetry.
Pachycam
The OCULUS Pachycam is compact and portable noncontact pachymeter with built-in keratometer. It can be mounted on slit lamp. It automatically corrects the intraocular pressure in accordance with various correction tables to obtain the “real” IOP. Image acquisition is done with the help of a 3-D alignment screen.
Principle
It is also based on Scheimpflug principle of the horizontal 4 mm cut image, which is evaluated and represented. It also gives central k-values as well as the local K-readings on the 4 mm cut.
Advantages
- Noncontact
- Portable and lightweight.