Ординатура / Офтальмология / Английские материалы / Moorfields Manual of Ophthalmology_Jackson_2007
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Optometrists
For blepharitis, recommend lid hygiene b.d. (p. 113) for 1 month and refer if symptoms persist.
The following guide to referral urgency is not prescriptive, as clinical situations vary.
Immediate
■Chemical injury: instil anaesthetic eyedrops (if available) then rinse immediately with clean neutral
liquid, e.g. sterile saline. Following copious irrigation |
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transfer all serious injuries to an eye unit by |
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ambulance. |
p. 139 |
■ Suspected gonococcal conjunctivitis |
p. 117 |
Same day |
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■ Suspected chlamydial conjunctivitis |
p. 122 |
■ Stevens-Johnson syndrome (systemic features may |
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require more urgent referral to physicians) |
p. 130 |
Urgent (within 1 week) |
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■ Nonresolving conjunctivitis |
p. 117 |
■ Viral conjunctivitis with corneal involvement |
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(keratoconjunctivitis) |
p. 119 |
■ Vernal keratoconjunctivitis |
p. 124 |
■ Ocular mucous membrane pemphigoid |
p. 133 |
Soon (within 1 month) |
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■ Pyogenic granuloma |
p. 35 |
■ Inflamed pingueculum |
p. 137 |
Routine |
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■ Blepharitis |
p. 113 |
■ Uninflamed pingueculum causing ocular discomfort |
p. 137 |
■ Superior limbic keratoconjunctivitis |
p. 136 |
■ Recurrent subconjunctival haemorrhage |
p. 135 |
DISEASE EYE EXTERNAL 4 Chapter
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Chapter 5
CORNEA
Anatomy
The normal cornea
■The air–tear interface is the most powerful refracting interface of the eye.
■The cornea is aspheric, with the central cornea steeper than the periphery, i.e. prolate.
■ The refractive power of the optical zone (central 4 mm) is ≈43 D.
■The average adult transverse corneal diameter is 11–12 mm, and 9–11 mm vertically.
■Corneal clarity depends upon an ordered lamellar collagen
structure and relative dehydration, requiring a minimum endothelial cell density of ≈1000 cells/mm2. The mean endothelial cell density falls by ≈0.6% per year.
■The central corneal thickness remains constant between the second and sixth decades but varies with the time of day
(thickest in the morning) and race (Afro-Caribbeans and Chinese ≈530 μm, Caucasians ≈550 μm at 60 years).
■Normal corneal innervation is essential for the maintenance of an intact epithelial surface.
■The stem cells that generate the epithelium are located at the junction between the cornea and the sclera, probably in the palisades of Vogt.
■The normal cornea is avascular.
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History and Examination
History
Ask about the presenting complaint, past ocular and medical history, medications, allergies, family, and social history. Document the presence or absence of the following key symptoms and determine their severity, nature, duration, and any relieving or exacerbating factors.
■Reduced vision.
■Abnormal sensation: direct stimulation of the corneal nerves usually gives rise to severe, sharp pain. Inflammation produces a duller, aching pain. Photophobia is a spasmodic pain on exposure to light caused by ciliary spasm. Irritation or grittiness is a symptom of epithelial disturbance. Itch usually indicates allergy. A diseased anaesthetic cornea may be asymptomatic.
■Tearing: multiple causes including corneal epithelial breakdown and stromal inflammation (see also p. 55).
■Red eye: ask if friends or family are affected, suggesting infection.
■Discharge: ask if this is purulent, mucoid, or ropey. Ask if friends or family are affected. Discharge is most often secondary to ocular surface infection or allergy.
■Contact lens: ask about contact lens wear including the type, cleaning routine, pattern of wear (i.e. daily or overnight), and duration of use (see also p. 207).
Examination
Corneal structure and function are dependent upon the lids, tears, conjunctiva, and sclera. Examine as follows, documenting relevant positive and negative findings.
■General habitus, hands and face: for example, rheumatoid hand changes may help diagnose the cause of peripheral corneal thinning.
■Vision: record uncorrected, best corrected, and pinhole VA. High-contrast Snellen VA may be maintained despite
CORNEA 5 Chapter
reduced contrast sensitivity from corneal opacity or |
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astigmatism. |
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examination |
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tears appear abnormal, and tear volume with Shirmer I test |
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Eyelids: note inflammation, abnormal contour, incomplete |
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closure, misdirected lashes, and lid margin disease |
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(blepharitis, meibomian gland plugging). |
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■ Tears: note the quality (discharge, debris) and quantity |
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(meniscus height). Measure tear break-up time (p. 158) if the |
and |
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(p. 147). |
■ Bulbar conjunctiva: note scarring (location, extent) and |
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History |
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subconjunctival haemorrhage. If inflamed, note depth |
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(conjunctival, episleral, scleral) and location (perilimbal, sectoral, diffuse, localized).
■ Tarsal conjunctiva: evert the upper and lower lids to look for keratinization, scarring, foreign bodies (FBs), and the presence of membranes, papillae, and follicles.
■ Corneal epithelium: note any defects (size, location, pattern, e.g. superficial punctate keratitis, dendritic, geographic), FBs, infiltrates (pattern, size, location, depth), oedema (extent and severity, e.g. bullae), deposits (location, pattern, material, e.g. iron, calcium, filaments).
■ Corneal stroma: record the size, location, depth, and pattern of opacities. Infiltrates tend to have feathery margins. Scars have defined edges on retroillumination and often result in corneal thinning. Areas of oedema have soft, diffuse edges on retroillumination and increased corneal thickness. Note the depth, number of clock hours, and distance of corneal new vessels from the limbus. Ghost vessels carry no blood and appear grey. Inactive vessels may have blood flowing through them but appear to have sharp edges on retroillumination. Active vessels have a cuff of oedema producing a less-defined edge.
■ Corneal endothelium and Descemet’s membrane: Note thickening, guttata, folds or breaks in Descemet’s membrane, and the extent and distribution of deposits (KPs, pigment, blood).
■ Drawings: draw any corneal abnormities using a frontal and slit view. Baseline photographs may help quantify changes seen at subsequent examinations.
Further clinical tests may be indicated based on the history and examination (see next page).
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Investigations
Consider the following tests and investigations, as indicated by the history and examination.
Corneal sensation Corneal anaesthesia may be due to corneal disease or predispose to it. Warn the patient, and then lightly touch the peripheral cornea with a cotton-tipped bud. Compare sides. Be aware; testing can affect vital staining and spread infection between eyes. For quantitative assessment use a Cochet and Bonnet aesthesiometer.
Vital dyes
1.Fluorescein 2% Viewed with a cobalt blue light this fluorophore has multiple uses including:
■Tear film assessment Can be used to assess tear volume (meniscus height), quality (tear break-up time, p. 158), and drainage (Jones tests and fluorescein disappearance test, p. 58).
■Corneal disease Raised areas have a thinner layer of fluorescein over them and appear bluer. Thinned cornea with an intact epithelium ‘pools’ fluorescein, appearing green. Dead epithelial cells and exposed basement membrane stain green, e.g. corneal abrasion. Bowman’s layer stains slowly.
■Aqueous leakage (Seidel’s test) Fluorescein 2% is applied to areas of suspected leakage. The usually dark-orange 2% fluorescein is diluted by leaking aqueous, turning bright green under a blue light.
2.Rose Bengal 1% should only be applied after topical anaesthetics. Warn patients that it may sting. Desiccated, devitalised conjunctival or corneal epithelial cells stain pink. Wash out afterwards with normal saline.
Schirmer I test Bend a Schirmer strip at the preformed notch by 90° and place into the conjunctival sac at the junction of the mid and temporal thirds of the lower lid, without using topical anaesthetic. Conduct the test in ambient lighting, with the patient blinking normally, eyes open and looking slightly up, and away from bright lights. Allow the paper to wet by capillary action. Aqueous tear deficiency is diagnosed if the strip wets by ≤5 mm over 5 minutes. From 5 to 10 mm is borderline. The Schirmer II test uses topical anaesthetic before testing and has been used as a measure of ‘baseline’ tear production which is probably unphysiological, as all tear production is reflex.
CORNEA 5 Chapter
147
Investigations
Pachymetry Measuring the central corneal thickness (CCT) is useful for monitoring corneal grafts, prior to refractive surgery, and for normalizing IOP measurements. For normal values, see page 144. There are several devices that measure CCT.
■Orbscan® has the advantages of being noncontact and
producing a thickness map. Standard deviation (SD) of CCT is ≈10 μm centrally (greater in the periphery). It may not be possible to capture data with some corneal curvatures, the posterior corneal surface must be visible, it is time consuming, and requires good patient cooperation.
■Ultrasound requires topical anaesthesia. Any area of the cornea can be measured. Record which area was measured. Touch the probe gently on to the cornea, perpendicular to
the surface, to avoid overestimating corneal thickness. The SD of CCT is ≈3 μm, taking seven measurements and averaging the middle five. Alternatively, take the smallest value that is consistent with the others, since this is most likely to indicate perpendicular placement of the probe on the cornea.
■Partial coherence interferometry is fast, noncontact, precise (SD ≈ 0.75 μm) with low interobserver variability. It requires a transparent cornea, and peripheral measurements are hard to obtain.
■Optical slit lamp-based pachymetry can be used with a contact lens in situ and to measure the depth of corneal scars. It is less accurate than the methods above.
Keratometry Dedicated instruments for measuring central corneal curvature, such as the Javel-Schiotz and Bausch and Lomb keratometers express the corneal curvature as a radius of curvature or in dioptric power. Automated devices such as the IOL Master®, Orbscan® and videokeratoscopes also provide measures of corneal curvature.
Corneal topography Several devices measure the corneal shape or curvature.
■Placido-based videokeratoscopy measures the corneal reflection of circles of light (mires) of known radius. Tear film abnormalities degrade the image. Videokeratoscopy does not measure beyond the surface and therefore the corneal power is derived mathematically. The reconstruction algorithms vary.
Peripheral measurements are less accurate because the relationship between the corneal curvature and the refractive
148 power varies, and fewer points are analyzed.
CORNEA 5 Chapter
Fig. 5.1: Corneal topography showing with-the-rule astigmatism (Topcon videokeratoscope). Compare with Fig. 5.6 (p. 161).
■Scanning slit-beam systems The computerized corneal mapping devices that use a calibrated video and slit beam system, e.g. Orbscan®. A mathematical representation of the true topographic anterior and posterior corneal surfaces is formed, from which the surface curvature is calculated. The image must be captured in 30 msec to prevent degradation from eye movement.
All methods produce similar results in the visually important, central 3 mm of the cornea. Data points taken along meridians from centre to periphery are converted into colour-coded contour maps. Yellows and greens represent normal corneal powers, cool colours represent lower power, and warm colours, higher power. Scales are not standardized and need to be selected carefully for meaningful interpretation. Topography of fellow eyes usually produces mirror images (enantiomorphs). Videokeratography of regular astigmatism results in a bow-tie pattern (Fig. 5.1).
Wavefront scanning technology can be used to measure the total ocular aberrations. Data acquisition and manipulation vary between manufacturers. Maps show the difference between the ideal planar wavefront and the wavefront distorted by the eye’s aberrations, including lower-order aberrations (prism, sphere, and cylinder), higher-order aberrations (all the rest), and point spread function (light scatter).
Specular microscopy permits visualization and photography |
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of the endothelium. Computer analysis produces quantitative and |
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qualitative indices that can be used to monitor cell loss from |
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contact lens wear and intraocular surgery. |
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Corneal abrasions and foreign bodies
Corneal Abrasions and
Foreign Bodies
Background A shearing force or foreign body breaches the epithelium to expose the subepithelial nerve plexus.
Symptoms Pain, photophobia, foreign body (FB) sensation, and tearing.
History Ask about the material (organic, metallic, or other) and mechanism of injury. Could a high-velocity FB have entered the eye from metal striking metal? Would safety goggles have been appropriate?
Examination Instil proxymetacaine 0.5% prior to examination then evert both lids to exclude subtarsal FBs (Fig. 5.2). Use 2% fluorescein to show conjunctival defects. If these coexist with subconjunctival haemorrhage that obscures the sclera, dilate the pupil to exclude an intraocular FB. Subconjunctival pigment may indicate uveal prolapse from scleral penetration. Note the pattern of corneal fluorescein staining such as a geographic abrasion, superficial punctate keratitis or linear scratches. Draw the abrasion location, size (in millimetres), and depth (percentage of corneal thickness). Iron FBs may be surrounded by a rust ring or sterile infiltrate, especially if present >24 hr (Fig. 5.3). The base of uninfected abrasions should be clear. Mild anterior uveitis may be present but iris or lens defects suggest an intraocular FB.
Differential diagnosis Treat expanding or new infiltrates associated with purulent discharge, severe redness, or pain as
Fig. 5.2: Subtarsal foreign body with associated corneal 150 fluorescein uptake.
CORNEA 5 Chapter
Fig. 5.3: Corneal FB with rust ring.
bacterial keratitis (p. 171). For apparently spontaneous epithelial defects consider corneal dystrophy and recurrent erosion syndrome. Exclude chemical and penetrating injury.
Management Remove foreign bodies (Box 5.1). Stat G. homatropine 1% or cyclopentolate 1% relieve pain from ciliary spasm. Offer oral analgesics as required.
Box 5.1: Removal of superficial corneal foreign body
1.Explain the procedure and warn that discomfort may increase when the anaesthetic wears off.
2.Instil topical anaesthesia, e.g. 3 drops of proxymetacaine 0.5%.
3.Multiple superficial FBs are sometimes more easily removed by irrigation.
4.Remove embedded FBs at the slit lamp using a 20-gauge needle with a bent tip. Remove any rust ring with a needle or dental burr. Deep rust rings in the visual axis that cannot easily be removed should be left to resolve spontaneously. Mild iron staining is not visually significant.
5.Document the epithelial defect size (post-removal).
6.Apply Oc. chloramphenicol and paraffin embedded mesh, e.g. Jellonet, and pressure pad unless there is a risk of infection, e.g.
organic matter, finger nail scratch, or contact lens wearer. |
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Corneal abrasions and foreign bodies
Follow-up
1.Contact lens wearers
■In contact lens wearers, without a clear history of a fingernail injury, an ‘abrasion’ is often the first sign of bacterial keratitis. If in doubt about the diagnosis, treat as bacterial keratitis. Otherwise, prescribe G. ofloxacin four times daily until the eye is comfortable, indicating epithelialization. Ask the patient to return as an emergency if symptoms deteriorate, suggesting infection.
■Re-start contact lens wearing 1 week after the epithelium has healed.
2.Non-contact lens wearers
■Small abrasion at low risk of infection: remove the pad after 24 hours. Use Oc. chloramphenicol q.d.s. for five days then stop. Review only if symptoms persist.
■Larger abrasions or high risk of infection: use Oc.
chloramphenicol q.d.s. and review at 24 hr. If the abrasion is healing well and the patient is comfortable, continue ointment for five days then stop. If not, instil G. homatropine 1%, continue Oc. chlorampheniol q.d.s. and review in 24 hr.
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