Keratoconus is a common corneal disorder where the central or paracentral cornea undergoes progressive thinning and steepening causing irregular astigmatism.
- 1 Disease Entity
- 2 Diagnosis
- 3 Management
- 4 Additional Resources
- 5 References
Disease Entity[edit | edit source]
Keratoconus adult eye (ICD-9 #371.60).
Disease[edit | edit source]
Keratoconus is a common corneal disorder where the central or paracentral cornea undergoes progressive thinning and steepening causing irregular astigmatism.
Etiology[edit | edit source]
Etiology is unknown. However, it is associated with atopy, Down’s Syndrome, Leber’s congenital amaurosis, and Ehler’s Danlos/connective discorders. The hereditary pattern is neither prominent nor predictable, but positive family histories have been reported. The incidence of keratoconus if often reported to be 1 in 2000 people.
Risk Factors[edit | edit source]
Eye rubbing, associated with atopy
Floppy Lid Syndrome
General Pathology[edit | edit source]
Keratoconus can show the following pathologic findings, including, fragmentation of Bowman’s layer, thinning of stroma and overlying epithelium, folds or breaks in Descemet’s membrane, and variable amounts of diffuse corneal scarring.
Pathophysiology[edit | edit source]
Histopathology studies demonstrate breaks in or complete absence of Bowman’s layer, collagen disorganization, scarring and thinning. The etiology of these changes is not known, though some suspect changes in enzymes that lead to breakdown of collagen in the cornea. While a genetic predispostion to keratoconus is suggested, a specific gene has not been identified.
Primary prevention[edit | edit source]
No preventive strategy has been proven effective to date. Some feel that eye rubbing or pressure (eg. sleeping with the hand against the eye) can cause and/or lead to progression of keratoconus, so patients should be informed not to rub the eyes. In some patients, avoidance of allergens may help decrease eye irritaion and therefore decrease eye rubbing.
Diagnosis[edit | edit source]
Diagnosis can be made by slit-lamp examination and observation of central or inferior corneal thinning. Computerized videokeratography is also useful in detecting early keratoconus and allows following its progression. Ultrasound pachymetry can also be used to measure the thinnest zone on the cornea. New algorithms using computerized videokeratopgraphy have been devised which now allow the detection of forme fruste, subclinical or suspected keratoconus. These devices may allow better screening of patients for prospective refractive surgery.
History[edit | edit source]
The majority of cases of keratoconus are bilateral, and often asymmetric. The less affected eye may show a high amount of astigmatism or mild steepening. Onset is typically in early adolescence and progresses into the mid-20’s and 30’s. However, cases may begin much earlier or later in life. There is variable progression for each individual. There is often a history of frequent changes in eye glasses which do not adequately correct vision. Another common progression is from soft contact lenses, to Toric or astigmatism correcting contact lens, to rigid gas permeable contact lenes.
Physical examination[edit | edit source]
A thorough and complete eye exam should be performed on any patient suspected of having keratoconus.
The general health of the eye should be assessed and appropriate ancillary tests should be done to assess corneal curviture, astigmatism and thickness. The best potential vision should also be evaluated. Many of the potential exam components are listed below:
• Proper history, including change in eye glass prescription, decreased vision, history of eye rubbing, medical problems, allergies, sleep patterns
• Assessment of relevant aspects of the patient’s mental and physical status.
• Visual acuity with current correction (the power of the present correction recorded) at distance and when appropriate at near.
• Measurement of best corrected visual acuity with spectacles and hard or gas permeaable contact lenses (with refraction when indicated)
• Measurement of pinhole visual acuity
• External examination (lids, lashes, lacrimal apparatus, orbit)
• Examination of ocular alignment and motility
• Assessment of pupillary function
• Measurement of intraocular pressure (IOP)
• Slit-lamp biomicroscopy of the anterior segment
• Dilated examination of the lens, macula, peripheral retina, optic nerve, and vitreous
• Keratomety/Computerized Topography/Computerized Tomography/Ultrasound Pachymetry
Signs[edit | edit source]
Early signs of Keratoconus include:
• Asymmetric refracive error with high or progressive astigmatism
• Keratometry showing high astigmatism and irreguality (axis that do not add to 180 degrees)
• Scissoring of the red reflex on ophthalmoscopy or retinoscopy.
• Inferior steepening, skewed axis, or elevated keratometry values on K reading and computerized corneal topography
• Corneal thinning, especially in inferior cornea. Maximum corneal thinning corresponds to the site of maximum steepening or prominence.
• Rizutti’s sign or a conical reflecton on nasal cornea when a penlight is shone from the temporal side
• Fleischer ring, an iron deposit often present within the epithelium around the base of the cone. It is brown in color and best visualized with a cobalt blue filter
• Vogt’s striae, fine, roughly vertically parallel striations in the stroma. These generally disappear with firm pressure applied over the eyeball and re-appear when pressure is discontinued.
Later signs of Keratoconus include:
• Munson’s sign, a protrusion of the lower eyelid in downgaze.
• Superficial scarring • Break’s in Bowman’s membrane
• Acute hydrop’s, a condition where a break in Descemet’s membrane allows aqueous into the stoma causing severe corneal thickening, decreased vision and pain.
• Stromal scarring after resolution of acute hydrops, which paradoxically may improve vision in some cases by changing corneal curvature and reducing the irregular astigmatism.
Symptoms[edit | edit source]
Progressively poor vision not easily corrected with eye glasses.
Clinical diagnosis[edit | edit source]
Diagnosis is made based on history of changing refraction, poor best spectacle corrected vision, abnormalities in keratometry, corneal topography, and corneal tomography, in association with corneal thinning; characateristic slit lamp findings can often be seen.
Diagnostic procedures[edit | edit source]
Diagnostic procedures include:
• Ophthalmic history, including family eye history, heritable disease, history of allergies, etc
• Slit-lamp examination
• Hard or gas permeable contact lens trial because good vision with lenses eliminates other sources of poor vision, including amploypia
• Measurement of K values
• Ultrasound Pachymetry
• Computerized corneal topographyExample of Early Keratoconus as evidenced on a Placido-disc based topographic evaluation
• Computerized corneal tomography (Rotating Scheimpflug, Rotating Slit Beam Photography)
Example of Keratoconus as evidenced on a Pentacam Scheimpflug evaluation
Laboratory test[edit | edit source]
Assessment of heritable diseases including • Down’s Syndrome • Leber’s congenital amaurosis • Ehlers Danlos/connective disorder. • Other medical conditions including sleep apnea
Differential diagnosis[edit | edit source]
Differential Diagnosis: • Keratoconus • Pellucid marginal degeneration • Keratoglobus • Contact lens induced corneal warpage • Coneal Ectasia post excimer laser treatment
Management[edit | edit source]
General treatment[edit | edit source]
General treatment The goal of treatment is primarily to provide functional visual acutiy. More recently attention has been directed to halting changes in the corrneal shape.
Initially, spcetacles or soft toric contact lenses in mild cases can be used. Rigid gas permeable contact lenses are needed in the majority of cases to neutralize the irregular corneal astigmatism. The majority of patients that can wear hard or gas-permeable contact lenses have a dramatic improvement in their vision. Specialty contact lenses have been developed to better fit the irregular and steep corneas found in keratoconus; these include ( but not limited to) RoseK, cutom designed contact lenses ( based on topography and/or wavefront measurements), semi-scleral contant lenses, piggy back lens use ( soft and hard lens used at the same time), and sceral lenses. Those patients that become contact lens intolerant or do not have acceptable vision, typically from central scaring, can proceed to surgical alternatives.
Medical therapy for patients who have an episode of corneal hydrops involves acute management of the pain and swelling. Patients are usually given a cycloplegic agent, sodium chloride (Muro) 5% ointment and may be offered a pressure patch. After the pressure patch is removed patients may still need to continue sodium chloride drops or ointment for several weeks to months until the episode of hydrops has resolved. Patients are advised to avoid vigorous eye rubbing or trauma.
Patients are usually followed on a 6-month to yearly basis to monitor the progression of the corneal thinning, steepening, the resultant visual changes, and to re-evaluate contact lens fit and care. Patients with hydrops are seen more frequently until it resolves.
Medical therapy[edit | edit source]
Patients with associated medical conditions should have these attended to properly.
Since the incidence of sleep apnea is very high in keratoconus, and because this condition may contribute to the pathogenesis of keratoconus and other medical conditions, all keratoconus patients should be questioned regarding sleep habits. Sleep studies and CPAP are recommended for at risk patients.
Medical follow up[edit | edit source]
Patients are usually followed on a 6-month to yearly basis to monitor the progression of the corneal thinning and steepening and the resultant visual changes and to re-evaluate contact lens fit and care. Patients with hydrops are seen more frequently.
Surgery[edit | edit source]
The majority of patients with keratoconus can be fitted with contact lenses if they are without a central corneal scarring. However, when patients become intolerant or no longer benefit from contact lenses, surgery is the next option. Surgical options can include: INTACS, Anterior lamellar keratoplasty, penetrating keratoplasty.
Non- FDA approved treatments, which typically have less evidence based information available on safety and efficacy, include use of UV/riboflavin collagen cross-linking of the cornea to stiffen the cornea and possibly prevent progressive changes in shape. This treatment has also be experimentally combined with excimer laser treatment, conductive keratoplasty, and/or INTACS. Some surgeons will use phakic IOLs to address high myopia and some of the asitigmatism.
Intracorneal ring segments (INTACS from Addition Technology) have also been approved for the treatmen tof mild to moderate keratoconus in patients who are contact lens intolerant. In these cases, patients must have a clear central cornea and a corneal thickness of > 450 microns where the segments are inserted, approximately at 7 mm optical zone. The advantage of INTACS is that they requrie no removal of corneal tissue, no intraocular incision, and leave the central cornea untouched. Most patients will need spectacles and/or contact lenses post-operatively for best vision, but will have flatter corneas and easier use of lenses. If a patient does not gain the expected results, the INTACS can be removed and then other surgical options can be considered.
Anterior lamellary keratoplasty has resurfaced as an option for treating keratoconus. It involves replacement of the central anterior cornea, leaving the patient’s endothelium intact. The advantags are that the risk of endothelial graft rejection is eliminated, and there is less risk of traumatic rupture of the globe in the incision, since the endothelium and Descemet’s and some stroma are left intact, and faster visual rehabilitation. There are several techniques utilitzed including, deep anterior lamellar keratoplasty (DALK) and big bubble keratoplasty(BBK) to remove the anterior stroma, while leaving Descemet’s layer and endothelium untouched. However, the procedures can be technically challenging requiring converstion to a penetating keratoplasty, and post-operatively there is the possiblity of interface haze leading to a decrease in BCVA; it is not clear if astigmatism is better treated with anterior vs penetrating keratoplasty. Penetrating keratoplasty has a high success rate and is the standard surgical treatament with a long track record of safety and effiicacy. Risks of this procedure include infection and cornea rejection and risk of traumatic rupture at wound margin. Many patients after PK may still need hard or gas-permeable contact lenses due to residual irregular astigmatism Any type of refractive procedure is considered a contraindication in keratoconic patients due to the unpredictability of the outcome and risk of leading to increased and unstable irregular astigmatism.
Surgical follow up[edit | edit source]
Following any corneal surgical procedure, patients need to be followed to complete visual rehabilitation. Most patients still require vision correction with spectacles or contact lenses, and often hard or gas permeable lenses are required if high levels of astigmatismare present.
All surgical patients need to be followed to ensure wound healing, evaulation for infection, suture removal and other routine eye care, such as testing for glaucoma, cataracts and retinal disesase. Graft rejection can occur after penetrating keratoplasty, requiring prompt diganosis and treatment to ensure graft survival.
Complications[edit | edit source]
Infection, poor wound healing, cornea transplant rejection,corneal neovasculariztion, glare, irregular asitgmatism, high refractive error.
Prognosis[edit | edit source]
The prognosis for penetrating keratoplasty in a keratoconic patient is excellent, with most patients able to return to an active lifestyle and the pursuit of personal goals. Intrastromal corneal ring segments (such as Intacs) can provide long-term success for patients with keratoconus, but this is typically in conjunction with contact lens use, and some may ultimately require corneal transplant to reach their goals of visual rehabilitation. "Progression" of keratoconus, even after corneal surgery, has been reported, but it is not clear how common or to what extent this can occur.
Additional Resources[edit | edit source]
http://www.aao.org/ ( American Academy of Ophthalmology)
http://www.nkcf.org/ (National Keratoconus Foundation)
http://www.nei.nih.gov/health/cornealdisease/index.asp#h (National Eye Institute)
http://clinicaltrials.gov/ct2/results?term=cornea+cross&cond=%22Corneal+Diseases%22 (National Institutes of Health Clinical Trial Registry)
References[edit | edit source]
Cornea Atlas, 2nd Edition. Krachmer, Palay. Elsevier, 2006.
H.E., Barron, L., McDonald, M.B., eds., The Cornea, 2d ed., 1996
External Disease and Cornea, Section 8. Basic and Clinical Science Course, AAO, 2006.
Refractive Surgery, Section 13. Basic and Clinical Science Course, AAO, 2006.
The Wills Eye Manual, 4th Edition. Kunimoto, Kanitkar, & Makar. Lippincott, 2004.
Bisbe L, Deveney T, Asbell PA. Big Bubble Keratoplasty, Expert Rev Ophthalmol. 2009; 4(5):553-561.
Asbell, P.A., Holmes-Higgin, D.K.: Intacs Corneal Rings Segments. In: Probst, L.E., Doane, J.F., Refractive Surgery: A color synopsis. Thieme, New York, 2001.
Colin J., Ertan A., eds. Intracorneal Ring Segments and Alternative Treatments for Corneal Ectatic Diseases. Kudret Eye Hosital, Kavaklidere-Ankara, Turkey.
Asbell, P.A. Is Conductive Keratoplasty the Treatment of Choice for Presbyopia? Expert Rev Ophthalmol. 2007; Feb:121-130.
- Ezra DG, Beaconsfield M, Sira M. et al. The Associations of Floppy Eyelid Syndrome: A Case Control Study. Ophthalmology. 2010; 117: 831-838.