Keratoconus: A Comprehensive Guide to Diagnosis and Treatment

Keratoconus (KCN) is a noninflammatory disease associated with progressive corneal ectasia and thinning. Visual loss occurs following progressive myopia, irregular astigmatism, and corneal scarring. Precise etiology of keratoconus is unknown, however, the corneal collagen structure is weak and is unable to withstand eye pressure so it starts bulging. It is a genetic disorder. Evidence of genetic etiology includes familial inheritance, discordance between dizygotic twins, and association with other known genetic disorders. Several loci responsible for a familial form of KCN have been mapped, however, no mutations in any genes have been identified for any of these loci. It is also seen to be associated with chronic eye rubbing, Down syndrome, atopic disease, connective tissue disease and inheritance.

Clinical features
Keratoconus usually develops in the second and third decade of life. Nowadays, because of increased awareness amongst Ophthalmic community, pediatric cases are on the rise, they are also aggressive in progressing and require close follow up after treatment.

In its early stages, the symptoms of keratoconus include frequent changes in refractive errors, doubling of vision and glare.

Signs associated with keratoconus are scissor reflex on retinoscopy, Charlouex’s oil droplet reflex during retro illumination with a dilated pupil, Fleischer’s ring ( subepithelial deposition of iron oxide hemosiderin within the posterior limiting lamina membrane that manifests as yellow–brown pigmentation in an arc or ring shape around the base of the cone) and Vogt’s striae (fine, vertical, stress lines within the posterior stroma and posterior limiting lamina of the cornea) ,increased visibility of corneal nerves, Munson’s sign in advanced keratoconus.

Diagnosis
Corneal topography is the primary diagnostic tool for KCN detection. Pentacam with Belin Ambrosio Display has been considered gold standard in the diagnosis of KCN, however newer devices like MS-39 which combine Tomography with SSOCT seem to be more promising.
Signs of early keratoconus seen on topography are irregular bowtie, sagging of astigmatism, skewing of axis, sup-inf asymmetry, isolated posterior KCN, Elevation >+17 in the central posterior cornea, Excessive downward shifting of thinnest pachymetry and abnormally high corneal higher order aberrations, especially coma.

Topography pictures showing 1. Asymmetric bow-tie with inferior steepning, 2.Asymmetric bow tie with skewing, 3. Posterior elevation

Forme Fruste keratoconus – Here the topography does not reveal frank keratoconus, however there are signs of keratoconus seen either isolated or in groups of two or three. Lasik surgeons need to be very careful about picking up such cases as they have higher chances of progression to frank keratoconus or post lasik ectasia. Classifications like Belin Ambrosio display are very helpful tools in such cases.

Classifications of Keratoconus

1.  Belin/Ambrosio enhanced ectasia display

The ABCD classification is measured at the cone

A – Anterior radius of curvature from 3 mm zone centered at the thinnest point

B – Posterior (back) radius of curvature from a 3mm zone centered at the thinnest point

C – Minimal corneal thickness (not apical)

D – Best spectacle corrected visual acuity

Newer Classifications

2. Alphonso classification of keratoconus

3. RETICS classification

Traditionally, classification of KCN is mainly topographic/tomographic. However, Retics classification appears to be more practical as it gives weightage to other important parameters like CDVA, amount of astigmatism, Q value & HOA. These parameters have a role in deciding the plan for management.

Management – There are two objectives in the management of keratoconus.

  1. Halting progression of KCN
  2. Visual Rehabilitation
    Previously, the two objectives were managed separately which prolonged the treatment period. Recent trends include combining the two objectives at least to a certain extent. This approach has also shown better results in certain cases.

Halting the progression – Corneal collagen cross-linking

It is a modality for increasing the corneal biomechanical strength to halt disease progression. The Dresden protocol includes removal of the corneal epithelium in a diameter of 9 mm, followed by saturation of the corneal stroma using 0.1% isotonic riboflavin solution in 20% dextran and UVA irradiation of the corneal stroma, the total dose given being 5.4 J/cm2 (3 mW/cm2 for 30 minutes).
Newer protocols such Athens protocol, Epi on protocols have been attempted with some success, with a view to cut down treatment time, recovery time and to improve patient comfort, but Dresden is still the most trusted protocol.

Traditionally, C3R was done only when the corneal thickness was at least 400 microns, with isotonic Riboflavin. Nowadays, however, C3R can be done in thinner cornea by adjusting the timing according to Jscalculator for C3R for thin cornea.

Methods for visual rehabilitation
A. In the initial stages, spectacle correction can be done.
B. In patients with irregular astigmatism, contact lenses may provide better visual rehabilitation. In early stages soft lenses can be used, as the disease progresses, rigid gas-permeable (RGP) lenses or Super Cone, Rose K, hybrid lenses, scleral lenses give much better quality and quantity of vision.

C.T-PRK -Topography Guided-photorefractive Keratectomy. Patients who are intolerant to contact lenses will need T-PRK with adjunctive CXL to smoothen the corneal surface. Best suited for Kmax upto 55, most important point achieved is improvement in BCVA because of treatment of higher order aberrations & smoothening of the corneal surface. Typically, only upto 40um tissue is removed. It is quite essential to pay in detail attention to the calculations for ablation.

D. INTACS – Intrastromal Corneal Ring Segments (ICRS)
ICRS implantation with CXL can be done for moderate to advanced disease with a pachymetry of > 450 microns. Objectives for this modality of treatment can be summarized as – 1. halting progression, 2. reducing cylindrical component to improve contact lens fitting, 3. converting eccentric cone to central cone, 4. smoothening cornea, 5. reducing Kmax and 6. reducing refractive error.

Picture showing pre-op and post op picture of patient with ICRS

E.ICL – once the KCN is stabilized, ICL is an excellent surgery to reduce the refractive error, including astigmatism and improve UCVA.

F. DALK, PKP. – Patients with advanced keratoconus with stromal scarring often require keratoplasty. In deep full thickness scar-PKP should be done, In deep scar not involving posterior stroma -DALK (Less chances of rejection, excellent visual rehabilitation, quick recovery as compared to PKP) can be considered. Because of early diagnosis and excellent management of early KCN, the incidence of corneal transplant in KCN has reduced considerably.

Management protocol based on the risk of progression given by Narayana Nethralaya- An excellent decision tree.

Shetty, Rohit, Current review and a simplified “five-point management algorithm” for keratoconus, IJO: January 2015 – Volume 63 – Issue 1 – p 46-53

Cataract surgery in Keratoconus patients

Cataract surgery in such patients is challenging in terms of IOL power calculation. A customized approach depending on grade of keratoconus should be done (RETICS classification is better suited here). The principles in management of cataract in KCN patients can be summarized as follows.
A. If not done already, these patients rarely require corneal cross linking due to less chances of progression.

B. Higher the grade of the KCN (Retics Classification), more difficult is the case to manage. Therefore, whenever possible, a smoothening procedure should be attempted to decrease the grade of Keratoconus as a first step.

C. Cases with regular astigmatism in the central 3mm circle give predictable results with toric IOL. The aim should be to debulk the astigmatism, and the patients need to be counselled accordingly.

D. Patients happy with speciality contact lenses, should be considered for simple, even non aspheric, IOL implantation and contact lenses can be continued post op.

E. Patients oblivious to the KCN problem and well-adjusted to the KCN vision can be managed liked option D.

F. For IOL power calculation, two approaches can be considered. 1. Estimating the true corneal power using methods like EKR and using regression formulae like Holladay. 2. Actually measuring the corneal power using SSOCT and using formulae like Barrett TK or Ray Tracing.