## Keratoconus

- Most common corneal ectasia

<HotTopic>
Clinical signs of keratoconus are a hot topic in exams!
</HotTopic>

- Progressive conical distortion of the cornea
    - Corneal steepening centrally
    - Vogt striae: disappear on pressure
    - Fleischer ring (iron deposition at the base of the cone)
    - Munson’s sign: conical distortion of the lid on downgaze
    - Rizzuti sign: sharply focussed conical reflection on the nasal cornea from a temporal light source due to total internal reflection
    - Charleaux’s sign: dark reflex at the cone on observing the cornea against a dilated pupil with direct ophthalmoscopy
    - Scissoring reflex on retinoscopy
    - Oil droplet reflex

- Irregular astigmatism: bilateral but asymmetric
- Axial stromal thinning
- Axial protrusion
- Increasing myopia
- Usually stabilises by mid-30s

### Aetiology
- Eye rubbing (and contact lens wear also a risk factor) 
- Stromal abnormality/connective tissue disorder
- Familial possibly autosomal dominant element but most are sporadic
- Fragmentation of Bowman’s layer leading to forward bowing of the cornea
- There is evidence of familial inheritance in 15% of cases (?abnormal superoxide dismutase)

### Associations

- Primary/idiopathic
- Allergic
    - Vernal keratoconjunctivitis
    - Eczema/asthma/hayfever/atopy
    - Floppy eyelid syndrome

- Congenital disorders:
    - Retinitis pigmentosa
    - ROP
    - Aniridia
    - Lebers congenital amaurosis

- Connective tissue disease
    - Ehlers-danlos syndrome
    - Marfan’s syndrome
    - Osteogenesis imperfecta/blue sclera

- Down syndrome: higher incidence of hydrops

- Apert’s syndrome (craniosynostosis)

### Complications
- Acute hydrops: Descemet's rupture leading to acute oedema and scarring

### Tomography
- Inferior steepening or asymmetrical bow-tie appearance 
- High central power (esp. Compared to fellow eye)
- Large difference between centre and periphery
- Steepness disparity between the eyes
- Inferotemporally displaced steep cone
- Mild: &lt;48D; moderate: 48-54D; severe: >54D
- Thinnest portion is at the apex of the cone

<HotTopic>
Corneal topography vs tomography

- Topography: maps the corneal surface

- Tomography: additionally provides a 3-dimensional representation of the cornea, eg. including back surface
</HotTopic>


### Management
- Counselling
- Spectacle or CL correction
  - Rigid gas permeable lens for moderate astigmatism
  - Intracorneal ring segment insertion: support the ectatic area to reduce steepening but do not affect progression

- Collagen crosslinking: riboflavin photosensitises the eye and then UV-A light is applied at a wavelength of 365nm. The interaction between these two leads to crosslinking within the collagen and ECM of the stroma (and avoids affecting the endothelium).
    - Limited use to corneas with >400microns in thickness to avoid endothelial toxicity
    - Has also been shown to reverse astigmatism

- DALK: is Descemet’s intact, ie. never had hydrops
- PK

- Supportive management (steroid, lubricants, hypertonic saline) of hydrops: injection of SF6 or C3F6 gas into the AC has been shown to promote closure of the rupture in Descemet’s.

- LASIK is **contraindicated** (can lead to corneal ectasia)
- Compare to contact lens warpage: long-term contact lens wear inducing irregular astigmatism with a cone but no other feature of keratoconus. Can be managed by discontinuing contact lens wear

## Pellucid marginal degeneration
- Rare bilateral progressive ectasia
    - Inferior thinning of the cornea
    - Protrusion **above** the area of thinning (cp. keratoconus)

- Non-hereditary, non-inflammatory
- M = F
- Affects the peripheral cornea
- Presents in the 3rd to 5th decades

### Clinical features
- Painless visual distortion
- Crescentic thinning (no vascularisation or lipid deposition)
- Especially affects the inferior cornea: cornea protrudes **above** the area of maximal **inferior thinning**
- Marked against-the-rule astigmatism
- Hydrops is rare
- Topography: crab-claw appearance

### Management
- Rigid CLs
- Surgery can be tried but is often disappointing

## Keratoglobus
- Globular overall appearance of the cornea
- Thinnest in the periphery
- Generalised protrusion of the cornea
- Typically present at birth

- Complications: corneal perforation/rupture

- Associations
    - Ehlers-danlos
    - Blue sclera
    - Flexible joints
    - Fractures
    - Deafness

Keratoconus Most common corneal ectasia Progressive conical distortion of the cornea Corneal steepening centrally Vogt striae: disappear on pressure…

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