Corneal Epithelial Thickness Mapping via SD-OCT: Epithelial Remodeling, Masking Physics & Ectasia Diagnostics

Professor Dan Reinstein demonstrated that corneal epithelial mitosis and shedding respond to eyelid shearing forces and stromal curvature gradients:

$$\frac{d(ET)}{dt} \propto -\nabla^2(\text{Stromal Curvature}) \implies \begin{cases} \text{Over Steep Stromal Apex:} & \mathbf{\text{Epithelium Thins (down to } 30\text{--}35\ \mu\text{m})} \\ \text{Around Annular Valley:} & \mathbf{\text{Epithelium Thickens (up to } 65\text{--}75\ \mu\text{m})} \end{cases}$$

By filling in stromal dips with extra cell layers, the epithelium smooths the front surface, masking up to $1.50\ \text{D}$ of subclinical stromal keratoconus from standard Placido topographers.

High-resolution Spectral-Domain OCT (SD-OCT) utilizes optical coherence interferometry to segment the tear film/epithelium and epithelium/Bowman interfaces with $1\ \mu\text{m}$ axial resolution. In early ectasia, the epithelial map reveals a pathognomonic Doughnut Pattern:

• A central, focal localized hot-spot of epithelial thinning directly overlying the peak of the stromal cone.
• Surrounded by a thick hyperplastic annular ring (doughnut) of compensatory epithelial thickening.

When an excimer laser ablates a deep central bed to correct $-8.00\ \text{D}$ of myopia, the abrupt transition zone triggers epithelial hyperplasia. The epithelium thickens centrally by $+10\text{ to }+15\ \mu\text{m}$, adding positive optical power that accounts for mild late myopic regression ($+0.50\text{ to }+0.75\ \text{D}$) months after PRK.