Diagnostic classification of corneal shape abnormalities

Abstract

Disclosed are systems and methods for characterizing corneal shape abnormalities. These methods may be used to differentiate corneas having subclinical keratoconus from other conditions which cause distortion of corneal shape, including warpage of the cornea due to contact lens wear. Also disclosed is classification scheme to aid diagnosis of corneal conditions and thereby guide clinical decision making regarding patient treatment. This classification scheme is based on computed properties of corneal shape, is amenable to automation, and may be implemented in an integrated system or provided in the form of software encoded on a computer-readable medium.

1 . A computer-based method of classifying corneal shape abnormalities, said method comprising:
 generating a corneal topography map of a subject's cornea;   generating an epithelial thickness map of the subject's cornea;   calculating an epithelial thickness pattern standard deviation (PSD) value based on the epithelial thickness map;   calculating a warpage index based on the corneal topography map and the epithelial thickness map; and   classifying the subject's cornea using the epithelial PSD value and the warpage index.   
     
     
         2 . The method of  claim 1 , wherein the corneal topography map is a mean curvature map of an anterior surface of the cornea. 
     
     
         3 . The method of  claim 1 , wherein the epithelial thickness map is generated using optical coherence tomography. 
     
     
         4 . The method of  claim 1 , further comprising:
 generating a pachymetry map of the subject's cornea; and   calculating an ectasia index based on the corneal topography map and the pachymetry map;   wherein the classifying the subject's cornea comprises classifying the subject's cornea using the epithelial PSD value, the warpage index, and the ectasia index.   
     
     
         5 . The method of  claim 4 , wherein the pachymetry map is generated using optical coherence tomography. 
     
     
         6 . The method of  claim 4 , wherein the corneal topography map, epithelial thickness map and pachymetry map are pattern deviation (PD) maps. 
     
     
         7 . The method of  claim 6 , wherein the warpage index is calculated according to:
   Warpage Index=100*sign(PD Ant ·PD Epi )*√|PD Ant ·PD Epi |
   
       where PD Ant  is the corneal topgraphy map, and PD Epi  is the epithelial thickness map. 
     
     
         8 . The method of  claim 6 , wherein the ectasia index is calculated according to:
   Anterior Ectasia Index=100*√Max( G   Ant   *G   Pachy )
   
       where G Ant  and G Pachy  are best-fit Gaussian waveforms for the corneal topography map and the pachymetry map, respectively. 
     
     
         9 . The method of  claim 4 , wherein classifying the subject's cornea comprises:
 classifying the cornea as abnormal, provided that epithelial PSD value exceeds a predetermined epithelial PSD threshold;   classifying the cornea as keratoconus without warpage, provided that the cornea is classified as abnormal and provided that the warpage index is below a predetermined warpage threshold;   classifying the cornea as warpage without keratoconus, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold and provided that the ectasia index is below a predetermined ectasia threshold; and   classifying the cornea as keratoconus plus warpage, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold and provided that the ectasia index exceeds a predetermined ectasia threshold.   
     
     
         10 . The method of  claim 9 , further comprising classifying the cornea as normal provided that the epithelial PSD value is below the predetermined epithelial PSD threshold. 
     
     
         11 . The method of  claim 9 , wherein the predetermined warpage threshold is 0. 
     
     
         12 . The method of  claim 1 , further comprising:
 generating a pachymetry map of the subject's cornea; and   calculating an ectasia index based on the corneal topography map and the pachymetry map;   calculating a coincident-thinning (CT) index based on the pachymetry map and the epithelial thickness map; and   calculating a pachymetry-anterior topography (PAT) index based on the pachymetry map and the corneal topography map;   wherein the classifying the subject's cornea comprises classifying the subject's cornea using the epithelial PSD value, the warpage index, and one or more of the ectasia index, the CT index, or the PAT index.   
     
     
         13 . The method of  claim 12 , wherein classifying the subject's cornea comprises:
 classifying the cornea as abnormal, provided that epithelial PSD value exceeds a predetermined epithelial PSD threshold;   classifying the cornea as compensatory epithelial modulation, provided that the cornea is classified as abnormal and provided that the warpage index is below a predetermined warpage threshold; and   classifying the cornea as primary epithelial disease, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold.   
     
     
         14 . The method of  claim 13 , further comprising:
 classifying the cornea as warpage plus keratoconus provided that the cornea is classified as primary epithelial disease and provided that the ectasia index is above a predetermined ectasia threshold; and   classifying the cornea as having warpage without keratoconus, erosion, or deposits provided that the cornea is classified as primary epithelial disease and provided that the ectasia index is below the predetermined ectasia threshold.   
     
     
         15 . The method of  claim 14 , further comprising:
 classifying the cornea as having keratoconus and one or more other ectasias provided that the cornea is classified as compensatory epithelial modulation and provided that the CT index is above a CT threshold; and   classifying the cornea based on the PAT index provided that the cornea is classified as compensatory epithelial modulation and provided that the CT index is below the CT threshold.   
     
     
         16 . The method of  claim 15 , wherein classifying the cornea based on the PAT index comprises:
 classifying the cornea as stromal addition or subtraction provided that the PAT index is above a PAT threshold; and   classifying the cornea as stromal distortion provided that the PAT index is below the PAT threshold.   
     
     
         17 . A system for classifying corneal shape abnormalities, the system comprising:
 an optical coherence tomography (OCT) system to acquire an OCT dataset for a cornea;   a logic subsystem; and   a data holding subsystem comprising machine-readable instructions stored thereon that are executable by the logic subsystem to:
 generate a corneal topography map of the cornea; 
 generate an epithelial thickness map of the cornea from the OCT dataset; 
 calculate an epithelial thickness pattern standard deviation (PSD) value based on the epithelial thickness map; 
 calculating a warpage index based on the corneal topography map and the epithelial thickness map; and 
 classify the cornea using the epithelial PSD value and the warpage index. 
   
     
     
         18 . The system of  claim 17 , wherein the instructions are further executable by the logic subsystem to:
 generate a pachymetry map of the cornea; and   calculate an ectasia index based on the corneal topography map and the pachymetry map;   wherein, to classify the cornea, the logic subsystem is to classify the cornea using the epithelial PSD value, the warpage index, and the ectasia index.   
     
     
         19 . The system of  claim 18 , wherein the corneal topography map, epithelial thickness map and pachymetry map are pattern deviation (PD) maps 
     
     
         20 . The system of  claim 19 , wherein the warpage index is calculated according to:
   Warpage Index=100*sign(PD Ant ·PD Epi )*√|PD Ant ·PD Epi |
   where PD Ant  is the corneal topgraphy map, and PD Epi  is the epithelial thickness map; and
 wherein the ectasia index is calculated according to:
   Anterior Ectasia Index=100*√Max( G   Ant   *G   Pachy )
 
 
   where G Ant  and G Pachy  are best-fit Gaussian waveforms for the corneal topography map and the pachymetry map, respectively.   
     
     
         21 . The system of  claim 17 , wherein, to classify the cornea, the instructions are executable by the logic subsystem to:
 classify the cornea as abnormal, provided that epithelial PSD value exceeds a predetermined epithelial PSD threshold;   classify the cornea as keratoconus without warpage, provided that the cornea is classified as abnormal and provided that the warpage index is below a predetermined warpage threshold;   classify the cornea as warpage without keratoconus, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold and provided that the ectasia index is below a predetermined ectasia threshold; and   classify the cornea as keratoconus plus warpage, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold and provided that the ectasia index exceeds a predetermined ectasia threshold.   
     
     
         22 . The system of  claim 17 , wherein the instructions are further executable by the logic subsystem to:
 generate a pachymetry map of the subject's cornea; and   calculate an ectasia index based on the corneal topography map and the pachymetry map;   calculate a coincident-thinning (CT) index based on the pachymetry map and the epithelial thickness map; and   calculate a pachymetry-anterior topography (PAT) index based on the pachymetry map and the corneal topography map;   wherein, to classify the cornea, the logic subsystem is to classify the cornea using the epithelial PSD value, the warpage index, and one or more of the ectasia index, the CT index, or the PAT index.   
     
     
         23 . The system of  claim 22 , wherein, to classify the cornea, the logic subsystem is to:
 classify the cornea as abnormal, provided that epithelial PSD value exceeds a predetermined epithelial PSD threshold;   classify the cornea as compensatory epithelial modulation, provided that the cornea is classified as abnormal and provided that the warpage index is below a predetermined warpage threshold; and   classify the cornea as primary epithelial disease, provided that the cornea is classified as abnormal and provided that the warpage index exceeds a predetermined warpage threshold.   
     
     
         24 . The system of  claim 23 , wherein, to classify the cornea, the logic subsystem is further to:
 classify the cornea as warpage plus keratoconus provided that the cornea is classified as primary epithelial disease and provided that the ectasia index is above a predetermined ectasia threshold; and   classify the cornea as having warpage without keratoconus, erosion, or deposits provided that the cornea is classified as primary epithelial disease and provided that the ectasia index is below the predetermined ectasia threshold.   
     
     
         25 . The system of  claim 24 , wherein, to classify the cornea, the logic subsystem is further to:
 classify the cornea as having keratoconus and one or more other ectasias provided that the cornea is classified as compensatory epithelial modulation and provided that the CT index is above a CT threshold; and   classify the cornea based on the PAT index provided that the cornea is classified as compensatory epithelial modulation and provided that the CT index is below the CT threshold.   
     
     
         26 . The system of  claim 25 , wherein, to classify the cornea based on the PAT index, the logic subsystem is to:
 classify the cornea as stromal addition or subtraction provided that the PAT index is above a PAT threshold; and   classify the cornea as stromal distortion provided that the PAT index is below the PAT threshold.