abnormal variation of the posterior corneal curvature may occur in two forms : the generalized posterior keratoconus , characterized by an regular increase of the curvature of the entire posterior corneal surface has , and the circumscribed posterior keratoconus , in which a localized paracentral or central posterior corneal indentation is seen . in the generalized form , the corneal stroma typically remains clear .
in contrast , the circumscribed posterior keratoconus shows stromal opacities overlying the localized anterior ectasia of the posterior surface , which may occupy the full stromal thickness .
circumscribed posterior keratoconus is usually bilateral and sporadic , but familial cases have been also documented . despite the anterior protrusion in some cases ,
the 60-year - old white male of mediterranean origin presented for a cataract extraction on his left eye . visual acuity was 20/25 in the right eye and light perception in the left eye due to cataract formation .
the patient denied history of injury , reporting only a bilateral ocular infection in childhood was reported .
slitlamp examination revealed a bilateral paracentrally localized depression of the posterior curvature measuring 3  mm in diameter .
there was scarring in the overlying corneal stroma ( figures 1 , 2 , and 3 ) .
the posterior depression was clearly detectable using ultrasound biomicroscopy ( humphrey , zeiss , oberkochen ) ( figure 5 ) and slit - scanning topography analysis ( orbscan , bausch and lomb ) ( figure 6 ) .
corneal thickness measured 450  m within the lesion and 540  m in the adjacent healthy cornea using the orbscan system .
the refractive power of both the posterior and anterior corneal curvature was 50 to 56 diopters within the paracentral area .
following phacoemulsification and posterior chamber lens implantation visual acuity increased to 20/50 in the left eye .
the clinical and topographic findings in this patient are consistent with the paracentral keratoconus posterior circumscriptus .
this is the first report on ultrasound biomicroscopy to visualise the local anterior bulging of the posterior corneal surface with concomitant thinning of the stroma .
light microscopy of this abnormality has shown focal disorganization of basal epithelium and basement membrane , a replacement of bowman 's layer by fibrous tissue , a thinned stroma with an irregular arrangement of the central collagen lamellae , and a variable appearance of descemet 's membrane   with posterior excrescences indentating the vacuolated endothelium correspond to the corneal guttae seen in specular reflection .
iron deposits are present in the basal and suprabasal epithelium , corresponding to the brownish epithelial line observed clinically , indicating an irregularity of the anterior corneal surface .
visualisation of the posterior keratoconus using corneal topography analysis has been reported so far in a few cases [ 7 , 8 ] .
the light microscopy findings suggest an early pathogenic mechanism probably originated in the fifth or sixth month of gestation .
it is classified as one of the anterior chamber cleavage anomalies ( mesenchymal dysgenesis ) , as there are other anterior segment and systemic developmental abnormalities , as well as melanin depositions surrounding the posterior depression and iridocorneal adhesions . however , not all cases share this phenomenon . acquired cases occur and are usually associated with trauma [ 9 , 10 ] .
the mechanism in such cases involves an oblique penetrating injury with splitting of the inner corneal layers .
differential diagnosis also includes congenital disorders as peter 's anomaly and congenital hereditary endothelial dystrophy but they are usually found in new borns .
inflammation process as perforated corneal ulcer may also be taken into consideration , but it is usually unilateral . in most of the cases of posterior keratoconus
the vision is not affected , rarely it may be associated with other ocular abnormalities as polar cataract , lenticonus , and ectopia lentis .
 this paper documents a rare nonprogressive developmental disorder  bilateral circumscribed posterior keratoconus  in a 60-year - old man referred for a cataract surgery . for the first time 
 ultrasound biomicroscopy was used to visualise the local anterior bulging of the posterior corneal surface with concomitant thinning of the stroma . the amount of localized posterior depression , corneal thickness and the refractive power of both the posterior and anterior corneal curvature were measured using slit - scanning topography analysis ( orbscan ) . 
1. Introduction
2. Case Report
3. Discussion