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 .