A case of unilateral circumscribed posterior keratoconus evaluated by three different imaging tools: optical coherence tomography, videokeratography, and Scheimpflug corneal tomography. 
Posterior keratoconus is a rare corneal anomaly which is part of the ectatic corneal disorders.  We report a clinical presentation of a unilateral posterior keratoconus in a 42-year-old man.  At the time of presentation, corrected distance visual acuity (CDVA) was 20/20 with a correction of +2.50 +2.50 × 90° in the right eye and 20/40 with +1 +3.00 × 105° in the left eye.  Slit lamp microscopy showed in the left eye an evidence of corneal thinning with a mild anterior protrusion and a remarkable posterior excavation.  The intraocular pressure was 19 mmHg in right eye and 16 mmHg in left eye.  Ultrasound pachymetry showed a minimum corneal thickness of 556 μ in right eye and 289 μ in left eye.  The anterior segment optical coherence tomography (AS-OCT) revealed central corneal thinning and showed a reduced epithelial thickness.  Videokeratography showed an increase of the corneal curvature in a defined area with central steepening in the area of the posterior corneal depression with gradual paracentral flattening.  The description of this case underlines the importance of this instruments such us AS-OCT and corneal topography in diagnosis of posterior keratoconus.  It can also be observed that in the contralateral eye there are no signs of ectasia as in the rare condition of unilateral keratoconus.