CCM Technique

Corneal confocal microscopy (CCM) is a non-invasive ophthalmic imaging technique. We use the Heidelberg Retina Tomograph 3 with the Rostock Cornea Module (Heidelberg Engineering GmbH). The CCM utilizes a 670 nm diode laser and provides digital images of the cornea. Both eyes are anesthetized using Oxybuprocainehydrochloride 0.4%(Conjuncain EDO; Fabrik GmbH) followed by a drop of carbomer 0.2% eye gel (Blumont Healthcare Ltd.). Study participants are instructed to fixate on a target. Several scans of the subbasal nerve plexus in the central cornea are captured per eye for 2 min. Adjacent images are separated by approximately 1-4 µm. The field of view of each image is 400x400 µm. CCM image extraction is carried out at a separate time by one investigator unaware of the study group. Three high-clarity CCM images per eye are selected using a validated protocol and criteria. Criteria for image selection are depth, focus position and contrast. CCM measures are quantified by either using CCMetrics, a validated image analysis software for manual nerve tracing or ACCMetrics, a validated image analysis software for automated nerve tracing. There are three CCM measures:

  • Corneal nerve fiber density (CNFD) (fibers/mm2)
  • Corneal nerve branch density (CNBD) (branches/mm2)
  • Corneal nerve fiber length (CNFL) (mm/mm2)

Video showing the CCM technique: