Optical coherence tomography (OCT) is rapidly  becoming a must-have technology for all ophthalmic  practices, not just those that focus on  retinal pathology. OCT scans have progressed  from fuzzy, grainy images of the macula to images that  yield nearly histologic-quality depictions of ocular structures.  With the development of the Fourier-domain OCT,  which uses a faster scanning rate than older time-domain  OCT systems, ocular tissue can be examined like never  before. In addition to better images of the macula, clinicians  can use OCT technology to help them manage glaucoma  and view the cornea. The RTVue OCT (Optovue  Inc., Fremont, CA) allows opthalmologists to enhance their  care of patients. This device has an axial resolution of  approximately 5 µm and a scanning speed of 26,000  A-scans per second, which provides better resolution than  is available with other OCT instruments.¹

HOW OCT OBTAINS IMAGES
  OCT is a noncontact technology that obtains images  by measuring the time delay of reflected near-infrared  light. Each individual axial reflection is combined over a  transverse dimension to yield the image of structures  such as the cornea (Figure 1). Similarly, clinicians can  obtain information about the optic nerve head and  ganglion cells when evaluating glaucoma or the macula  when assessing macular disease.

GLAUCOMA EVALUATION AND MANAGEMENT
  Perhaps the most valuable use of the RTVue OCT in a  nonretinal practice is for the management of glaucoma,  which entails a combination of clinical evaluations and  objective tests that allow physicians to follow the health of  the optic nerve. Practitioners’ clinical evaluation of the optic  nerve is ultimately limited by what they can view with the  slit lamp. One determines the optic nerve’s health by following  trends such as optic nerve cupping, standard automated  perimetry or visual fields, IOP, and corneal thickness.  Clinicians can monitor optic nerve head cupping through  drawings, but there can be drastic variability from observer  to observer.² Optic nerve head photographs are useful but  can be limited in their ability to pick up subtle details.  Glaucoma patients are typically older and are not always  able to accurately perform the current gold standard visual  field test. It can be extremely frustrating for physicians to  make long-term clinical decisions regarding glaucoma management  with visual field data that have large numbers of  fixation losses and false positive and negative errors. Eye care  professionals then base their target IOP on potentially limited  clinical findings of optic nerve health and visual fields.

With OCT, practitioners can quickly and accurately  measure the thickness of the optic nerve head at all  points circumferentially and the peripapillary retinal nerve  fiber layer (RNFL). These measurements are independent  of the patient’s ability to press a button when he or she  sees a dim light, as is required with standard automated perimetry   testing. Thus, the sensitivity and specificity are  unrelated to a patient’s ability to perform the test. This  more standardized way to document the health of the  optic nerve aids clinicians in choosing a target IOP. OCT is  also less affected by cataracts than standard automated  perimetry visual field testing.

Reduced macular thickness in glaucoma has been reported  by Ziemer et al.³ Measuring the inner RNFL, ganglion cell  layer, and inner plexiform layer (referred to as the ganglion  cell complex [GCC]), has been shown to increase diagnostic  accuracy for glaucoma.^4,5 RNFL measurements combined  with changes in the optic nerve head thickness provide  more data that allow us to better follow glaucoma progression.  ^6,7sup> OCT can provide information about changes in the  health of the optic nerve and the GCC earlier than clinicians  can detect them with the slit lamp. Changes in the RNFL  and optic nerve head’s thickness may precede visual field  loss. OCT can detect changes in the RNFL and optic nerve  head’s thickness earlier, ultimately helping clinicians to slow  or arrest visual field loss and provide better glaucoma management  (Figure 2).

ANTERIOR SEGMENT AND CORNEAL  IMAGING, KERATOCONUS DETECTION
  Anterior segment and corneal imaging have obvious  uses in evaluating corneal opacities, LASIK flaps, and  Descemet’s stripping endothelial keratoplasty buttons as  well as for measuring the anterior chamber angle. Lesserknown  uses for OCT include the evaluation of corneas for  keratoconus and the determination of true corneal power  to aid in post-LASIK IOL calculations.

OCT can be used to detect abnormal corneal thinning  in keratoconus.⁸ David Huang, MD, PhD and his colleagues  have derived OCT corneal thickness parameters  that are predictive of the cornal ectasia seen in keratoconus.  Evaluations can be performed by comparing  thickness relationships between the superonasal octant  and the inferotemporal octant, superior and inferior  octants, and thinnest corneal thickness as well as the  magnitude of the differences in corneal thickness. Using  these diagnostic parameters and cutoff values, the results  of the corneal scan can be used to determine the likelihood  of keratoconus.

In Figure 3, the superior-nasal octant thickness minus  the inferotemporal octant thickness value of 78 µm is  greater than the cutoff of 45 µm, which would not necessarily  point to keratoconus. However, the superior octant  thickness minus the inferior octant thickness value of -26 µm  is much less than 45 µm, the minimum thickness value of  414 µm is less than the cutoff value of 470 µm, and the  minimum corneal thickness minus the maximum corneal  thickness value of -157 µm is less than the cutoff value of  -100 µm. This cornea is abnormal in three of the four  parameters, which is highly suggestive of keratoconus.

One problem facing ophthalmologists is calculating  IOL powers after LASIK. The true corneal power can be  calculated using OCT to measure corneal pachymetry  and curvature. This is a promising technique that is currently  being studied.

CONCLUSION
  OCT greatly helps anterior segment surgeons and general  ophthalmologists. With straightforward training of technicians,  the OCT scans can be obtained quickly and with  great accuracy. Having digitized, repeatable optic nerve  parameters along with a traditional clinical diagnosis helps  physicians to determine glaucomatous progression. When  the patient cannot perform standard visual field testing, the  OCT is invaluable. Having the ability to image the cornea  with OCT provides one more technique by which to diagnose  and manage corneal disease. OCT has always been a  great way to image the macula. The technology allows clinicians  to evaluate patients quickly and easily for retinal thickening  related to an epiretinal membrane, central serous  retinopathy, cystoid macular edema, or diabetes, to name  a few. OCT has many other uses beyond imaging of the  macula!

Robert Brass, MD, is an associate clinical professor  of ophthalmology at Albany Medical College,  and he is in private practice at Brass Eye Center in  Albany, New York. He is a speaker for and is  involved in clinical research with Optovue, Inc., but  he acknowledged no other financial interest in the company  or its products. Dr. Brass may be reached at (518) 782-7827;  drbrass@drbrassonline.com.

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