OD News Articles

3rd January 2011

OCT—Looking Below the Surface

by Reid Mamiya, OD Chehalis, WA

Ocular coherence tomography (OCT) is increasingly becoming part of optometry’s repertoire of diagnostic tests. More and more practitioners are purchasing or gaining access to these instruments. A desire to avoid overlooked subclinical problems, and the ability to explore and monitor suspicious presentations, drives the need for this high tech imaging.

As OCT instruments improve, faster imaging and higher resolution provide better coverage and enhanced visualization of fine ocular pathologies. While the original focus of OCT was limited to layers and pathologies of the retina, the most current instruments also provide high resolution images and analysis of the cornea, anterior segment, and optic nerve head.

Common Pathologies

OCT technology is relatively easy to use and provides remarkably detailed information. Abnormalities not visible on regular clinical examination can easily be seen. Some of the common retinal pathologies detected with OCT include:

  • Epiretinal membranes
  • Macular holes
  • Drusen
  • Macular edema
  • Pigment epithelial detachments (PED)
  • Vitreomacular traction
  • Subretinal fluid
  • Choroidal neovascular membranes (CNVM)
Retina Review

OCT images sometimes present opportunities for treatment before problems become clinically significant. However, to take advantage of this sophisticated engineering, we must know the instrument’s capabilities and be able to interpret the data it produces. Following is a brief review of retinal layers and how they should appear on OCT scans.

1.  NFL — Nerve Fiber Layer

  • Formed by fibers of the optic nerve
  • Composed of axons from the ganglion cells below
  • High reflectance of OCT beam

2.  GCL — Ganglion Cell Layer

  • Contains cell bodies of ganglion cells
  • Low reflectance of OCT beam

3.  IPL — Inner Plexiform Layer

  • Contain dendrites of ganglion cells and cells of the inner nuclear layer
  • High reflectance of OCT beam

4.  INL — Inner Nuclear Layer

  • Contains bipolar, horizontal and amacrine cells
  • Low reflectance of OCT beam

5.  OPL — Outer Plexiform Layer

  • Synapses of the photoreceptors and the cells of the inner plexiform layer
  • High reflectance of OCT beam

6.  ONL — Outer Nuclear Layer

  • Contains the cell bodies of the photoreceptors
  • Helps in light detection
  • Low reflectance of OCT beam

7.  RPE — Retinal Pigment Epithelium

  • Pigmented cell layer outside the neurosensory retina
  • Composed of hexagonal cells that nourish the retinal visual cells
  • Provides phagocytosis of photoreceptor cells

8.  C — Choroid

  • Vascular layer provides oxygen and nutrients to outer layers of the retina
  • Located between the retina and sclera
  • Bruch's membrane is the innermost layer of the choroid

 
Which Layer?

Knowing which retinal layer is damaged by pathology is an important part of determining the likely diagnosis. For example, how might a lamellar macular hole be differentiated from a full thickness hole? On OCT, lamellar holes show the layer of photoreceptors still attached to the RPE—while full thickness holes appear with a depression of the internal limiting membrane through to the outer segment of the photoreceptor layer.

Sometimes it may be challenging to tell if a patient has a PED or CNVM—or both. On OCT, subretinal fluid or a break in Bruch's membrane points to a choroidal neovascular membrane. Sometimes fluorescein angiography is needed to make a definitive diagnosis, but knowing the retinal layer of involvement is crucial.

Moving Forward

While the original focus of OCT was limited to layers and pathologies of the retina, some of the most current instruments also provide high resolution images and analysis of the cornea and anterior segment including:

  • Corneal pattern pachymetry
  • Epithelium measurements
  • Stroma and overall corneal thickness
  • Anterior chamber angle
Cornea

Some of our OCT instruments perform 8 high-definition meridional scans of the cornea that form a 6mm diameter pachymetry map. Pattern pachymetry is useful in:

  • Diagnosing and monitoring several corneal pathologies
  • Screening refractive surgery candidates

Screening for early or forme fruste keratoconus by detecting focal thinning is critical with potential refractive surgery patients. If there is thinning of the inferior cornea, we proceed with great caution. Patients interested in LASIK enhancements also need to have sufficient corneal thickness for retreatment. To reduce the risk of ectasia, our standard is to maintain at least a 250 micron stromal bed.

AC Angle

The anterior chamber angle can be imaged with the newest OCTs, which is useful for patients at risk for angle closure. Baseline images can help determine if patients’ conditions are worsening over time. While gonioscopy remains the standard, objective measurements with OCT provide visual supportive information.

Proper Perspective

OCT is currently perceived as the hot, must-have instrument. But, while scans can provide great information, we encourage you to keep this technology in perspective. By itself, OCT cannot diagnose glaucoma or any other condition. It does not substitute for a thorough history, comprehensive exam, and above all, clinical judgment.

OCT and Other Diagnostic Services at PCLI

We have carefully compared OCT models and none are perfect. Some of our offices, but not all, are equipped with OCT instruments—and we will continue to purchase more as the systems improve. Where available, we are happy to provide OCT scans and other diagnostic services at your request. Our doctors can provide interpretation or we can forward the images for your analysis. Diagnostic service referral forms outlining specific services at participating PCLI offices are available in the "For Doctors" section of our website at www.od.pcli.com.

View Three OCT Case Studies

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