OCT artifacts and pitfalls: In the eye of the beholder

Attention to detail is key when reviewing optic coherence tomography (OCT) to get a real picture of what is occurring in the eye. 

This article was reviewed by Richard K. Lee, MD, PhD

Diagnosing and managing glaucoma now commonly involves the aid of optical coherence tomography (OCT) scans to correlate structure with function of the optic nerve. Sounds simple enough.

For example, a patient with an inferior optic nerve head notch has a superior visual field defect that can then be correlated and quantified on the OCT scans and confirm that there really is loss of the nerve fiber layer, according to Richard K. Lee, MD, PhD, associate professor and the Walter G. Ross Distinguished Chair in Ophthalmology at the Bascom Palmer Eye Institute,  University of Miami Miller School of Medicine, Miami.

Clinicians should not be concerned only with structure to follow or identify glaucomatous disease. He noted that in the Ocular Hypertension Treatment Study (Ophthalmology. 2006;113:1603-12), the optic nerve head (ONH) showed the first signs of disease progression in almost 60% of patients. In a third of patients the first sign of glaucoma development was visual field deterioration. Both should be followed. Optic nerve dysfunction with concomitant cupping of the ONH was observed in only 7% of patients. 

Related: OCT finds niche in glaucoma diagnosis, management 

Nothing is perfect
No matter how much a perfect instrument is desired, OCT machines have their shortcomings. 

“Clinicians can see ONH hemorrhages, pallor, edema, peripapillary crescents white, black, or grey, or other features of the optic nerve anatomy that cannot be observed in or identified by OCT,” Dr. Lee said. “There is no reliable information at all in eyes with high myopia where the OCT signal strength is so poor that no imaging is possible.”

OCT can be invaluable when tracking glaucoma progression. If physicians rely only on the green color coding, and don’t look more closely, they can easily miss the fact that the nerve fiber layer (NFL) may be continuously becoming thinner and thinner. The OCT printout tells the clinician that the NFL thickness is in the normal range relative to the normative database, but the clinician may miss the thickness issue. 

Evaluation of the VF in such a patient may show continued progression of a glaucomatous VF defect that can happen despite aggressive medical therapy. 

Related: Measuring RNFL with OCT detects subtle differences in glaucoma progression 

“This patient could be seen as normal, but functionally getting much worse,” Dr. Lee commented.

Physicians used to review all the data manually. OCT can provide all of this information with an automated comparison of current to prior OCTs to determine if there is change which, Dr. Lee believes, should be reviewed for all patients on a regular basis.

“The lesson here is that in some eyes no signal can be obtained, and the eye can be absent and a normal signal can be obtained,” he explained. “The caveat is that clinicians must look closely at their scans and correlated to the clinical examination. The fact that there were no OCT NFL scan results should tell the ophthalmologist that something is wrong.”

Related: Macular OCT: A must for treating glaucoma patients 

A number of variables affect OCT instruments that are machine-, operator-, and patient-dependent. 

“We always obtain duplicate or triplicate OCT scans or more at the same sittings for comparisons of minute details among the scans,” Dr. Lee said. 

Clinicians should check the signal strength and segmentation, review the right and left eye and superior and inferior NFL density butterfly pattern and the average NFL thickness, and look for asymmetry and progression of the NFL loss. 

Read more by Lynda Charters
 

Richard K. Lee, MD, PhD
E: rlee@med.miami.edu
Dr. Lee has no financial interest in any aspect of this report.