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Optical Coherence Tomography in Neuro-Ophthalmology: Do We Really Need It?

Monteiro, Mario L. R. MD

doi: 10.1097/WNO.0000000000000465
Editorial
Free

Ophthalmology and Otolaringology, University of São Paulo Medical School, São Paulo, Brazil.

Address correspondence to Mario L. R. Monteiro, MD, Av Angélica 1757 conj. 61, 01227-200, São Paulo, Brazil; E-mail: mlrmonteiro@terra.com.br

In the field of medicine, we are constantly searching for ways to improve diagnostic accuracy and patient management. Despite awareness of the costs involved, the introduction of new technologies capable of achieving this goal is naturally appealing. Unfortunately, enthusiasm for new technologies can lead professionals to overestimate the value of automated procedures to the detriment to a meticulous clinical examination. Because of the nature of neuro-ophthalmology, a subspecialty overlapping many areas of medicine, we often evaluate patients with complicated signs and symptoms and rely on a variety of diagnostic techniques that require proper interpretation of test results.

A Point Counter-Point discussion published in this issue of the Journal of Neuro-Ophthalmology features a debate on the use of optical coherence tomography (OCT) as a diagnostic tool in our subspecialty. Drs. Chen and Trobe were asked whether OCT should be used routinely to evaluate and monitor patients with idiopathic intracranial hypertension (IIH) (1). Actually, the same question applies to a number of other disorders of the visual apparatus affecting the optic nerve, chiasm, and retrochiasmal pathways. In fact, when the question was asked at the last two North American Neuro-Ophthalmology Society meetings, the audience was split between those in favor and those against the routine use of OCT. An experienced clinician and advocate of the nonuse of OCT, Dr. Trobe admits OCT is noninvasive and simple to perform but considers it dispensable in neuro-ophthalmic practice. According to him, to be truly useful, OCT must generate clinically relevant information unobtainable by conventional methods, be inexpensive and not distract the clinician from the major findings on physical examination. Many of us are tempted to agree with him because we have performed quite well with other more conventional forms of testing. Furthermore, several new technologies have been introduced that have not survived the test of time and became restricted to research or very occasional clinical use.

Like Dr. Trobe, until recently, I was very skeptical about the routine clinical use of any new technology claiming to measure the retinal nerve fiber layer (RNFL) in evaluating the optic disc. When scanning laser polarimetry (SLP) and OCT were introduced in the late 1990s – causing considerable excitement among glaucoma and retinal specialists – I had mixed feelings about the advertised statement that these machines could make better evaluations of the peripapillary RNFL than I could. Living in a developing country, I decided to use SLP and OCT primarily as research tools in patients with temporal hemianopia from chiasmal compression. Because such patients have RNFL loss in the temporal and nasal aspects of the disc (band atrophy of the optic nerve (2)), we and other investigators used this fundus finding as a model to evaluate these new technologies (3–7). As expected (and, I admit, with some relief), I found that SLP was not as good at detecting temporal and nasal RNFL loss as I was (3,8,9). However, I was surprised to find that OCT, even in its first generation, detected the expected patterns of RNFL loss (4). Needless to say, subsequent versions have even greater accuracy (10–12). A few years ago, I was surprised once again when OCT was shown to have the ability to measure macular thickness well enough to map band atrophy in patients with axonal loss (12,13) or the progressive macular thinning consistent with ganglion cell loss in a case of indirect optic nerve trauma (14,15).

For many years, OCT was used primarily as a research tool in neuro-ophthalmology, correlating OCT with clinical findings, perimetry, and electrophysiology test results (5,16–19). However, more recently, a number of investigators have demonstrated its practical usefulness in a wide range of neuro-ophthalmic disorders (20–26). OCT technology continues to improve with advances in image resolution and acquisition speed, the combination of OCT with fundus photography, the introduction of automated segmentation of retinal layers, three-dimensional reconstruction of scanned structures, deep optic nerve structure evaluations (with enhanced depth imaging), and tracking systems to improve accuracy and repeatability of data acquisition (18,26–31). OCT makes it possible to detect subtle axonal loss which in conditions such as subclinical demyelinating disease, mild optic disc swelling, and (to our astonishment) retinal ganglion cell loss in retrochiasmal disease (32–34). Although still an expensive technology, the wide array of potential uses of OCT in clinical practice, including the diagnosis of retinal diseases and glaucoma and assessment of anterior segment structures, justifies its use in patient evaluation.

Dr. Trobe is correct when he states that OCT rarely is necessary to evaluate patients with papilledema from increased intracranial pressure, the diagnosis, and treatment of which pose little difficulty (1). However, it is certainly useful in the follow-up of patients with IIH (as long as it is combined with clinical data and visual field testing). It may minimize the need for more invasive procedures such as lumbar puncture, especially in patients undergoing medical treatment.

Although I believe OCT can be useful in many clinical scenarios, I absolutely agree it should not distract clinicians from careful neuro-ophthalmic examination, perimetry, and other important aspects of management. This caveat is not restricted to OCT but applies to many other ancillary studies (e.g., neuroimaging, psychophysical and electrophysiological testing). As clinicians, we are used to investing much effort in identifying what is and what is not a useful supplement to clinical findings. OCT is just one more such instrument in that regard.

Also in this issue of the Journal of Neuro-Ophthalmology is a study by Drs. Chen and Kardon (35). They “hit the nail on the head” when listing the fundamental problems encountered in acquiring and analyzing OCT data. The greatest challenge of introducing OCT into routine practice is to understand how artifacts of acquisition and interpretation can interfere with establishing the correct diagnosis. Awareness of artifacts and biases is indispensable in clinical practice, whether the tool is OCT, neuroimaging, or automated perimetry. Even history taking has its pitfalls, and we often have to ask the same question on more than one occasion to obtain accurate information. In other words, it would be unfair to compare OCT, performed by a physician unaware of its potential artifacts and limitations, to a careful clinical examination performed by an experienced neuro-ophthalmologist.

Dr. Chen and Dr. Kardon do a superb job identifying the confounding factors related to patient age, RNFL segmentation (media opacity, optic nerve edema, and image truncation), refractive errors, axial length, angular distribution of the RNFL in myopic or tilted discs, cyclotorsion, artificially increased RNFL thickness, interindividual variation in RNFL thickness, and interpretation of the optic disc area, rim, and cup. They also share their insights regarding segmentation errors which may occur when analyzing the macular ganglion cell/inner plexiform layer on OCT and provide a systematic approach to interpreting OCT of the optic nerve focusing on the ganglion cell population. Chen and Kardon demonstrate how much effort it takes to master OCT data acquisition and interpretation, that is, to make OCT a truly useful tool.

For clinicians adopting OCT in their daily practice, the advantages and possibilities of the technology are evident. However, the large amount of data which can be acquired with OCT should not divert attention from the clinical context or lessen reliance on more well-established diagnostic tests. Moreover, clinicians looking to use OCT as a reliable and handy tool should be willing to take up the challenge of a long, but fascinating learning curve.

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© 2016 by North American Neuro-Ophthalmology Society