Optical Coherence Tomography in Glaucoma

1 Introduction: Practical Guide, OCT for Glaucoma

Jullia A. Rosdahl

Summary

This is an introduction with a reader's guide for our book on optical coherence tomography (OCT) for glaucoma. OCT is an invaluable tool for the diagnosis and management of glaucoma. This textbook provides a practical guide for the use of OCT in the clinical care of glaucoma patients: Including background on the development of OCT; in-depth descriptions of OCT of the optic nerve and retina in glaucoma patients, with chapters dedicated to illustrative case examples, artifacts, structure-function correlations, comparison of common devices, and anterior segment OCT; special considerations for OCT for childhood glaucomas and patients with high refractive errors; and future directions, namely, OCT angiography, swept-source OCT, and artificial intelligence. This introductory chapter also includes suggestions on how to use this guide depending on the reader's background and interests.

Keywords: optical coherence tomography, glaucoma, readers guide, optic nerve, retina, retinal nerve fiber layer

1.1 Introduction

Glaucoma is a group of eye diseases characterized by the loss of neural tissue at the optic nerve head, with "cupping" first visualized in the 1800s with Dr Helmholtz' ophthalmoscope, and the associated loss of peripheral vision. Over the last several decades, advances in computerized imaging have enabled doctors to visualize and quantify the optic nerve tissue to an astounding degree. In the 2000s, optical coherence tomography (OCT) became an integral part of the care of glaucoma patients, from screening glaucoma suspects, to the diagnosis of glaucoma, and for following patients with glaucoma to assess for progression of the disease.

The purpose of this guide is to serve as both a reference for understanding how OCT is used for the diagnosis and treatment of glaucoma, as well as a practical guide for "everyday" use to help doctors use this technology with greater skill and confidence.

1.2 Overview of the Guide

Summaries of each chapter provide an overview of the guide.

1.2.1 Development of OCT

OCT is a now a fixture in eye clinics around the world but only came to exist less than 30 years ago. Following decades of research on how evolving laser technologies could have clinical applications, in 1991 the first OCT captured an image of the eye. The history of OCT captured an research and development is discussed by summarizing the science, sharing insights on the economic risks and successes, and highlighting clinical impacts.1

1.2.2 OCT of the Optic Nerve

Assessing the optic nerve is critical in the evaluation of glaucoma patients. Computerized imaging technologies such as OCT provide quantitative measurements of optic nerve head parameters, including the retinal nerve fiber layer (RNFL). A systematic approach to OCT interpretation is discussed, including attention to potential limitations and artifacts.2

1.2.3 OCT of the Macula

Retinal imaging of the macula, with attention to the retinal ganglion cell layer, inner plexiform layer, and nerve fiber layer, can supplement the information obtained with the peripapillary RNFL. Applications of macular imaging for glaucoma, advantages and disadvantages, and pitfalls to avoid are discussed.3

1.2.4 Illustrative Case Examples

Case examples illustrate the use of OCT in glaucoma diagnosis and management. Cases spanning the spectrum of glaucoma severity are discussed, from glaucoma suspect, early to advanced glaucoma, as well as examples of glaucomatous progression. Characteristic findings from other chapters (3, 4, 6, and 7 in particular) are reinforced.4

1.2.5 Structure-Function Relationship

The relationship between structure and function of the optic nerve is the basis of our pathophysiological understanding of glaucoma. This chapter describes structure-function mapping, the temporal relationship between structural damage and functional defects, and how structural changes are linked to functional changes in glaucoma.5

1.2.6 Comparison of Common Devices

OCT devices produced by several manufacturers are available for clinical use. Differences in imaging specifications, analysis techniques, normative databases, and diagnostic capabilities are discussed for the Cirrus 6000 (Carl Zeiss Meditec AG, Jena, Germany), Spectralis (Heidelberg Engineering GmbH, Heidelberg, Germany), Avanti RTVue XR (Optovue, Inc., Fremont, CA, USA), and 3D OCT (Topcon Corporation, Tokyo, Japan).6

1.2.7 Artifacts and Masqueraders

All OCT machines have artifacts. Critical assessment for artifacts and attention to ocular pathology unrelated to glaucoma are discussed, with relevant clinical examples of "red" and "green" "OCT diseases" and future directions.7

1.2.8 Anterior Segment OCT in Glaucoma

OCT of the anterior segment provides noninvasive, high-resolution, cross-sectional images of the anterior segment structures. This technology can provide a useful supplement for the diagnosis and management of glaucomas, particularly primary angle closure disease.8

1.2.9 Special Considerations: OCT in Childhood Glaucoma

OCT is an important tool for the management of childhood glaucoma, especially since children with glaucoma may not be able to perform visual field testing. Special considerations for the use of OCT in children are discussed, including how to acquire OCT images and suggestions for interpreting OCT images from pediatric eyes.9

1.2.10 Special Considerations: High Refractive Errors

Special care should be taken when interpreting OCT scans in eyes with high refractive errors, given the rising prevalence of myopia and that myopia is a risk factor for glaucoma. Causes of OCT scan errors are discussed as well as newer parameters to improve the accuracy of glaucoma diagnosis in myopic eyes.10

1.2.11 Future Directions: OCT Angiography for Glaucoma

OCT angiography is an emerging technology giving detailed images of the microvasculature of the optic nerve and retina. Although its role in the diagnosis and management of glaucoma is still unclear, growing evidence shows good correlation between OCT angiography of the optic nerve and surrounding structures, and tissue loss and visual field loss from glaucoma.11

1.2.12 Future Directions: Swept-source OCT for Glaucoma

Swept-source OCT is an emerging technology using a tunable, longer wave of light than spectral domain OCT, allowing for high resolution with improved range of depth and increased scan speed. How this new technology fits in the framework of clinical glaucoma care is not yet clear, but current research is investigating the role of swept-source OCT in imaging the optic nerve and macula, as well as the anterior segment and choroid, for glaucoma.12

1.2.13 Future Directions: Artificial Intelligence Applications

Artificial intelligence refers to the development of computer programs to automate tasks to mimic human behavior. Deep learning is a type of artificial intelligence and uses a neural network to learn from a training dataset; these algorithms can process complex data such as ophthalmic images. Although research is still needed to study the performance of deep learning algorithms in real-world settings, these algorithms can be trained to distinguish between eyes with glaucoma and control eyes on OCT.13

1.3 How to Use the Guide

Depending on the background of the reader, this book can be approached in a variety of ways. Certainly, it can be read starting from Chapter 2 through Chapter 14, as a comprehensive summary of how OCT can be used for the care of glaucoma patients, which is recommended for trainees and doctors new to OCT. Readers with some general knowledge and experience with OCT may find individual chapters to be more useful; and starting with Chapter 5 to review the Illustrative Cases may help to reveal knowledge gaps to be filled by the reading specific chapters. Readers with an already significant background in OCT will likely find the chapters on Anterior Segment OCT (Chapter 9), special considerations (Chapters 10 and 11), and future directions (Chapters 12, 13, and Chapter 14) most useful. Anyone with an interest in the history of medicine will find Chapter 2 to have unique insights into the development of this technology.

1.4 Conclusion

This book is designed to be a practical guide, summarizing the clinical utility of the OCT technology, covering the basics and more advanced analyses, common clinical scenarios as well as more rare situations such as pediatric cases and high refractive errors, and aspects of OCT that are on the cutting edge such as OCT angiography, swept-source OCT, and artificial intelligence. We hope this guide helps the reader to use OCT with greater skill and confidence to care for their patients with glaucoma.

References

[1]Schuman JS. Spectral domain optical coherence tomography for glaucoma (an AOS thesis). Trans Am Ophthalmol Soc. 2008; 106:426-458

[2]Gracitelli CPB, Abe RY, Tatham AJ, et al. Association between progressive retinal nerve fiber layer loss and longitudinal change in quality of life in glaucoma. JAMA Ophthalmol. 2015; 133(4):384-390

[3]Tan O, Chopra V, Lu AT, et al. Detection of macular ganglion cell loss in glaucoma by Fourier-domain optical coherence tomography. Ophthalmology. 2009; 116(12):2305-14.e1, 2

[4]American Academy of Ophthalmology Preferred Practice Patterns. Primary Open Angle Glaucoma PPP,...