Spectral domain OCT can be utilized for pupillometry

Issue: November 10, 2022

ByDhivya Ashok Kumar, MD, FRCS, FICO, FAICO

ByDeepika Soundararajan, MBBS, DNB

ByAmar Agarwal, MS, FRCS, FRCOphth

Fact checked byChristine Klimanskis, ELS

OCT is a noninvasive, noncontact imaging system providing real-time, high-resolution, cross-sectional images using near infrared light interferometry.

Pupil assessment can be performed using OCT to assess pupil size, amplitude of constriction and relative afferent defect. Both time domain and Fourier domain OCT can be utilized for this purpose. However, many physicians do not use it due to limitations in measurement details and analysis. In this column, we would like to share our experience in pupil assessment using OCT.

1. Scaling performed using ImageJ analysis.

*Source: Dhivya Ashok Kumar, MD, FRCS, FICO, FAICO, Deepika Soundararajan, MBBS, DNB, and Amar Agarwal, MS, FRCS, FRCOphth*

Spectral domain OCT

The system we used for analysis was the Cirrus 5000 spectral domain OCT (Zeiss) with 5 µm resolution. The patient was seated in front of the OCT machine with the necessary chinrest and headrest. The room light was dimmed, and the patient was given 30 seconds to adapt to the dark environment. The illumination source from the OCT machine was used as the light source for the initial examination, based on which pupillary reactions were noted in both eyes. The illumination source was situated 30 cm from the examined eye; hence, accommodation was prevented. Because there were moment-to-moment variations in pupillary response to light, three readings from each eye were taken at the same interval for both eyes simultaneously, and the average of the three readings was used for analysis. In the second part of the examination, readings were also taken with the effect of external light (standard indirect ophthalmoscope light) on pupil reactions of both eyes. Anterior segment five-line raster technique was used with length of line 3 mm and spacing 0.25 mm with 1:1 scaling (ie, 1 mm of pupillary diameter corresponds to 1 mm on display screen).

OCT images were taken in JPEG file format and evaluated by ImageJ analysis software for the measurement of pupil diameter in millimeters. Scaling was performed as the first step before processing and was calculated as 1 mm being equivalent to 42 pixels (3 mm equals 126 pixels) using the set scale option (Figure 1). Subsequently, the files for each eye were selected, and the pupillary diameter measurement was made in millimeters.

OCT pupillometry

Eyes with a relative afferent pupillary defect (RAPD) had larger pupil diameters than that of the fellow eyes (Figure 2). There was a significant difference (P < .05) between fellow eye pupil diameter (3.8 ± 0.99 mm) and the affected eye diameter (4.3 ± 1.16 mm) (Figure 3). There was also a significant difference in the pupil size with respect to the severity of RAPD (P < .05).

2. OCT showing the affected eye with a larger pupil size (right) as compared with the normal fellow eye (left).

3. Bar diagram showing the difference in control and affected eye mean pupil size measured by OCT pupillometry.

On real-time assessment of the pupil with external light source, there was a significant difference between fellow eye pupil diameter (2.9 ± 0.89 mm) and RAPD eye diameter (3.7 ± 1.06 mm) (P < .05).

Infrared video pupillometry

The results of the analysis by OCT are almost similar to infrared video pupillometry (IVP). In a study by Volpe and colleagues, a significant correlation between neutral density filter strength and intereye differences was seen for all measurement parameters in volunteers with simulated afferent pupil defects. IVP allows for a marginally more detailed analysis of the pupillary light response, but IVP devices are sophisticated tools with limited availability due to high acquisitions costs. In contrast, OCT is a widely available standard diagnostic tool in most hospitals and medical practices.

Pupillometry in clinical practice

Quantification of RAPD is important as it aids in management and prognosis, especially in conditions with retinal or optic nerve damage. Common clinical scenarios such as glaucoma, optic neuropathy, drug-induced optic nerve toxicity, optic neuritis and routine neuro-ophthalmological disorders require serial analysis of the pupil. Bedside testing, namely the swinging flashlight method (SFM), has been the gold standard. However, the technological improvement of other devices has increased the ease of documentation. Neutral density filters have been used to quantify SFM-based RAPD in retinal diseases. The densities of the filters are measured in log units ranging from 0.3 to 3.0 in steps of 0.3. The Sbisa bar method using SFM is also noted as a subjective evaluation using neutral density filters and can depend on other described factors.

Conclusion

OCT-based pupillometry is a fast, simple, user-friendly and well-tolerated method for evaluation, and it enables unambiguous clinical detection of RAPD. We can also use the method in hazy or edematous corneas. Telemedicine and serial documentation are other advantages of OCT pupillometry. However, comparison with other devices and the gold standard method may be needed in various clinical scenarios. Nevertheless, we believe it can be a useful method of pupil measurement in a day-to-day busy outpatient clinical practice.

References:
Bremner FD. Eye (Lond). 2004;doi:10.1038/sj.eye.6701560.
Fernández E, et al. Opt Express. 2005;doi:10.1364/opex.13.008184.
ImageJ: Image processing and analysis in Java. http://rsb.info.nih.gov/ij/.
Kumar DA, et al. Indian J Ophthalmol. 2019;doi:10.4103/ijo.IJO_885_18.
McCormick A, et al. Br J Ophthalmol. 2002;doi:10.1136/bjo.86.9.985.
Ramsay A, et al. Eye (Lond). 1995;doi:10.1038/eye.1995.151.
Schuster AK, et al. Acta Ophthalmol. 2017;doi:10.1111/aos.13184.
Thompson HS, et al. Surv Ophthalmol. 1981;doi:10.1016/0039-6257(81)90124-7.
Volpe NJ, et al. Ophthalmology. 2000;doi:10.1016/s0161-6420(00)00354-7.
Wilhelm H. J Neurol. 1998;doi:10.1007/s004150050248.

For more information:
Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books published by SLACK Books, sister company of Healio publisher Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; email: aehl19c@gmail.com; website: www.dragarwal.com.

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Disclosures: The authors report no relevant financial disclosures.

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