Smartphone-Acquired Image Photogrammetry for Detection of Shallow Anterior Chamber

Abstract

Purpose: This study aimed to explore the role of smartphone imaging of the eye using two perspectives - anterior and temporal - in the detection of a shallow anterior chamber (AC). The AC depth (ACD) of an eye can be used as a surrogate marker for identification of eyes at risk of developing angle-closure disease.

Methods: A prospective observational study was conducted at a university teaching hospital in South India. Each eye was photographed with a smartphone using the two perspectives, followed by quantitative measurement of ACD using optical biometry. The percentage of nasal iris illuminated was measured from the image acquired using the flashlight method (anterior perspective), whereas pupil position relative to the cornea was measured from the image acquired using the temporal perpendicular method (temporal perspective). The receiver-operating characteristic curve and area under the curve (AUC) were studied for both perspectives independently for overall predictive accuracy in detection of shallow AC (ACD <2.7 mm, obtained by IOL Master).

Results: A total of 275 eyes were examined, of which 77 (28%) had an ACD <2.7 mm. The accuracy of detection of shallow AC was found to be 95.2% for both perspectives when used alone or in combination. AUC of the anterior perspective was 0.99 (95% CI 0.982-0.997). The AUC for the temporal perspective was 0.993 (95% CI 0.988-0.999).

Conclusion: Smartphone-acquired image photogrammetry of an eye with anterior and temporal perspectives independently and in combination provided accuracy nearing 95% in the detection of shallow AC (ACD <2.7 mm).

Registration: This trial was registered with the Clinical Trial Registry of India (CTRI/2018/09/015867, September 28, 2018).

Keywords: angle-closure glaucoma; community screening; flashlight test; shallow anterior chamber; smartphone teleophthalmology; telemedicine.

Figure 1

Photogrammetry of images on anterior and temporal perspectives. (A) Estimation of nasal iris illumination on anterior perspective (image acquired using flashlight method). Corneal reflex at temporal limbus surrounded by blue circle. I, distance between nasal pupillary border and start of iris shadow; S, distance between start of iris shadow and nasal limbus. (B) Relative position of midpoint of pupil to corneal apex and temporal limbus (image acquired using the temporal perpendicular method). C, distance of midpoint of pupil from corneal apex; L, distance of midpoint of pupil from temporal limbus. Midpoint of pupil determined as midpoint of vertical line joining superior and inferior pupillary edges.

Figure 2

Samples obtained in each perspective and average ACD measured using IOL Master. (A) Photogrammetry of anterior-perspective images. The x-axis represents ratios obtained from the images, and the y-axis represents the number of samples. Mean ACD in each category is on the orange line. (B) Photogrammetry of temporal-perspective images. The x-axis represents ratios obtained from the images, and the y-axis represents the number of samples. Mean ACD in each category is on the orange line.

Figure 3

Receiver-operating characteristic (ROC) curve and area under the curve (AUC) of anterior and temporal perspectives. The green diagonal line is the reference (0.5), indicating the null hypothesis. The dark-blue line marks the ROC curve for the anterior perspective and the orange line the ROC curve for the temporal perspective.

Figure 4

Scatterplots depicting correlations among the three variables. R, Pearson’s correlation coefficient. The diagonal black line corresponds to linear R². (A) ACD vs anterior-perspective ratio; (B) ACD vs temporal-perspective ratio. (C) Anterior-perspective ratio vs temporal-perspective ratio.

Figure 5

Clinical determination of shallow AC on anterior perspective (flashlight method). The horizontal line denotes the total width of the nasal iris traced from the nasal pupillary edge to the nasal limbus. The vertical line demarcates a temporal third of the total width. The dot on the horizontal line marks the start of the iris shadow. (A) Normal ACD on anterior perspective. The start of the iris shadow is on the nasal side of the vertical line depicting >two-thirds of nasal iris being illuminated. (B) Shallow ACD on anterior perspective. The start of the iris shadow is on the temporal side of the vertical line depicting less than a third of the nasal iris being illuminated.

Figure 6

Clinical determination of shallow AC on temporal perspective (temporal perpendicular method). The horizontal line connecting the corneal apex, midpoint of pupil, and temporal limbus denotes the anteroposterior diameter of anterior chamber. The longer vertical line demarcates the anterior third of the anterior chamber. The dot on the horizontal line demarcates the midpoint of the pupil (determined by the shorter vertical line joining the superior and inferior pupillary edge). (A) Normal ACD on temporal perspective. The midpoint of the pupil lies in the posterior two-thirds of the eye. (B) Shallow ACD on temporal perspective. The midpoint of the pupil lies in the anterior third of the eye.