On the axial location of Gunn's dots

Abstract

Purpose: To determine the axial location of Gunn's dots in the retina.

Methods: Adaptive optics scanning laser ophthalmoscopy (AOSLO) images and adaptive optics - optical coherence tomography (AO-OCT) volumes were collected from a region where Gunn's dots were found inferior to the optic disc from a subject determined by clinical examination to be a glaucoma suspect. AO-OCT volumes were also collected along the horizontal and vertical meridians from six healthy subjects and one glaucoma subject to identify and document other occurrences of Gunn's dots. AO-OCT volumes were registered in three-dimensions and averaged. Gunn's dots were segmented, and their volume, area, and diameter were measured.

Results: All Gunn's dots imaged in this study from all subjects were confined to the inner limiting membrane, neither extending into the vitreous nor into the nerve fiber layer. The size of the dots was highly variable. The measured volume, area, and diameter (mean ± standard deviation) were 1119.9 ± 590.9 μm³, 220.2 ± 105.5 μm², and 14.3 ± 3.1 μm, the latter within the range as previously published reports.

Conclusions: Based upon evidence from this study and others, Gunn's dots are not thought to be Müller cell end-feet or hyalocytes. We hypothesize that they are related to microglia, either as the by-product of their phagocytosis function, or are actual dead ameboid-shaped microglia who have fulfilled their scavenger role in retinal pathology. Further studies are needed in diseased eyes to determine if they have predictive value.

Keywords: AO-OCT; AOSLO; Gunn's dots; Hyalocyte; Microglia; Müller cells.

Fig. 1

AOSLO images of Gunn's dots in the inferior nasal region of a 43-year-old glaucoma suspect subject. (a) Clinical OCT scan with overlay showing regions of NFL thinning (in pink) inferior to the disc where the AOSLO data were acquired (blue box). (b–c) Two AOSLO montages taken at timepoints separated by ~16 months. Yellow box indicates region shown in (d–i). Red box indicates region where AO-OCT volumes were collected in Apr. 2019. (d–g) Confocal and (h–i) split detection AOSLO images at four imaging sessions separated by ~6 years showing stability of Gunn's dots. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

AO-OCT imaging of Gunn's dots in inferior nasal region of a glaucoma suspect patient. (a) En-face slice at ILM. Dashed lines show location of cross-sectional scans shown in (e–f). Arrow indicates location of a hyperreflective structure that has similar appearance as microglia (central soma and radiating processing). En-face slices: (b) 8.2 μm below the ILM, (c) at the approximate mid-point of the NFL (25.3 μm below the ILM), and (d) at the base of the NFL (49.3 μm below the ILM). Cross-sectional views: (e) vertical and (f) horizontal through the OCT volume corresponding to the dashed lines shown in (a). Dashed lines indicate the en-face planes shown in (a–d). Inset in (e) shows 2 × zoomed region of single Gunn's dot. Segmentation results are shown in (g) cross-sectional, (h) en-face, and (i) en-face binary views. Scalebars = 50 μm. Accompanying videos (media) captures entire volume.

Fig. 3

En-face (top row) and cross-sectional (bottom row, lateral location of dashed line) AO-OCT views of single Gunn's dots (arrows) in four different healthy subjects (aged 28–72 years) at the retinal eccentricity indicated. En-face sections were maximum projections over 10 pixels (6.85 μm) centered on the ILM. Scalebars = 50 μm.

Fig. 4

En-face (top row) and cross-sectional (bottom row) AO-OCT views of: (a) Relatively dense region of ILM microglia in a 28-year old healthy subject, (b) ILM microglia in various stages of activation in a 58-year old healthy subject, (c) cellular hyperreflective structures in a 72-year old healthy subject and (d) cellular hyperreflective structures in a 54-year old subject with glaucoma. AO-OCT en-face projections are at a depth just above the ILM. Inset of cross-sectional view in (a) shows 3 × zoomed view of activated microglia. Scalebars = 50 μm.