Ciliary body length revisited by anterior segment optical coherence tomography: implications for safe access to the pars plana for intravitreal injections

Abstract

Purpose: To investigate the dependence of the ciliary body length (CBL) on the axial length (AL) and to draw conclusions on implications regarding safe pars plana access for intravitreal injections and vitreoretinal surgery.

Methods: A total of 200 individuals (mean age 42 years, SD ± 15.4) were enrolled in the study. Objective refraction and AL were obtained. Spherical equivalent (SE) was calculated. Anterior segment optical coherence tomography (ASOCT) was used to image and measure the CBL.

Results: The mean SE was - 1.64 diopters (SD ± 3.15, range - 14.5 to + 9 diopters) and the mean AL was 24.19 mm (SD ± 1.65, range 19.8-32.2 mm). There was a significant correlation between SE and AL (r² = 0.62, p < 0.0001). Mean CBL correlated significantly with age (r² = 0.11, p < 0.0001), AL (r² = 0.23, p < 0.0001) and SE (r² = 0.25, p < 0.0001). The mean CBL was 3351 μm (SD ± 459, range 2184-4451 μm). Three separate groups were defined by their AL with a normal AL group (AL 22.5 to 25 mm), a short AL group (AL < 22.5 mm) and a long AL group (AL > 25 mm). The mean CBL in the normal AL group was 3311 μm (SD ± 427), in the short AL group 2936 μm (SD ± 335) and in the long AL group 3715 μm (SD ± 365), and differed significantly (p < 0.0001) when compared.

Conclusion: For interventions requiring pars plana access (as an intravitreal injection or vitreoretinal surgery), an incision distance of 3.5-4.0 mm posterior to the limbus is recommended. In our research, however, a difference of 0.77 mm in mean CBL between the group with short AL and the group with long AL is demonstrated, implying that the mean CBL in very short and very long eyes differs significantly. These findings suggest that the AL should be taken into account for pars plana access and that it would be advisable to prefer the shorter or longer recommended distance (3.5 and 4.0 mm, respectively) from the limbus, which correlates with the AL. If AL is > 25 mm, a distance of 4.0 mm from the limbus should be chosen; and if AL is < 22.5 mm, a distance of 3.5 mm seems adequate.

Trial registration number and date: NCT00564291, 27 Nov 2007.

Keywords: Anterior segment optical coherence tomography; Axial length; Ciliary body; Intravitreal injection; Pars plana.

Fig. 1

Two representative ASOCT images with infrared superficial en face images of the conjunctiva and underlying sclera near the limbus on the left (A) with the arrow indicating the cross section of the anterior segment optical coherence tomography (ASOCT) image on the right. The limbus (L) and the size of the ciliary body (CBL) are marked as measured on image (B), which illustrates the measured length of the ciliary body (in red), the ciliary body (CB) itself and the anterior chamber angle (ACA). L, limbus; CBL, ciliary body length; C, cornea; Scl, sclera; Conj, conjunctiva; SS, scleral spur; Art, artefact (inverted iris)

Fig. 2

Correlations of the mean ciliary body length (CBL) with age, axial length (AL) and spherical equivalent (SE) from left to right, which showed a significant linear correlation. The Y-axis represents in all 3 graphs the mean ciliary body length (CBL). The 3 AL groups are depicted in colour on each graph (colour association is depicted on the vertical bar on the right)

Fig. 3

Graph showing the 3 groups defined by their axial length (short, normal and long) and the corresponding mean ciliary body lengths (CBL) of the individual eyes on the left with 3 anterior segment optical coherence tomography (ASOCT) slides on the right illustrating the CBL measurements. Each ASOCT image belongs to one sample of the 3 groups, whereby its exact position on the graph is indicated with thin lines. (A) ASOCT image of an eye in the long AL (> 25 mm) group with a rather long CBL. (B) ASOCT image of an eye in the moral AL (22.5–25 mm) group with a normal-sized CBL. (C) ASOCT image of an eye in the short AL (< 22.5 mm) group with a fairly short CBL

Fig. 4

Comparison of the mean ciliary body length (CBL) in the 4 quadrants (superotemporal, inferotemporal, inferonasal and superonasal) between the 3 groups, which are defined by their axial length (AL). Each graph depicts the distribution of the mean CBL in the respective quadrant between the 3 AL groups. Below the graphs, a colour-coded bar depicts the colour association to the 3 AL groups