Drawbridge elevation versus tangential separation of the inner retina as the mechanism of full thickness macular hole formation.

Abstract

 

Purpose

 

This study is to test one aspect of a novel bistable hypothesis of macular hole formation and closure. The inner retina at the normal fovea has the shape of an inverted shallow dome with a central hole. The bistable hypothesis postulates that this is a bistable structure which can be triggered to flip inside out and back again like an umbrella. Flipping the inner retina inside out causes a macular hole due to the oblique path of the Muller cells through the retina. An antero-posterior (A-P) movement of the inner retina would then cause an A-P and tangential movement of the outer retina. In cross section, this movement of the inner retina is seen as a drawbridge elevation of the inner retina and this causes the outer retina to curl up and away from the central fovea (Fig. 1). In this hypothesis the size of the macular hole would be related to the degree to which the annulus of inner retina has been flipped through and there is no requirement for tangential separation of the inner retina.

 

Methods

 

This is a retrospective study of developing full thickness macular holes that were observed to change morphology prior to surgery. For each eye, the following was determined from aligned OCT scans: dA - average of the change in angle of elevation of the inner retina on either side of the hole (defined using tangents to the inner retina at a distance of 700 microns from the axis of the hole) dI - change in separation of the inner retina (minimum hole diameter at the level of the inner retina) dD - change in the base diameter of the macular hole

 

Results

 

17 eyes were identified. One eye progressed through a sequence of 2 changes in morphology allowing 18 measurements to be made. In 2 eyes, enlargement of the macular hole with further elevation of the inner retina was observed despite complete separation of the parafoveal vitreoretinal attachments at presentation. Correlation coefficient between dA and dD: 0.63 (p = 0.005) Correlation coefficient between dI and dD: -0.08 (p = 0.75) Scatter plots are shown in Fig. 2

 

Conclusions

 

This study suggests that the mechanism of macular hole formation involves the elevation of the inner retina in a drawbridge manner. These findings are consistent with the bistable hypothesis of macular hole formation

 

 

Schematic of oblique Muller fibres connecting inner and outer retina. In reality the fibres have a Z-configuration.

 

 

Scatter plots

 

Keywords: 586 macular holes • 550 imaging/image analysis: clinical