Regarding ‘Advances of optical coherence tomography in myopia and pathologic myopia’

Sir,

We read with interest the article ‘Advances of optical coherence tomography in myopia and pathologic myopia.'¹ In this article, the authors cite the original definition of dome-shaped macula (DSM) by Gaucher et al² as ‘an inward bulge of the macula within the concavity of a posterior staphyloma in highly myopic eyes,’ and go on to expound the ‘many uncertainties’ regarding DSM. Specifically, the authors outline several theories for the development of DSM: ‘resistance to deformation of scleral staphyloma, scleral infolding through the collapse of the posterior portion of the eye wall, and tangential vitreoretinal traction.’¹ We would like to present a novel mechanical explanation for the development and pathophysiology of DSM based on the fundamental physical principles of Pascal’s principle and Laplace’s law.

Pascal's principle³ tells us that the internal pressure inside of a closed vessel, such as the eye as a whole (or a blood vessel or a tube containing fluid), is the same throughout, despite variations in radius or diameter of the vessel. Laplace's law⁴ goes on to define the relationship between the radius of that vessel and wall tension, given the constant internal pressure (Figure 1). In an enclosed vessel such as a theoretical eye with uniform elasticity and thickness, wall tension increases with increased radius, although the radius cannot rapidly increase if the wall is not very elastic. However, if the very makeup of the wall varies from place to place, thicker or less elastic in one area as opposed to another, one would expect variation to develop over time in the radius of that portion of the vessel with varying wall thickness (Figure 2a). In DSM, it is known that there is an annulus of scleral thinning around a central area of macular scleral thickening.⁵ The annular area of scleral thinning would be expected to stretch to a greater degree than the central (sub-macular sclera), where the sclera is known to be thickened. The result is a central area that is less distended than the area of scleral thinning surrounding it, producing the characteristic dome-shaped appearance of DSM (Figure 2b).

Figure 1

Laplace's Law.

Figure 2

Pascal's Principle and Laplace's Law in Dome-Shaped Macula (DSM).