New corneal layer identified – significant surgically

Tentatively called Dua’s Layer, it is about 10 microns thick (10.15 +/-3.6 microns) and is defined by the group as being ‘pre-Descet’s’. It is located between the most posterior row of stromal keratocytes (deepest stroma) and Descet’s mbrane (see accompanying diagram).
Its significance relates to the many deep corneal surgical procedures now relatively common among corneal surgeons, e.g. deep anterior lamellar keratoplasty (DALK). The most common variants use intra-corneal bubble formation as a corneal delaminating technique, the most popular of which is the so-called big bubble technique (BB) using gas (usually air) injection.
It was careful observation of the outcomes of those delaminations that led to the discovery of Dua’s layer and investigations into its existence were first aired by the team at ophthalmology conferences in Italy and the UK in 2007. Earlier reports by others referred to ‘residual stroma’ on Descet’s mbrane or delamination of banded and non-banded layers of Descet’s mbrane in that context.
In initial studies in older corneas, the team identified three types of BB formations. Type-1 (the most common) was defined as: well-circumscribed, central, dome-shaped, and up to 8.5 mm in diameter. Type-2: thin-walled, larger (10.5 mm), that started at the periphery but enlarged centrally as more gas/gas pressure was applied. Type-3: a mixture of the characteristics of Types 1 and 2.
A major clue to the existence of the new layer came from observations of Type-1 BBs – after separation of the layers using the BB technique, Descet’s Mbrane could be dissected from the posterior surface of the ‘bubble’ without deflating it, suggesting strongly the existence of a competent but separate layer impervious to air, forming the posterior wall of the bubble. That layer is Dua’s Layer. Type-2 BBs deflate when Descet’s is dissected suggesting an absent or defective Dua’s Layer.
In a separate experimental series Descet’s was roved first and then a BB created. It too showed that a BB could still be formed in Type-1 cases. The difference between Types 1 and 2 BBs led the authors to suggest that Dua’s layer ends before Descet’s mbrane does.
Dua et al., suggest that the new layer can be delaminated up to a diameter of 9mm but outside that dimension the adherence is too strong to allow cleavage although wrinkling can be seen to confirm forces in the layer beyond that dimension. The authors go so far as to recommend that large BBs greater than about 8.5 mm not be attpted.
The belief/observation that DALKs result in ‘stronger’ wounds that those resulting from penetrating keratoplasties was attributed to the strength of an intact Dua’s layer. The air-tight behaviour of Dua’s layer was attributed to the greater space between its collagen fibrils accommodating a greater amount of proteoglycans.
Although not used in their investigations the authors believe that similar results would have been achieved if a viscoelastic (liquid) delaminator was used in place of air.
Transmission and scanning electron microscopy as well as immunostaining were used in the histological analysis of the corneal tissues.
In addition to its surgical ramifications the authors believe the discovery has significance to our understanding of corneal biomechanics, posterior corneal pathology such as acute hydrops, Descatocele, and pre-Descet’s dystrophies. Due to a lack of suitable eyes, the team could not extrapolate their findings confidently to the very young.
Dua HS et al., 2013. Humas Corneal Anatomy Redefined: A Novel Pre-Descet’s Layer (Dua’s Layer). Ophthalmology.

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