DUA'S LAYER

Recognition of the previously unrecognised corneal layer situated in the deep stroma just anterior of Descemet’s membrane eponymous, now known as Dua’s layer, will have considerable effect on posterior corneal surgery, and contributes to the understanding of lamellar corneal surgery, especially deep anterior lamellar keratoplasty and posterior corneal pathology, said its discoverer Harminder S Dua MD, PhD.

Dua’s layer is well-defined, acellular and strong, consisting of five to eight lamellae of type-1 collagen bundles totalling about six to 15 microns thickness. The bundles are coarse and arranged in transverse, longitudinal and oblique directions. Bundle spacing is similar to that in stromal tissue, but Dua’s layer is entirely free of keratocytes in the zone that forms the posterior wall of the bubble. The rest has to be investigated. These histological features also distinguish Dua’s layer from the adjacent Descemet’s membrane, which consists of finer, closer spaced, parallel collagen bundles in banded and non- banded layers with endothelial cells.

Existence of Dua’s layer could explain why, in big-bubble deep anterior lamellar keratoplasty, some bubbles can be expanded peripherally and some cannot, said Jose Guell MD, Barcelona, Spain. Dr Dua’s research suggests that larger peripheral bubbles separate between Dua’s layer and Descemet’s membrane, whereas smaller, central bubbles form between Dua’s layer and the posterior stroma. Based on clinical experience, Dr Guell inflates from the periphery to get a larger bubble in cases where a larger-diameter dissection is required. “It draws a very nice understanding of what we observe in posterior lamellar surgery.”

The plane between Dua’s layer and the stroma may also be exploited for generating tissue for endothelial transplant, according to Prof Dua. In a May 2013 article in Ophthalmology, he wrote that with Dua’s layer splinting Descemet’s membrane, donor endothelial tissue does not scroll as much as when it is attached to Descemet’s only.

Dua’s layer may also be involved in posterior corneal pathology such as acute hydrops, Descemetocoele and pre-Descemet's dystrophies. The layer may also add to the understanding of the biomechanical properties of the cornea, which are currently attributed mostly to the compact anterior lamellae and Bowman zone, he added.

Demonstrating Dua’s layer   

The existence of Dua’s layer as a distinct anatomical feature, especially relevant to the surgical anatomy of the cornea, was confirmed in a series of experiments conducted at Nottingham by Prof Dua and his team. Air was injected, as in clinical deep anterior lamellar keratoplasty using the big bubble technique, into the corneas of four donor globes and 21 sclerocorneal discs, some of which were mounted on artificial anterior chambers. Following creation of a big bubble, Descemet’s membrane was peeled off, creation of a big bubble was attempted after peeling Descemet’s, and bubbles were inflated until they popped to measure popping pressure.

These experiments demonstrated that upon air injection, in most cases Dua’s layer separates cleanly from the posterior stroma centrally but adheres strongly peripherally. These type-1 bubbles between Dua’s layer and the posterior stroma occurred in 14 of the 25 corneas examined. They inflated from the centre, extended to a diameter of about 7.0mm to 8.5mm, and had a mean bursting pressure of approximately 700mm of mercury. Descemet’s membrane could be peeled entirely without deflating these bubbles, proving the integrity of Dua’s layer.

By contrast, type-2 bubbles, which occurred in five of the 25 corneas, formed between Dua’s layer and Descemet’s membrane. They inflated from the periphery, extended up to 10.5mm and had a bursting pressure of up to 300mm of mercury. Peeling Descemet’s membrane caused these bubbles to deflate. However, in all cases a type-1 bubble could be inflated between the stroma and intact Dua’s layer after removal of Descemet’s membrane, again indicating that Dua’s is a distinct anatomical feature separate from Descemet’s. The fact that air permeated through the entire corneal stroma till it reached Dua’s layer, which was lifted off as a ‘bubble’ indicates that this layer is impervious to air, which is another important feature.

Three corneas also formed mixed bubbles, which were a combination of the central and peripheral types. That air injected into the stroma is able to reach Descemet’s membrane only at the periphery suggests that Dua’s layer does not extend to the edge of the cornea. However, with type-1 bubbles, stress induced striae or wrinkles extend beyond the edge of the bubble into the attached part of the layer suggesting that Dua’s layer extends beyond the bubble edge.

Nonetheless, existence of a distinct Dua’s layer remains controversial in the field, Dr Guell noted. The layer is difficult to dissect and its thickness varies from patient to patient. “I personally believe the layer does exist, but these differences are why there is no consensus on its existence.”

Prof Dua acknowledged that his study showed variation in the layer, and it was based on tissue samples from older adults, ranging from age 53 to 94 with a mean of 77.7 years. Therefore, it cannot be directly extrapolated to younger eyes. However, most DALK procedures are carried out for keratoconus and clinical observations are consistent with existence of Dua’s layer, suggesting it is well-defined in this younger age group as well, he pointed out. More study is required to define its characteristics in very young children. Recently, Dr Vinay Phillips from Trivandrum, South India, sent histology and OCT images to Prof Dua, clearly demonstrating the presence of the layer in a 9.5-year-old girl who was operated for keratoconus.