Sir,

We welcome the interesting work by Drs Moore et al1and want to applaud the authors for their novel ideas. However, we must take exception to the statement that ‘an adequate air bubble is not commonly seen’. Both Dr Anwar and myself are able to produce the ‘big bubble’ consistently: in 40–60% of eyes on the first try, in 80–90% of eyes on the aggregate of the first two tries, and in nearly all eyes when various additional manoeuvers (such as additional air injections, or preliminary anterior keratectomy followed by fluid injection and further air injection) are used in the initially resistant cases.

We maintain that the ‘Big Bubble’ technique2, 3 is the most efficient way of performing maximum depth lamellar keratoplasty—an essential feature of which is the baring of the host's Descemet's membrane over the central region of the cornea. Two prerequisites for successful use of the ‘Big Bubble’ technique are

  1. a)

    that a ‘big bubble’ has actually been generated, and

  2. b)

    that the surgeon is aware of this fact.

Success of (a) depends on close observation of several details, several of which were ignored in this experimental work:

  1. 1)

    In their paper, Dr. Moore and coauthors do not mention that they trephined the cornea prior to injecting the air. This important first step of the ‘Big Bubble’ technique serves to ‘isolate’ the central cornea (to a large extent) from the peripheral cornea. Failure to perform this step may aid excessive spread of air into the corneal periphery, to the trabecular meshwork and into the anterior chamber instead of deep spread towards Descemet's membrane.

  2. 2)

    The authors made an opening into the eye (to insert the endoscope) before injecting the air. Again, this would facilitate air entry into the anterior chamber. Air inside the anterior chamber directly competes for space with the ‘big bubble’. The more air that is present in the anterior chamber (and the higher the pressure), the smaller will be the room available for the bubble of Descemet's detachment.

  3. 3)

    A 26 gauge needle was used for injecting air into the cornea instead of a 27 or 30 gauge needle. (At this time, the relevance of this difference in technique is uncertain.)

  4. 4)

    The force of the initial air injection may not have been sufficient.

  5. 5)

    As the authors of the paper conceded, it is possible that cadaver eyes react different from live eyes. Further, it is conceivable that the pathological conditions for which this surgery is performed actually predispose these eyes to the formation of a central detachment of Descemet's membrane.

  6. 6)

    Finally, we want to stress that here too, as in other skills, a certain learning curve is natural.

  7. 7)

    Despite the differences in technique listed above, the authors did record the formation of several small bubbles of air between Descemet's membrane and deep stroma. Hence, it seems that some areas of detachment were generated, albeit not a confluent central region.

Regarding point (b) above, we diagnose a ‘big bubble’ by several characteristic features: the first indication is that the air (the blanching of the corneal stroma) spreads in a wave-like manner—like waves spreading over water when a drop falls on a calm surface—in a circular fashion. A completed bubble frequently exhibits a feathery white band at its (circular) periphery—offset, by a band of darker cornea, from the whitened region of air-insufflated stroma near the needle tip. (In some very rare cases, a ‘big bubble’ can be achieved without any air infiltrating/whitening the corneal stroma.) The anterior surface of the cornea ‘rises,’ that is, moves anteriorly as the bubble takes up some space in the central cornea. This bulging is accentuated after the performance of an anterior keratectomy.

The endoscope may certainly be able to contribute proof. This would be particularly important if the surgeon is not yet experienced with this procedure. As point (2) above indicates, however, the endoscope should only be used as a last step, that is, to confirm the presence of a big bubble. As use of an endoscope very likely compromises the outcome of the air injection, it should not be carried out too early. It should never be employed to confirm the impression that a big bubble had not yet formed. (A final consideration is that the insertion of an endoscope could prove risky in phakic eyes unless the anterior chamber first be stabilised with viscoelastic substances.)