Fig. 10.1
(a) Peripheral tears can be removed by radial pulling of Descemet’s membrane to the periphery (red dotted arrow) on one side of the tear. (b) This will turn the tip of the tear into a rounded edge
Fig. 10.2
In larger radially directed tears, two forceps can be used to pull Descemet’s membrane from both sides of the tear in parallel (red dotted arrow) without further tear enlargement. Before approaching the tear, Descemet’s membrane should be stripped close to the tip of the tear by pulling almost orthogonally to the tear direction (green dotted arrow). The advantage of the two forceps technique is that no additional loss of Descemet’s membrane will result
In general stripping close to the tear needs intense observation. Further enlargement of the tear during pulling has to be excluded. Tangential pulling will enlarge the tear and must be avoided.
Even grafts which have been completely torn into two pieces can be used for transplantation as long as a certain amount of donor endothelial cells is preserved [10]. However, torn DMs are by far more difficult to graft and overlapping of DM from the same tissue can occur reducing the total number of viable grafted corneal endothelial cells.
Peripheral tears barely cause problems once the different techniques how to approach these tears have been adopted. Central tears are more challenging to manage since one has to avoid further damage of this central tissue which will result in endothelial cell damage right in the centre of the optical zone. To strip DM with central adhesions, stripping has to be stopped as soon as tearing is recognised. Stripping should then be proceeded from the two sites orthogonal to the initial stripping direction. When approaching the adhesive zone, DM still attached to the corneal stroma has to be grasped with a forceps to avoid further enlarging of the tear. Technically, implantation and unfolding of grafts with central tears do not differ from intact grafts. However, the graft should be placed with the defect as peripheral as possible preferable towards the 12 o’clock position. Depending on the graft preparation technique, DM becomes trephined before stripping is completed. Trephination can be performed eccentrical to shift the central tear towards the periphery of the graft (Fig. 10.3).
Fig. 10.3
(a) If a high number of tears approaching the trephination line (red dotted circle) or centrally located tears occur, eccentrical trephination (b; red dotted circle) results in a sharp outline of the graft without tears in the peripheral edge and can also shift a central tear towards the periphery of the graft
10.3 Intraoperative Complications
A smooth course of DMEK surgery can be hindered by factors of which many of them can be identified preoperatively already. Small, hyperopic eyes with high vitreous pressure offer only little space for unfolding manoeuvres after graft implantation. A cornea appearing cloudy at the slit lamp is in general even less transparent under the surgery microscope. Anterior segment abnormalities, e.g. in patients with a history of glaucoma drainage device implantation, in aphakic eyes or in eyes with larger iris defects or anterior synechia increase the risk of having difficulties to unfold and place the graft correctly.
These factors should be taken into account when planning the surgery. The first decision to be made is whether the risk of complications outperforms the benefits of DMEK instead of DS(A)EK. Frequently patients with complex anterior segment disorders are on a higher risk for graft rejections which are barely seen after DMEK (1 % within 2 years) but occur more frequently after DS(A)EK (15-fold higher) [11]. Moreover, the functional results and the healing process are more favourable making DMEK the method of choice in most patients [12]. However, surgeries on higher risk for intraoperative complications are more likely to result in higher endothelial cell loss due to mechanical stress and are thus on a higher risk of (primary) graft failures. With careful planning of the surgery and with growing experience of the surgeon, even corneal endothelial diseases in the face of complex anterior segment disorders can be successfully treated with DMEK.
When high vitreous pressure is expected, the following measures should be considered:
Oculopression before surgery
Administration of drugs with lowering effect on vitreous pressure (acetazolamide, mannitol)
Low systolic and mean arterial blood pressure during surgery
Cataract surgery in combination with DMEK (Triple-DMEK) in phakic eyes [2]
Eyes with a small corneal diameter and a shallow anterior chamber give little space for unfolding the graft. Therefore, adapting the graft size to the anterior chamber depth and the corneal white-to-white distance facilitates the unfolding process. To the authors’ experience, graft diameters of as little as 7 mm can be helpful for DMEK in very small eyes. Intracameral unfolding of the DMEK graft can also be hampered by anterior synechia or by intracameral tubes from drainage devices [13]. In such cases the mechanical barrier should be removed by lysis of the anterior synechia or surgical shortening of the intracameral tube.
Not only small eyes but also very large eyes or eyes after vitrectomy with a deep anterior chamber without vitreous pressure can make unfolding of the DMEK graft difficult. Once the graft is partially unfolded by external manoeuvres or by an intracameral air bubble, the graft will spontaneously form a roll again in case the anterior chamber is very deep (Fig. 10.4a). In an eye where the anterior chamber depth can be adjusted and flattened by fluid release, the peripheral parts of an unfolded graft are trapped between iris and cornea which keeps the graft unfolded and prevents it from spontaneous rolling (Fig. 10.4b). In eyes after vitrectomy or large eyes with very deep anterior chambers, the donor size should be enlarged (depending on the recipient’s diameter of the cornea), and again, corneas from an older donor should be used since the rolling behaviour in these grafts is much less pronounced.
Fig. 10.4
(a) Very large eyes or eyes after vitrectomy with a deep anterior chamber without vitreous pressure can make unfolding of the DMEK graft difficult. After partial unfolding of the graft (e.g. by an intracameral air bubble), the graft will almost always spontaneously form a roll again. (b) In normal-sized eyes with normal vitreous pressure, anterior chamber depth can be adjusted and flattened by fluid release. Thus, the peripheral parts of an unfolded graft can be trapped between the iris and the cornea which keeps the graft unfolded and prevents it from spontaneous rolling
If during DMEK surgery, unexpected rolling behaviours are noted in pseudophakic eyes which have been operated on before elsewhere, care should be taken to rule out vitreous prolapse and such prolapse should be removed to avoid incarceration between the recipients’ cornea and the DMEK role.
Correct unfolding and positioning of the graft is substantially influenced by the transparency of the recipient’s cornea. Graft orientation during unfolding in eyes with cloudy corneas can easily be lost. Certain techniques have been described to confirm the orientation of the unfolding process.
Orientation marks are an easy measure to confirm the orientation not only during the surgery but also thereafter during the postoperative course [14]. Three marks can be set at the margin of the graft using a 1-mm trephine during graft preparation of which two of them are set close together and the third one is placed with little distance. This results in a clockwise order of the marks of 1–2 when the graft is seen from the endothelial site. The order changes when the graft is turned which is the case if the graft is placed correctly at the backsite of the corneal stroma, and it is seen from the epithelial site. Here the clockwise order of the marks is 2–1, meaning that one mark is placed with little distance to the other two marks which sit closer together (Fig 10.5).
Fig. 10.5
Orientation marks set by small trephinations of the edge of Descemet’s membrane help to identify the graft orientation. (a) Two marks are set close together and the third one is placed with little distance. This results in a clockwise order of the marks of 1–2 when the graft is seen from the endothelial site. (b) The order changes if the unfolded graft is directed correctly inside the anterior chamber when it is seen from the epithelial site. Here the clockwise order of the marks is 2–1, meaning that one mark is placed with little distance to the other two marks which sit closer together (Reproduced with the kind permission of Elsevier Inc. from Ref. [14])
To confirm the correct orientation of a partially unfolded graft with curled edges, the tip of a cannula can be placed between graft and recipient’s cornea. When the cannula is moved from the centre of graft into the peripheral roll, one can recognise the blue shimmer of the stained graft. In contrast, when the graft orientation is false, the tip of the cannula cannot be placed inside the roll when starting the movement from the centre since the roll opens downwards (Moutsouris sign) [15]. A (handheld) slit beam is another option to visualise the graft orientation by identifying the direction towards the rolled graft edges directed [16].
Novel microscopes offer online OCT imaging during surgery. Hereby, all steps of preparation and implantation can be visualised even in very cloudy recipient corneas [17]. These devices not only help to identify orientation and localisation of the graft but also help to visualise the remaining amount of fluid after gas tamponade.
Intraoperative bleeding after iridectomy during DMEK is a rare but possibly serious complication which can lead to early termination of the surgery. An iridectomy should therefore always be performed before the recipient’s DM stripping. Otherwise, massive corneal edema will be the result in case the surgery has to be finished after stripping and before correct graft placement. To avoid intraoperative bleeding, an iridotomy with a Nd:YAG laser can be placed (at 6 o’clock position) before surgery. Even if the iridotomy is too small at the beginning of the surgery, widening of the opening by blunt spreading with a microscissor offers a reduced and only minimal risk of bleeding.
10.4 Postoperative Complications
DMEK-specific early postoperative complications comprise a rise in intraocular pressure (IOP) due to gas tamponade, graft detachment and primary graft failure.
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