Limbus

Limbus, m. graduated eircle or arc. Hmbi Limes, m. Hmit. 

The cornea is avascular and is thus immunologically privileged to some extent. Inflammatory cells and antibodies can only enter the cornea from the corneoscleral limbus or from the aqueous. However, if inflammation occurs, the almost inevitable result is opacification of the cornea with visual deterioration, which is frequently permanent. 

A new antiviral agent, developed for treatment of CMV retinitis, can be administered by intravitreal injection. Formivirsen sodium is a phosphorothioate oligonucleotide that inhibits CMV replication through an antisense mechanism. It is formulated as a sterile and preservative-free solution and supplied in single-use vials (Vitravene ). The product is administered directly into the vitreous cavity posterior to the limbus through a 30-gauge needle. This procedure can be performed on an... 

Vasoconstriction and pupil dilatation Topical A drop of a suitable topical anesthetic maybe applied, followed in a few minutes by 1 drop of the phenylephrine HCl 2.5% on the upper limbus. 

A new systematic approach started with the classihca-tion of eye bums introduced by Roper Hall in 1978, giving hints from the initial clinical presentation to the later outcome. The modihed classihcation from Reim rehected the observation that limbus affection and circulation in the limbus area in the early posttraumatic phase gives major information on the later outcome of eye bums. 

With a size of about 1.3 cm and an average diameter of 11.5 mm in adulthood, the cornea has an ovoid shape with a horizontal axis on its front side and is circular on its back side. It is 0.5-mm thick at the center and 1-mm thick on the edge. As shown by modem measurements, the shape of the cornea can differ for different individuals. On its edge there is the limbus that partly has the same characteristics and ensures the junction with the totally opaque sclera, which surrounds the entire eyeball. 

The stem cells of the basal cells are located at the level of the limbus, which come from centripetal migration. The daughter cells migrate to form the intermediate cells. Their cytoplasm, which is rich in glycogen and mitochondria, shows their high metabolic activity. It also contains a Golgi s apparatus, some microtubules, and some keratin filaments connected to each other by desmosomes and hemidesmo-somes. Most of all this cytoplasm contains some actin... 

The corneal stroma also contains Schwann cells, surrounding the corneal axons, and immunocompetent cells (T andB lymphocytes, monocytes, and Langerhans cells). These latter cells are very numerous at the level of the limbus close to small vessels, but there are also a few of them at the level of the one third at the front of the central corneal stroma [4]. 

The limbus is a very important zone when an eye bum occurs it is the region in which concentrate the stem cells that might be destroyed by the bum. 

The ability of the cornea to regenerate (including endothelium and epithelium) is thus concentrated at the level of the limbus. This has clinical consequences already foreseen by Hughes as early as 1946 [7] when he made Umbo-conjunctival ischemia the main forecasting sign of the chemical eye bum (check cUnical considerations in Chap. 7). 

These vessels end in arcade-like structures at the limbus. The corneal stroma is made of three different main layers that differ in density of collagen and type of packing. The Bowman s membrane of the anterior stroma is part of the basal membrane of the corneal epithelium and accounts for 5% of the thickness of the central 500-600 pm cornea. The corneal stroma consists of highly ordered, horizontally organized and noninterconnected coUagen I and X fibriUae that are kept in a hydrated state with a water content of 72-78% and an osmolarity of 420 mOsmol/kg [1]. 

There are major key issues on stem cells that explain poor prognosis of Grade HI bums including ischemia of the limbus to an extent of more than 50%, according to the classification of Reim [45]. The overall concept of stem cell survival on the cornea was pubhshed by Tseng [46]. This concept gives a clear insight that survival of stem cells is cmcial for successful comeal reepithehalization. 

The perilimbal ischemia is the breaking of the conjunctival and episcleral vessels resulting in a plus or minus spread white avascular areas around the edge of the limbus (Fig. 7.3). 

Fig. 7.3 The limbus is the junction between the transparent cornea and the conjunctiva (vascular zone)...

The limbus is the anatomic junction between the transparent cornea and the conjunctiva, a tissue in which the vessels circulate. At this level, there would be the limbal stem cells, cells generating the differentiated epithelial cells of the cornea. The essential property of the cornea is transparency. The seriousness of the ocular bum consists in the loss of the comeal transparency. Actually the limbus is a real barrier to the conjunctiva. In the following months, a serious ocular bum will result in the development of a conjunctival cover leading to a loss of vision. 

A bum is very serious with reserved prognosis when ischemia spreads on more than half of the limbal circumference (Fig. 7.6). Most of the times, this ischemia predominates on the lower half of the limbus, because it is where the chemical, in general a liquid, concentrates. It is necessary to emphasize that the maximum quantity of liquid which can cover the surface of the eye is 200 pL. 

In severe ocular bums with complete loss of the limbal vascularization, other than the predictable impossibility of secondary re-epithelialization, there is an immediate risk of necrosis for the anterior segment. In order to restore the limbal circulation and to block the evolution towards a necrosis or an aseptic ulceration, a Tenon s plastics may be realized. It consists in the making of a Tenon s advancement flap located at the level of the limbus [4-8]. The intervention must be realized as soon as the necrotic tissues have been removed. The dissection starts in the equatorial region and continues at the back of the conjunctival sacs. The flaps must be 1-2 mm thick. Their elastic consistency helps their advancement. The flap is sutured to the... 

The limbus autograft requires a good quality corneal reepithelialization for 75-100% patients and the constitution of a barrier preventing the neovascular cicatricial phenomena of conjunctival origin [18, 21, 22]. The date of intervention from the date of bum is a subject of argument. Most authors consider that it is better to wait several months for the inflammatory reaction to decrease. However, some authors recommend an earlier intervention, before the development of complications due to the LSC deficit [16, 23]. 

A TK does not provide any LSC therefore, it does not suffice to treat extended Umbal ischemia. It must be coupled with a limbus transplantation [46]. 

A TK can be practiced in the same operatory step as a limbus allograft. The intervention begins with a Um-bal peritectomy, followed by the TK [47]. However, the epithelial cicatrization and the comeal transparency are better when the TK is secondarily realized (from 1 to 13 months). The endothelial rejection is less important too 0% against 53% for a transplantation occurring in the first month [18]. 

An auto-TK coupled with a limbus autograft may exceptionally be practiced as illustrated in Figs. 8.1-8.4. 

Fig. 8.4 Left eye. One month postsurgery. Clear right cornea sutured with 16 separated stitches of 10/0 nylon. 360° limbus autograft taken from right eye and sutured with 8 separated stitches of 10/0 nylon on the cornea and with 8 separated stitches of 8/0 vicryl on the conjunctiva...

The big diameter LK was first introduced in the year 2000 by Vajpayee [50] for a use in the surgical treatment of sequelae due to comeal bums. Vajpayee has recorded the results of nine ocular operations. The intervention begins with a conjunctival peritectomy over 360°. The conjunctiva is reclined backwards. The recipient cornea is trepanned to a 12-13 mm diameter and to a 300 pm depth. The lamellar graft is sampled via a trepanation at 1.5 mm back to the limbus in order to include LSC. It is then sutured by 24 10/0 nylon stitches. Despite the limbus allograft, no immunosuppressive therapy has been prescribed. The operation was practiced about 30 months after the occurrence of the bum. Results are recorded after a 7.4 month observation. The visual acuity has improved in six cases. No recurrence of the comeal neovascularization and no... 

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