EYE TRAUMA

 

Overview

For a person that suffers severe eye trauma as a result of a chemical or thermal burn, the resulting scar tissue in the cornea can be a cause of acute visual impairment, including legal blindness. As little as five years ago, there was little chance that an accident victim's sight could be restored. The only option would have been a corneal transplant, which has a very high rate of failure in a patient whose eye(s) have no functional natural repair mechanisms. [1]

Presently limbal stem cell transplants are used to restore the natural re-epithelialization of an injured eye. Limbal stem cells, and the cells into which they evolve, transiently amplifying cells (TAC) remain in a proliferating state without evidence of differentiation. The corneal epithelium is then maintained by the migration of these cells from the limbal region, along the periphery of the cornea, to the central surface of the cornea where they become terminally differentiated. If this process of migration and differentiation proceeds uninhibited, the ocular surface remains transparent and smooth.

Damage to Existing Limbal Stem Cells

In a case where the corneal-limbal epithelium is damaged or completely lost, the surface of the eye is re-epithelialized by bulbar conjuctival cells. Bulbar conjunctiva is continuous with the cornea and composes the epithelial layer that covers the visible white of the eye, outside of the iris. These cells represent a completely different cell line and conjunctival re-epithelialisation of the cornea leads to scarring, decreased visual acuity, and significant pain.

Limbal Stem Cell Transplantation

The transplant procedure was first performed by Harvard researcher, James Zieske, in 1989 and has subsequently been more carefully studied and refined. [2] The basics of the transplantation involve the removal of the damaged cornea from the injured eye with a surgical knife. A 1 millimeter square portion of limbal tissue is grafted from the patient's healthy eye, and is placed onto the prepared surface of the injured eye. The graft is covered with a protective soft lens and the eye is placed under gauze for 3 days. Artificial tears and anti-microbial washes are applied twice a day for 10 days following the procedure. Within 2 weeks, the protective lens is removed and re-epithelialisation by limbal stem cells has resumed. [3]

 

While 60% of patients will show a significant improvement in vision compared to preoperative measures, the patient's vision is not fully restored. [4] At this point the recipient of the limbal graft has functional limbal stem cells and subsequent re-epithelialisation of the cornea, yet the corneal surface is still misshapen. Approximately 6 months after the limbal graft, the ocular surface is stabilized to the point where vision can be fully restored with the transplantation of a smooth corneal surface, clinically known as keratoplasty.

New Developments

Several variations to the procedure have been studied over the last decade and significant improvements have been made. Researchers have focused on finding ways to transplant more limbal tissue so that the patient receives more stem cells during the procedure. Other issues involve avoiding transplant rejection through the use of immune suppressing drugs. [5]

The cultivation of autologous corneal-limbal epithelial cells is related to both of the aforementioned topics. The restoration of the corneal surface with autologous corneal epithelial sheets produced by serial cultivation of limbal cells serves as a method to improve limbal stem cell transplants. The use of cultured stem cells assures that an ample number of cells are available for transplantation. Also, in the case of severe bilateral lesions, in which an autologous transplant is not an option, the use of cultured cells provides a route through which a patient can easily receive a graft of limbal tissue.

The cultivation process involves biopsy samples of limbal tissue from a healthy eye that have been minced and treated with trypsin (0.05%). The cells were plated on Dulbecco- Vogt Eagle's and Ham's media in the presence of several amino acids, enzymes, and growth factors at 37 degrees celsius. The cultures showed confluent growth in approximately five days, and could be used effectively for transplantation. [6]

Japanese scientists have worked on other refinements to the procedure. Current areas of study involve optimizing the dosage and duration of immuno-suppressive therapy, and reducing the effects of "dry-eye" by making eye drops from a patient's own tears. [7]