Limbal Stem Cell Transplant

Introduction

Limbus forms the border between the transparent cornea and the opaque sclera. Limbus contains pathways for the aqueous humor outflow, and it is the surgical incision site for cataracts and glaucoma.

What Is a Limbal Stem Cell?

Limbal stem cells are present in the basal epithelial layer of the limbus. These are adult stem cells that help in the renewal and maintenance of the epithelium of the cornea. The stem cells have the potential to differentiate into specific cell types.

Adult stem cells are involved in the internal repair mechanism and replace the lost cells through the wound-healing process. Corneal stem cells are located at the corneoscleral limbus. The stem cells help maintain corneal transparency.

What Is Limbal Stem Cell Deficiency?

The cornea of the eye is a highly organized, transparent tissue with cellular and non-cellular layers. The corneal epithelium helps protect the eye from external trauma. Epithelial cells are shed regularly and replaced with stem cells located at the limbus of the eye. Limbus consists of adult stem cells. Damage to the stem cells results in limbal stem cell deficiency. The absence of a healthy cornea will result in opacification and vascularization of the cornea. These alterations in the cornea lead to reduced vision and pain.

What Causes Limbal Stem Cell Deficiency?

1. Acquired Causes:

• Inflammatory / Autoimmune Diseases:

  • Steven-Johnson Syndrome - It is a rare disease of the skin and mucous membranes causing skin lesions.

  • Ocular Cicatricial Pemphigoid - An autoimmune disease affecting eyes, showing severe dry eyes and conjunctival scarring.

  • Graft Versus Host Disease - It is a systemic condition where the host immune cells attack the recipient cells. Clinically presenting symptoms include dry eyes, dry mouth, mouth soreness, rashes, shortness of breath, jaundice, nausea, and vomiting.

  • Vernal Keratoconjunctivitis - It is a severe form of allergic response causing an increased itching sensation. Other ocular changes are foreign body sensation, mucous discharge, tearing, photophobia, and burning sensation.

  • Bullous Keratopathy - An eye disease that causes blister-like swelling in the cornea. Clinical features include photophobia, blurred vision, and foreign body sensation.

• Infections:

  • Fungal infections are caused by Candida species.

  • Bacterial infections like infectious keratitis.

  • Viral infections like Herpes simplex, Varicella zoster, and adenovirus.

• Traumatic/Toxic:

  • Chemotherapy.

  • Benzalkonium chloride (for example, glaucoma medications).

  • Contact lens overuse.

  • Limbal surgery - surgery involving the limbus of the eye.

• Other Causes:

  • Ocular surface tumors/ limbal tumors.

  • Pterygium/ pseudo pterygium neurotrophic keratopathy - A degenerative disease of corneal epithelium.

2. Genetic Causes:

Associated with certain genetic disorders like

• Ectrodactyly-Ectodermal-Dysplasia-Clefting Syndrome - A rare genetic disorder with a congenital absence of fingers or toes.

• Keratitis-Ichthyosis-Deafness (KID) Syndrome - A syndrome with characteristic ocular problems, hearing loss, and skin problems.

• Xeroderma Pigmentosum - Genetic condition showing extreme sensitivity to Ultraviolet radiations.

• Turner Syndrome -A chromosomal abnormality in females, where they are born with only one X chromosome.

• Dyskeratosis Congenita - It is an inherited bone marrow failure syndrome with a triad of clinical symptoms of abnormal nails, reticular skin pigmentation, and oral leukoplakia.

3. Idiopathic Causes

• Unknown causes.

What Are the Clinical Signs of Limbal Stem Cell Deficiency?

• Decreased vision.

• Redness.

• Watering.

• Photophobia.

• Recurrent pain attacks.

• Corneal vascularization.

• Conjunctival epithelial ingrowth.

• Recurrent erosion.

• Inflammation of stroma.

• Poor corneal epithelialization.

What Is a Limbal Stem Cell Transplant?

It is a surgical procedure to replenish the ocular surface epithelium in the eye and a treatment option for limbal stem cell deficiency. Conventional treatment procedures like corneal transplants will not renew the outer epithelium of the cornea using limbal stem cells. Finally, limbal stem cell transplantation is the only successful treatment modality.

In this procedure, a donor limbal tissue is grafted into the eye. Donor tissue is taken through biopsy and grown to form an amniotic membrane. Later, this membrane, along with the stem cells, is transplanted into the eye.

What Are the Various Types of Graft Tissue?

Autologous Transplant:

In patients where one of the eyes is affected and the other healthy eye has a good population of stem cells. The stem cells are collected from the healthy eye and sent for cultivation. Thus autologous cultivated stem cell transplant is carried out.

Allogeneic Transplant:

In patients where both eyes are affected, and there is a deficiency of limbal stem cells, the limbal tissue is taken from the limbus of a close relative or a donor eye from the eye bank.

What Are the Steps in the Limbal Stem Cell Transplant Procedure?

1. Growing the Stem Cells:

The stem cells collected, either the autologous or allogeneic graft, are stimulated to grow under optimal conditions. Thus the graft is grown into an amniotic membrane. It takes about two complete weeks for the stem cells to expand and grow into a thin layer suitable for transplantation.

2. Surgical Procedure:

The donor's eye is operated under general anesthesia under aseptic techniques. In the conjunctiva, a small incision of 3 to 4 mm is made from the limbus. The undermined limbus and conjunctiva are excised.

During the transplant procedure, the cultivated stem cells and the amniotic membrane will be transplanted into the eye. The donor tissue is sutured in the correct anatomical position and sutured. After the surgery, the patient has to wear a bandage contact lens to protect the transplanted cells. Dislodging of donor tissue by eyelids has prevented the growth of epithelial tissue.

3. Post-surgical Follow-up:

After successful transplant surgery, the doctor will closely monitor the patient. Contact the doctor if there are any of the symptoms

• Decrease in vision.

• Light sensitivity.

• Pain and discomfort.

Follow-up examinations are done after the completion of the treatment. A schedule for follow-up visits is given by the procedure.

What Are the Various Types of Transplant Procedures?

Keratolimbal Allograft:

• When a cadaveric tissue is used, the conjunctiva or cornea is the stem cell carrier. This technique is rarely used.

• Surgical Technique: Tissue from the youngest donor is selected. Do not select donors above age 50. Surgery is done within 72 hours to preserve the viability of the tissue.

Advantages and Disadvantages of Keratolimbal Allograft:

Advantages:

• Readily available tissue.

• 360 ° limbal coverage is possible.

• Normal, healthy eyes are spares.

Disadvantages:

• Surgery is required Immediately.

• The viability of stem cells is in doubt.

• Increased risk of graft rejection.

• Technically difficult.

Conjunctival Limbal Allograft:

• In this procedure, the allograft is harvested from healthy eyes. It is carried out in patients with unilateral or bilateral stem cell deficiency.

• Surgical Technique -In this procedure, the donor and receipt are operated simultaneously. The donor is operated on first, immediately followed by the recipient.

• Donor tissue preparation - harvesting of the donor tissue is done under peribulbar or topical anesthesia. Donor tissue is stored in a balanced salt solution or Ringer's lactate solution before transplantation.

Advantages and Disadvantages of Conjunctival Limbal Allograft:

Advantages:

• Donor tissue with more viable stem cells.

• Minimal stromal tissue is harvested.

• Reduced risk of graft rejection.

• Technically easier procedure.

Disadvantages:

• Risk of iatrogenic limbal stem cell deficiency.

• Transplantation of vascularised tissue might increase the risk of rejection.

Cultured Corneal Epithelial or Limbal Stem Cell Transplantation:

• Currently, it is a promising technique for limbal transplantation. In this technique, stem cells are cultured from a small amount of tissue, thus minimizing the damage to the donor's surface tissues.

• Surgical Technique- The cultured corneal epithelial grafts are mounted on petrolatum gauze or bandage soft contact lenses. The de-epithelialized amniotic membrane is used as a carrier.

• This promotes epithelialization and reduces inflammation and scarring of the donor tissue. It also serves as a natural substrate for the limbal stem cells to grow and proliferate.

• Also, the use of a carrier has made the handling of donor tissue easier. Another important advantage is it reduces the immune response and produces antiangiogenic proteins.

What Are the Complications of a Limbal Stem Cell Transplant?

• Intraoperative complication includes damage to the eye muscle.

• Epithelial breakdown.

• Corneal perforation.

• Severe inflammation.

• Eyelid abnormality.

• Contact lens-induced keratopathy.

• Limbal graft rejection.

Conclusion

Limbal stem cell transplantation is an effective method in people with limbal stem cell deficiency. The outcome of the transplant surgery depends on certain factors. Underlying ocular pathology can affect the success rate of transplant surgery. In patients with abnormal tear formation, limbal stem cell transplantation is poor. Immunosuppression is important in patients undergoing limbal transplant therapy.