The internal limiting membrane (ILM) is a critical structure within the eye, playing a significant role in maintaining ocular health and function. This detailed guide delves into the anatomy, function, and clinical significance of the ILM, offering valuable insights for both medical professionals and curious readers.

The ILM is the innermost layer of the retina, situated between the vitreous humor and the nerve fiber layer of the retina. Despite being only about 2-3 micrometers thick, this thin, transparent membrane is composed primarily of Müller cell footplates and a basal lamina. Its delicate structure belies its importance as a crucial barrier and scaffold within the eye.

• Composition
  • Müller Cells: These specialized glial cells extend through the entire thickness of the retina and contribute to the formation of the ILM by anchoring their end-feet to its surface. Müller cells are essential for the structural and functional support of retinal neurons, maintaining the homeostasis of the retinal environment. They also play roles in ion balance, neurotransmitter recycling, and response to injury.
  • Basal Lamina: This layer consists of an extracellular matrix (ECM) composed of collagen type IV, laminin, and other glycoproteins. The ECM provides structural integrity and serves as a substrate for Müller cell footplates. It also includes proteoglycans, which contribute to the membrane’s stability and function.
• Structure
  • Thickness and Variability: The thickness of the ILM varies across the retina, being thinnest near the fovea (the center of the macula) and thicker towards the periphery. This variation in thickness can influence surgical approaches and outcomes. The foveal ILM can be as thin as 0.5 micrometers, while in the periphery, it can be up to 3 micrometers.
  • Attachment to the Vitreous: The ILM is firmly attached to the vitreous body in younger individuals. With aging, the vitreous may detach from the ILM, leading to conditions such as posterior vitreous detachment (PVD). The attachment is mediated by collagen fibrils and proteoglycans, which weaken over time.

The ILM has several essential functions, including:

• Structural Support: The ILM acts as a scaffold, maintaining the architectural integrity of the retina. It provides a surface upon which retinal cells can adhere, ensuring proper alignment and function. This structural role is vital for the overall stability and organization of retinal layers.
• Barrier Function: The ILM serves as a selective barrier that regulates the movement of cells and molecules between the vitreous humor and the retina. This barrier function is crucial for maintaining the retinal environment’s stability and protecting retinal neurons from potentially harmful substances in the vitreous. It also prevents the migration of inflammatory cells into the retina, thus maintaining a controlled environment.
• Repair and Regeneration: Müller cells, integral to the ILM, play a pivotal role in retinal repair and regeneration. They can proliferate and migrate to injury sites, aiding in wound healing and tissue remodeling. This capacity is vital in response to retinal injuries and diseases. Müller cells also secrete growth factors and cytokines that modulate the retinal microenvironment.

The ILM is involved in various ocular conditions and is a focal point in several ophthalmic surgeries. Understanding its role is crucial for diagnosing and treating retinal diseases.

Common Disorders Involving the ILM

1. Macular Holes
   • Definition: A macular hole is a full-thickness defect in the macula, the central part of the retina responsible for sharp, detailed vision.
   • Causes: It can result from vitreomacular traction (where the vitreous pulls on the macula) or trauma. Other causes include high myopia and idiopathic origins.
   • Symptoms: Patients may experience central vision loss, visual distortion, or a dark spot in their vision. Early stages may present as blurred or wavy central vision (metamorphopsia).
   • Diagnosis: Optical coherence tomography (OCT) is the gold standard for diagnosing macular holes, providing detailed cross-sectional images of the retina.
   • Treatment: Surgical intervention often involves vitrectomy and ILM peeling to promote hole closure and restore vision. Postoperative face-down positioning can help improve outcomes by allowing the gas bubble inserted during surgery to tamponade the hole.
2. Epiretinal Membranes (ERM)
   • Definition: ERMs are fibrous tissues that grow on the surface of the retina, leading to vision distortion and blurring.
   • Causes: They can develop idiopathically (without a known cause) or secondary to conditions such as retinal detachment, inflammation, or trauma. Other risk factors include aging, diabetes, and uveitis.
   • Symptoms: Symptoms include visual distortion (metamorphopsia), blurred vision, and difficulty reading. Double vision (diplopia) and photopsia (flashes of light) can also occur.
   • Diagnosis: Diagnosis is typically confirmed with fundus examination and OCT imaging, which can show the extent and thickness of the membrane.
   • Treatment: Treatment involves vitrectomy and peeling of the ERM and ILM to relieve traction on the retina and improve visual outcomes. The decision to operate depends on the severity of symptoms and impact on the patient’s quality of life.
3. Diabetic Retinopathy
   • Definition: A complication of diabetes that affects the retinal blood vessels, leading to vision loss.
   • ILM Involvement: In advanced cases, proliferative diabetic retinopathy can lead to tractional retinal detachment, where fibrovascular tissue pulls on the ILM and retina.
   • Symptoms: Symptoms can include floaters, blurred vision, dark areas of vision, and difficulty seeing colors. In severe cases, sudden vision loss can occur.
   • Diagnosis: Fundus photography, fluorescein angiography, and OCT are used to diagnose and assess the severity of diabetic retinopathy.
   • Treatment: Treatment includes vitrectomy and ILM peeling to relieve traction and prevent further retinal damage. Laser photocoagulation and anti-VEGF injections are additional treatments to manage diabetic retinopathy.

• ILM Peeling
  • Procedure: During vitrectomy, surgeons use fine instruments to carefully peel away the ILM. This procedure is performed under high magnification and illumination. The use of microsurgical instruments and high-resolution imaging is essential for precision.
  • Purpose: ILM peeling relieves traction on the retina, which is crucial for treating conditions like macular holes and epiretinal membranes. It also stimulates retinal cells to migrate and proliferate, aiding in tissue repair. Removing the ILM can help prevent recurrence of ERMs.
  • Outcomes: Successful ILM peeling can result in significant visual improvement and reduced recurrence rates of retinal conditions. However, the procedure carries risks such as retinal tears, hemorrhage, and infection.
• Staining Techniques
  • Indocyanine Green (ICG): A dye that selectively stains the ILM, enhancing its visibility during surgery. However, concerns about its potential toxicity have led to the use of alternative dyes. ICG can cause phototoxicity and retinal pigment epithelial changes.
  • Brilliant Blue G (BBG): Another dye used to stain the ILM. BBG has a favorable safety profile and provides excellent contrast, aiding in the precise removal of the ILM during surgery. BBG has less phototoxicity compared to ICG.

Ongoing research into the ILM continues to yield new insights and improve treatment outcomes for retinal diseases. Innovations such as minimally invasive surgical techniques and advanced imaging technologies are enhancing our ability to diagnose and manage conditions involving the ILM.

• Future Directions:
  • Stem Cell Therapy: Exploring the potential of stem cells in repairing or regenerating damaged ILM and retinal tissues. Research is focused on developing methods to differentiate stem cells into retinal cells and integrate them into the existing retinal architecture.
  • Gene Therapy: Targeting genetic mutations that affect the ILM and retinal health, offering potential cures for inherited retinal diseases. Gene therapy aims to introduce, remove, or alter genetic material within retinal cells to treat or prevent disease.
  • Imaging Technologies: Enhanced imaging methods, such as optical coherence tomography (OCT), provide detailed views of the ILM, aiding in early diagnosis and precise surgical planning. Adaptive optics and OCT angiography are advancing our understanding of retinal microstructures and blood flow.

The internal limiting membrane is a vital component of the eye’s anatomy, playing a significant role in retinal health and disease. Advances in medical research and surgical techniques continue to improve our understanding and treatment of ILM-related conditions. By staying informed about the latest developments, we can better appreciate the complexities of this delicate structure and its impact on vision.

For more information on ocular health and related topics, explore our other articles and stay updated with the latest advancements in eye care.

World Eye Care Foundation’s eyecare.live brings you the latest information from various industry sources and experts in eye health and vision care. Please consult with your eye care provider for more general information and specific eye conditions. We do not provide any medical advice, suggestions or recommendations in any health conditions.