limbus Flashcards
(14 cards)
What is the function of the limbus in the eye?
The limbus is the transitional zone between the cornea and sclera.
It is crucial for aqueous humour outflow, with obstruction potentially leading to glaucoma.
What anatomical structures are found at the limbus?
Canal of Schlemm (CS)
Trabecular meshwork (TM)
Scleral spur (SS)
Ciliary muscle (CM)
How do the corneal layers transition at the limbus?
Corneal epithelium merges with conjunctival epithelium.
Bowman’s layer ends at the limbus.
Stroma merges with the sclera.
Descemet’s membrane ends at Schwalbe’s line (SL).
Endothelium lines the trabecular meshwork.
What is the structure and role of the trabecular meshwork (TM) at the limbus?
The trabecular meshwork is a sieve-like structure that facilitates the passage of aqueous humour.
With age, trabeculae thicken and debris can accumulate, potentially leading to primary open-angle glaucoma (POAG).
How does aqueous humour flow through the Canal of Schlemm (CS)?
The aqueous fluid is pushed from the trabecular meshwork into the Canal of Schlemm by intraocular pressure (IOP).
Most of the fluid passes through the endothelial cells via giant cytoplasmic vacuoles via passive transport, and a small amount is transported by pinocytosis or active transport.
What is the drainage route for aqueous humour from the Canal of Schlemm?
Aqueous drains from the Canal of Schlemm (CS) into collector channels (CC).
It then flows into the deep scleral plexus (DSP), intrascleral plexus (ISP), and episcleral plexus (ESP), and ultimately into the anterior ciliary veins and aqueous veins (AV).
What are the two main pathways for aqueous humour outflow?
Trabecular outflow (80-90%): Passes through the trabecular meshwork and Canal of Schlemm.
Uveoscleral outflow (10-15%): Passes through the ciliary body surface into the choroid veins.
Iris route: A small amount drains via the iris surface.
How do anti-glaucoma drugs and surgical procedures work to increase aqueous outflow?
Anti-glaucoma drugs: Prostaglandin analogs increase uveoscleral outflow.
Surgical procedures:
Laser trabeculoplasty: Opens the trabecular meshwork with a laser.
Trabeculectomy: Removes a small part of the trabecular meshwork to improve drainage.
How is the drainage angle assessed during a slit-lamp examination?
The Van Herrick’s method compares the ratio of corneal thickness (C) to the gap (G) between the back of the cornea and the front of the iris.
Angle open: Ratio of 1: ≥ 0.5
Angle closure: Ratio of 1: < 0.5
What is the eclipse method (pen-torch shadow method) for assessing the drainage angle?
A pen-torch is held at 90º to the eye.
More shadow (eclipse) occurs in shallow drainage angles, helping to assess the openness of the angle.
What is gonioscopy, and how is it used to assess the drainage angle?
Gonioscopy involves using a gonioscopic lens placed on the cornea to visualize the drainage angle.
The trabecular meshwork should be visible for an open angle.
If the trabecular meshwork is obscured, there is a risk of angle closure. If no structures are seen, angle-closure glaucoma may be present.
What clinical signs can indicate angle-closure glaucoma during gonioscopy?
Trabecular meshwork obscured: Indicates risk of angle closure.
No structures visible: Indicates angle-closure glaucoma.
What are the common treatment methods for glaucoma that improve aqueous outflow?
A:
Laser trabeculoplasty: Opens the trabecular meshwork to improve aqueous outflow.
Trabeculectomy: Surgical removal of a portion of the trabecular meshwork to enhance drainage.
What is the clinical significance of assessing the limbus in optometry?
The limbus is vital for aqueous humour drainage, and obstructions here can lead to glaucoma.
Evaluation of the drainage angle is key in detecting angle-closure glaucoma. Tests like Van Herrick’s, eclipse method, and gonioscopy are essential.
