Schlemm's canal, also known as the canal of Schlemm, and as the scleral venous sinus, is a circular

lymphatic Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the origi ...

-like vessel in the

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

. It collects

aqueous humor The aqueous humour is a transparent water-like fluid similar to blood plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the po ...

from the

anterior chamber The anterior chamber ( AC) is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. Hyphema, anterior uveitis and glaucoma are three main pathologies in this area. In hyphema, ...

and delivers it into the episcleral blood vessels. Canaloplasty may be used to widen it.

Structure

Schlemm's canal is an endothelium-lined tube, resembling that of a lymphatic vessel. On the inside of the canal, nearest to the aqueous humor, it is covered and held open by the

trabecular meshwork The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the ...

. This creates outflow resistance against the

Development

While Schlemm's canal has generally been considered as a vein or a scleral venous sinus, the canal is similar to the lymphatic vasculature. It is never filled with blood in physiological settings as it does not receive arterial blood circulation. Schlemm's canal displays several features of lymphatic endothelium, including the expression of PROX1, VEGFR3,

CCL21 Chemokine (C-C motif) ligand 21 (CCL21) is a small cytokine belonging to the CC chemokine family. This chemokine is also known as 6Ckine (because it has six conserved cysteine residues instead of the four cysteines typical to chemokines), exodus- ...

FOXC2 Forkhead box protein C2 (FOXC2) also known as forkhead-related protein FKHL14 (FKHL14), transcription factor FKH-14, or mesenchyme fork head protein 1 (MFH1) is a protein that in humans is encoded by the ''FOXC2'' gene. FOXC2 is a member of the ...

, but lacked the expression of LYVE1 and PDPN. It develops via a unique mechanism involving the transdifferentiation of venous endothelial cells in the eye into lymphatic-like endothelial cells. This developmental morphogenesis of the canal is sensitive to the inhibition of lymphangiogenic growth factors. In adults, the administration of the lymphangiogenic growth factor

VEGFC Vascular endothelial growth factor C (VEGF-C) is a protein that is a member of the platelet-derived growth factor / vascular endothelial growth factor (PDGF/VEGF) family. It is encoded in humans by the ''VEGFC'' gene, which is located on chromosom ...

enlarged the Schlemm's canal, which was associated with a reduction in

intraocular pressure Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated t ...

. In the combined absence of

angiopoietin 1 Angiopoietin 1 is a type of angiopoietin and is encoded by the gene ANGPT1. Angiopoietins are proteins with important roles in vascular development and angiogenesis. All angiopoietins bind with similar affinity to an endothelial cell-specific ty ...

and

angiopoietin 2 Angiopoietin-2 is a protein that in humans is encoded by the ''ANGPT2'' gene. Naturally occurring antagonist for both ANGPT1 and TIE2; expressed only at the sites of vascular remodeling; similar to angiopoietin-1 Function * See Angiopoietin#C ...

, Schlemm's canal and episcleral lymphatic vasculature completely failed to develop.

Function

Schlemm's canal collects

from the

. It delivers it into the episcleral blood vessels via aqueous veins.

Clinical significance

Canaloplasty

Canaloplasty is a procedure to restore the eye’s natural drainage system to provide sustained reduction of

. Micro

catheter In medicine, a catheter ( ) is a thin tubing (material), tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgical procedure. ...

s are used in a simple and minimally invasive procedure. A surgeon will create a tiny incision to gain access to Schlemm's canal. A microcatheter circumnavigates Schlemm's canal around the iris, enlarging the main drainage channel and its smaller collector channels through the injection of a sterile, gel-like material called

viscoelastic In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both Viscosity, viscous and Elasticity (physics), elastic characteristics when undergoing deformation (engineering), deformation. Viscous mate ...

. The catheter is then removed and a suture is placed within the canal and tightened. By opening Schlemm's canal, the pressure inside the eye is relieved.

Laser trabeculoplasty

trabeculoplasty Trabeculoplasty is a laser treatment for glaucoma. It is done on an argon laser equipped slit lamp, using a Goldmann gonioscope lens mirror. Specifically, an argon laser is used to improve drainage through the eye's trabecular meshwork, from which ...

is a modification of the

Laser trabeculoplasty Trabeculoplasty is a laser treatment for glaucoma. It is done on an argon laser equipped slit lamp, using a Goldmann gonioscope lens mirror. Specifically, an argon laser is used to improve drainage through the eye's trabecular meshwork, from which ...

(LTP) is the application of a laser beam to burn areas of the trabecular meshwork, located near the base of the iris, to increase fluid outflow. LTP is used in the treatment of various open-angle glaucomas. The two types of laser trabeculoplasty are ''Argon Laser Trabeculoplasty'' (ALT) and ''Selective Laser Trabeculoplasty'' (SLT). As its name suggests, Argon laser trabeculoplasty uses an

Argon laser An ion laser is a gas laser that uses an ionized gas as its lasing medium. Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike helium–neon lasers, the ...

to create tiny burns on the trabecular meshwork. Selective laser trabeculoplasty is newer technology that uses a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

to target specific cells within the trabecular meshwork and create less thermal damage than ALT. SLT shows promise as a long-term treatment."Review of Ophthalmology: SLT for Glaucoma Threapy"
Selective Laser Trabeculoplasty In SLT, a laser is used to selectively target the

melanocytes Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and hea ...

in the trabecular meshwork. Though the mechanism by which SLT functions is not well understood, it has been shown in trials to be as effective as the older Argon Laser Trabeculoplasty. However, because SLT is performed using a much lower power laser, it does not appear to affect the structure of the trabecular meshwork (based on electron microscopy) to the same extent, so retreatment may be possible if the effects from the original treatment should begin wear off, although this has not been proven in clinical studies. ALT is repeatable to some extent with measurable results possible. If SLT is successful, it will stimulate metabolic change in pigmented cells in the trabecular meshwork. This causes an immune response which, in turn, clears the meshwork channel/drain of cellular build up. This allows more aqueous humour to flow into Schlemm's canal from the anterior cavity, reducing the intraocular pressure and therefore lowering the risk of glaucoma, or further damage to the optic nerve, due to overpressure in the eye.

Angiopoietin deficiency

Schlemm's canal and episcleral lymphatic vasculature completely failed to develop in the combined absence of

and

. This causes

buphthalmos Buphthalmos (plural: buphthalmoses) is enlargement of the eyeball and is most commonly seen in infants and young children. It is sometimes referred to as buphthalmia (plural buphthalmias). It usually appears in the newborn period or the first 3 mo ...

and

glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

Glaucoma

Glaucoma may be related to drainage of aqueous humor from the eye. This is mostly related to resistance on the internal surface of Schlemm's canal.

History

Schlemm's canal is named after

Friedrich Schlemm Friedrich Schlemm (11 December 1795 – 27 May 1858) was a German anatomist who was professor at the University of Berlin. He was born in Salzgitter. As his family could not afford higher education, he was apprenticed to a barber-surgeon in Brauns ...

(1795–1858), a

German German(s) may refer to: * Germany, the country of the Germans and German things **Germania (Roman era) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizenship in Germany, see also Ge ...

anatomist Anatomy () is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old scien ...

Additional images

Gray870.png, Enlarged general view of the iridial angle. (Labeled with older label of 'sinus venosus scleræ' at center top.)

References

External links

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DiagramDiagram
{{Authority control Human eye anatomy Anatomy named for one who described it