ANATOMY OF ANGLE OF EYE

In this article we discuss in detail about the anatomy of the anterior chamber and the angle of the eye. If you are an audio visual learner you can visit my video on the anatomy of anterior chamber here. Also you may refer to the text below as a read along notes for the same. Knowledge about the angle structures is extremely important before once proceeds to gonioscopy.

What is Anterior Chamber ?

  • Anterior chamber is an angular space bounded anteriorly by the posterior (inner) surface of the cornea and posteriorly by the anterior surface of the iris and a part of ciliary body.
  • The peripheral most part of the anterior chamber where the cornea and the iris meet forms the Angle of the eye.
image showing the anterior chamber, posterior chamber and vitreous in the eye

Salient Features about the Anterior Chamber

•The Average Depth of the anterior chamber is about 3 mm 
•It contains 0.25 ml of the aqueous humour.
•Anterior chamber depth is shallower in the hypermetropic eye than the myopic eye.
•It is also shallower in children and in older people.

CAUSES OF SHALLOW ANTERIOR CHAMBER 

  • HYPERMETROPIA
  • ANGLE CLOSURE DISORDERS 

DEEP ANTERIOR CHAMBER 

  • PSEUDOPHAKIA
  • APHAKIA
  • MYOPIA
image showing comparison of deep and shallow anterior chamber

Contents of Anterior Chamber : Aqueous Humour

  • The aqueous humour is transparent fluid similar to blood plama but containing low protein and glucose  and high ascorbic and lactate concentrations.
  • The differences between the plasma and aqueous humor have been summarised below in the schematic. 
  • Water constitutes more than 99.9% of the total constituents of aqueous humor 
  • It is secreted from the Ciliary processes of the ciliary body. We shall discuss more about it in Aqueous humour dynamics .
image depictig aqueous humor composition v/s blood plasma composition

What is Angle of Eye?

  • The peripheral most part of the anterior chamber where the cornea and the iris meet forms the Angle of the eye.
  • Since the eye is more like a sphere, the angle is not a two dimensional but a three dimensional structure.
image depicting the angle of the eye and its structure

What STRUCTURES form the Angle?

From anterior to posterior ,the angle consists of the following structures: 

•SCHWALBES LINE
•ANTERIOR TRABECULAR MESHWORK
•POSTERIOR TRABECULAR MESHWORK
•SCLERAL SPUR
•CILIARY BODY BAND
•ROOT OF THE IRIS

magnified view of angle structures, namely schwalbes line, trabecular meshwork, scleral spur,,ciliary body band and iris root

Root of Iris/ Peripheral Iris

We will start our discussion with the most posterior structure i.e the root of the iris.(It is much easier to understand the structures when studied from posterior to anterior).

  • Root of the iris is the point where the iris inserts into the ciliary body.
  • The iris inserts at variable levels on the anterior face of the ciliary body.
  • The iris is THINNEST at this part.
  • The Root of Iris in Asians is thicker and therefore mydriasis leads to crowding of the angle in asians due to bunching up of the thicker iris root.
image showing the structure of iris and iris root

CLINICAL NUGGET : IRIDODIALYSIS

  • Disinsertion of the iris root is known a iridodialysis/coreodialysis.
  • Most common cause is trauma.
  • Could also be iatrogenic following a cataract surgery.
  • It leads to a “D” shape pupil configuration.
  • Smaller superior iridodialysis are usually asymptomatic, as they get covered by the upper eyelid.
  • Inferior, temporal location of iridodialysis cause symptoms like glare, photophobia and monocular diplopia.
image showing IRIDODIALYSIS and D shaped pupil in iridodialysis

Ciliary Body Band (CBB)

  • The ciliary body is attached to the sclera.
  • The iris inserts into the anterior surface of the ciliary body and the part of the ciliary body between root of iris & scleral spur is known as ciliary body band or the ciliary band.
  • There is a potential space between the ciliary body and the sclera is known as the supraciliary space.

CLINICAL NUGGET : ANGLE RECESSION

  • The ciliary body consists of three types of muscle fibres; circular, longitudinal and radial.
  • Following blunt trauma, there can be a split between the circular and longitudinal muscle fibres. This leads to apparent expansion of the ciliary body band on gonioscopy.
  • This split seen as a broadened ciliary body band zone on gonioscopy is known as the ANGLE RECESSION. 
  • Angle recession interferes with the normal outflow of aqueous leading to increased intraocular pressure. when this occurs, the pathology is called as angle recession glaucoma.
image showing enlarged ciliary body band in angle recession

CLINICAL NUGGET : CYCLODIALYSIS

  • Disinsertion of the ciliary body from the sclera is known as cyclodialysis.
  • This creates an alternate pathway for aqueous drainage by creating direct communication between the aqueous humor and the suprachoroidal space, leading to excessive filtration. 
  • This causes the intraocular pressure to fall down
  • Therefore in cyclodialysis, the IOP is usually low.(HYPOTONY)
  • Most common cause is blunt trauma.
  • Could also be iatrogenic following excessive manipulation during a cataract surgery.
image showing Cyclodialysis cleft

Scleral Spur (SS)

To understand the angle of the eye it is very important to understand the concept of scleral spur. It is very important to be able to visualize it for a better understanding.

  • The junction between the cornea and conjunctiva is known as the LIMBUS externally 
  • Internally however there is a inward groove present corresponding to the limbus 360 degrees. This groove or sulcus is known as the SCLERAL SULCUS.
  • The posterior wall of the sulcus has a projection/spur which is known as the SCLERAL SPUR.
  • Remember again that all these structures are present 360 degrees
  • Scleral spur is seen as a white line on gonioscopy
image showing scleral spur, scleral sulcus and trabecular meshwork

Trabecular Meshwork (TM) and Schlemms Canal

  • The scleral sulcus(grove) is covered by a sieve like structure known as a trabecular meshwork.
  • This trabecular meshwork,  converts the scleral sulcus it into a tube, which is known as schlemm’s canal.
  • It is a circular canal with a diameter of 190 to 350 microns.
  • EXTERNAL LANDMARK FOR SCHLEMMS CANAL : It lies posterior to the sclero-corneal junction.
  • It is lined by endothelium
  • It communicates with anterior chamber through the trabecular meshwork on the inside.
  • On its outer side the schlemm is perforated by approximately 25 to 30 aqueous collector channels which branch into intrascleral and deep scleral plexi finally opening into the episcleral veins .
image showing the sclemms canal with its connections,namely the collector channels, intrascleral plexus an deep scleral plexus

CLINICAL NUGGET : SCHLEMMS CANAL IS NOT A RIGID TUBE!

  • Note: The Schlemms canal is not a rigid tube.
  • The scleral spur, which has longitudinal muscle fibers attached to it, is pulled towards the ciliary body on contraction of the ciliary body.
  • This causes the Schlemms canal to open up, leading to more drainage of the fluid and decreased IOP.
  • In situations where there is excessive pressure inside the eye, the Schlemmer canal collapses due to excessive pressure from inside.

DETAILED ANATOMY OF TRABECULAR MESHWORK

  • The trabecular meshwork consists of endothelial-lined holes and lamellae.
  • From gonioscopic perspectives, the trabecular meshwork is divided into an anterior part and a posterior part.
  • The ANTERIOR TRABECULAR MESHWORK is defined as the zone from the Schwalbe’s line to the anterior edge of the Schlemm’s canal. This layer is not involved in filtration. It is non-pigmented.
  • The POSTERIOR TRABECULAR MESHWORK is the zone from the anterior edge of the Schlemm’s canal to the angle recess. It is associated with filtration. It is pigmented and has three layers, from innermost to outermost. These are as follows:
  1. Uveoscleral layer: innermost layer. This layer has bigger meshes and therefore provides almost no resistance to the flow of the aqueous humor.
  2. Corneoscleral layer: middle layer. Here, the circular holes of the uveoscleral meshwork get transitioned into elliptical holes.
  3. Juxtacanalicular layer: outermost layer in close proximity to the Schlemm canal. It has the tightest meshwork out of the three layers and provides the greatest resistance to aqueous outflow.
The trabecular meshwork looks triangular in cross-section. The apex of the triangle is towards the cornea, and the base is towards the scleral spur.
  • The trabecular meshwork consists of contractile spongy tissue and undergoes changes in shape along with contraction of the ciliary body.
  • As the ciliary mucle contracts, the longitudinal muscles pull the scleral spur towards it opening the schlemms canal and trabecular meshwork 
  • The JUXTACANALICULAR trabecular meshwork is the seat of maximum resistance in the trabecular meshwork due to least sized pores. It is also referred to as cribriform layer.
diagram depicting the detial anatomy of trabecular meshwork. Trabecular meshwork has three parts, uveoscleral, cornea and juxtacanalicular

AGE RELATED CHANGES IN TRABECULAR MESHWORK

•Increased pigmentation
•Reduced endothelial cells
•Thickened basement membrane
•Accumulation of debris
•Glycosaminoglycans deposition with age in extracellular space
•Increased resistance to outflow

Schwalbe’s Line

  • As the trabecular meshwork inserts in to the periphery of cornea, a ridge is created, known as Schwalbe’s line.
  • It also marks the prominent posterior end of Descemet’s membrane of the cornea.
  • Schwalbe’s line is the most anterior structure seen on gonioscopy.
  • It is seen as a glistening white line
  • Visualizing Schwalbe’s line requires an important technique of gonioscopy known as the “corneal wedge technique“
image showing the Schwalbes line and the corneal wedge useful in its identification

CLINICAL NUGGET : POSTERIOR EMBRYOTOXON

  • Normally the schwalbe’s line is not seen externally and is seen only on gonioscopy.
  • Sometimes it is more anteriroly placed than usually making it visible on external examination
  • This anteriorly displaced schwalbe’s line is referred to as the posterior embryotoxon.
  • It is associated with Axenfeld Rigers anomaly and syndrome.
image showing posterior embryotoxon which is anteriorly displaced schwalbes line

CLINICAL NUGGET : SAMPAOLESI LINE

  • The glistening white schwalbe’s line sometimes get pigmented.
  • This pigmented schwalbe’s line is known as the sampaolesi line.
  • It is seen in pseudo-exfoliation syndrome and pigment dispersion syndrome.
image showing the sampaolesi line, which is the pigmented schwalbes line

Normal blood vessels in angle

  • Radial vessels
  • Vertical branches of the anterior ciliary artery
  • Normal vessels never cross the scleral spur 
  • Abnormal vascularisation/neovascularisation of angle is seen in neovascular glaucoma

CONCLUSION

So that brings us to the end of important angle structures. If you have difficulty memorizing them in order , you can use the following mnemonic-

“Silly Tigers Snore, Cuddling In.“

  • Silly : Schwalbe’s Line
  • Tigers: Trabecular meshwork (anterior TM followed by Posterior TM)
  • Snore: Scleral Spur
  • Cuddling: Ciliary body band
  • In: Iris 
     
mnemonic for remembering angle structures

I hope that was useful! If you liked the article, share it with your friends. Have a cheerful day!

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