Chamber angle

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Iridocorneal Angle = chamber angle

The anatomical structure cornea ( cornea ) and iris in the anterior chamber of the eye form is referred to as chamber angle ( Angulus iridocornealis hereinafter). The aqueous humor drains through it . If it is pathologically changed, it can lead to a drainage disorder with an increase in intraocular pressure and the development of glaucoma . Degenerative diseases of the chamber angle are the pathological correlate for chronic open-angle glaucoma and are therefore among the most common causes of blindness .

anatomy

Gonioscopic view into the chamber angle: swallow line (1), pigmented and unpigmented trabecular meshwork (2), scleral spur (3), ciliary body ligament (4), anterior iris surface (5)

The following structures can be distinguished (from front to back) (see fig.):

  • Swallow line: it is furthest forward and appears as a delicate gray line; it is the boundary between the corneal endothelium and the trabecular meshwork.
  • Trabecular system: A distinction is made between an anterior, unpigmented part following the Schwalbe line with a whitish color and a rear functional, mostly pigmented part. In the posterior part, the aqueous humor is drained via Schlemm's canal.
  • Scleral spur: anterior part of the sclera; it appears as a prominent, white line between the functional trabecular structure and the ciliary body ligament, unless the structure is overlaid with strong pigmentation.
  • Ciliary body ligament: part of the ciliary muscle between the base of the iris and the scleral spur; it appears gray to dark brown.

Clinical significance

Width of the chamber angle

The width of the chamber angle is of clinical importance, since a narrow chamber angle is associated with an increased risk of developing an acute glaucoma attack or chronic narrow-angle glaucoma .

In the case of greater farsightedness , cataracts and in Asians , the chamber angle is often narrower.

Several systems have been proposed to classify the chamber angle, whereby the Shaffer classification has prevailed:

  • Degree 0 (0 °): closed chamber angle (irido-corneal contact).
  • Degree I (10 °): very narrow chamber angle (only Schwalbe line visible), closure very likely.
  • Grade II (20 °): moderately narrow chamber angle (trabecular structure visible), closure possible.
  • Degree III (20–35 °): open chamber angle (visible up to the scleral spur), closure unlikely.
  • Grade IV (35–45 °): very wide chamber angle (ciliary body ligament visible), closure impossible.

An analogous, alternative classification is based on Scheie . The aim of these classifications is essentially to estimate the risk of an attack of glaucoma by angle block.

Diseases

  • Embryonic development disorders: A prominent Schwalbe line ( embryotoxon posterius ) can be associated with malformations ( Axenfeld-Rieger anomaly ) and congenital glaucoma.
  • Degenerative changes in the trabecular meshwork are considered to be the cause of chronic open-angle glaucoma.
  • Pigment deposits in the chamber angle, which are usually located in the lower quadrant, can be an indication of pigment dispersion glaucoma or an angular block attack.
  • Abnormal vessels of the chamber angle are typically delicately ramified and run in various directions. They are an indication of diseases ( neovascular glaucoma , uveitis , fuchs heterochromic cyclitis ).
  • Anterior uveal tumors ( ciliary body , iris)
  • Cysts of the iris
  • Foreign body in the chamber angle

Operations of the chamber angle

The selective laser trabeculoplasty is an intervention at the chamber angle to lower the intraocular pressure. The fistulating, pressure-lowering surgical procedures also affect structures of the chamber angle. The chamber angle can also be used to hold intraocular artificial lenses.

examination

With the slit lamp alone, the chamber angle cannot be viewed directly due to the total reflection of the cornea. It is only possible to indirectly view the angular width of the chamber by estimating the distance between the posterior surface of the cornea and the iris ( van Herick's method ). With the help of a so-called contact glass , however, the chamber angle can be inspected directly with the slit lamp.

Ultrasonic biomicroscopy and the Scheimpflug camera are available as imaging methods for the morphological assessment of the chamber angle .

Gonioscopy

Three-mirror contact glass according to Goldmann, on the examiner side, with visible angle mirrors

The most common ophthalmological procedure for examining the chamber angle is gonioscopy . It is performed with a contact lens, which in this context is also called a gonioscope. A distinction is made between direct and indirect gonioscopy. Glasses for indirect gonioscopy (e.g. Goldmann three-mirror glass, Zeiss four-mirror glass) are the most common.

  • The Goldmann three-mirror contact lens has 3 mirrors: 2 mirrors for examining the retinal periphery and another for examining the chamber angle (inclination angle of 59 °). The examination is carried out on the slit lamp after surface anesthesia, whereby a viscous substance is required between the cornea and the contact glass. By rotating the three-mirror contact glass by 360 °, the chamber angle can be viewed in a circular manner.
  • The Zeiss glass has 4 mirrors with an inclination angle of 64 °. It is therefore not necessary to turn the glass during the examination. Due to the flatter radius of curvature, a viscous substance is not required. By applying pressure to the cornea, the Zeiss glass can be used to redistribute fluid in the anterior chamber so that the chamber angle is widened. As a result, an acute-angled, invisible chamber angle can be widened so far that goniosynechiae can be seen. This so-called single gonioscopy can be used to differentiate acute angle-closure glaucoma without goniosynechiae from chronic angle-closure glaucoma with goniosynechiae.

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