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Figure 2. Different anterior chamber depths as seen from the lateral perpendicular (profile) view. The more forward the pupil is, the shallower the anterior chamber. In the leftmost photo, the pupil is relatively posterior (set back), indicating an EZ ratio of < 0.5 and an anterior chamber deeper than 2.5 mm.
The Van Herick's technique has become the most commonly used qualitative method of assessing the size of the anterior chamber angle (ACA). Whereby, it involves comparing the depth of the peripheral anterior chamber to the thickness of the cornea, when a narrow beam is shone within the limbus at a 60°angle. [4]
The eye is divided ultrasonographically into four components:Cornea, Anterior chamber, Lens thickness and Vitreous cavity. The velocity of sound in these compartments are 1620, 1532, 1641, 1532 m/s respectively. [2] Through normal eyes an average velocity of 1555 m/s is accepted for calculation.
The anterior chamber angle is a part of the eye located between the cornea and iris which contains the trabecular meshwork. The size of this angle is an important determinant of the rate aqueous humour flows out of the eye, and thus, the intraocular pressure. The anterior chamber angle is the structure which determines the anterior chamber depth.
Multiple studies have shown a 12–17 μm/year decrease in the anterior chamber depth with aging. [ 6 ] [ 7 ] If a phakic IOL patient is assumed to have a 50-year lifespan, the overall decline in ACD may add up to 0.6–0.85 mm, long-term data about this effect are not available.
A good analogy to better understand the cup-to-disc ratio is the ratio of a donut hole to a donut. The hole represents the cup and the surrounding area the disc. If the cup fills 1/10 of the disc, the ratio will be 0.1. If it fills 7/10 of the disc, the ratio is 0.7. The normal cup-to-disc ratio is less than 0.5.
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It fills both the anterior and the posterior chambers of the eye, and is not to be confused with the vitreous humour, which is located in the space between the lens and the retina, also known as the posterior cavity or vitreous chamber. [2] Blood cannot normally enter the eyeball. [3]