System of anterior segment imaging

Due to the multiple and confusing terms currently used in anterior segment imaging, we suggest to use a more systematic approach. The system is based on several principles:

Our system preserves some terms that are currently used in practice, such as: WTW – white to white measurement, STS – sulcus to sulcus measurement, ATA – angle to angle measurement, ACD – anterior chamber depth and pACD – pseudophakic anterior chamber depth.

Horizontal and vertical distances in frontal plane are labeled in the format of “Y-to-Y“. In general, Y-to-Y (YTY) is the longest distance between two landmark points Y, such as STS – sulcus to sulcus measurement.  These parameters can be more specifically defined as horizontal YTY (hYTY) in horizontal directions and as vertical YTY (vYTY) in vertical direction.

Axial distances are generally measured from the anterior corneal vertex and labeled as „depth“, for example ACD – anterior chamber depth or RD – Scleral spuR depth.

The last letter of the multiple letter shortcut (such as ACD - anterior chamber depth) generally defines the category of the term: D – depth or can also mean distance in axial direction, A - angle, X - axis, S - segment, C - chamber, K- compartment, P - pressure, G - gradient, M - membrane, V – volume, H – vault height.

The first letters of the multiple letter shortcuts describes the landmark points or structures involved. The landmark points are generally, if possible, described by single letters:,  I - (IPE) peripheral edge of iris pigment epithelium, M - (Margin) pupil margin – defined as edge of central iris pigment epithelium, A - (Angle) point of anterior chamber angle recess, X - (apeX) apex of the ciliary body, S - (Sulcus) ciliary sulcus recess, B - SchwalBe’s line (the anatomical structure of Schwalbe’s line in a single frame of any image is a point and not a line), R - (spuR) scleral spur, O - (IOL Optic) most peripheral point of optic of IOL, and H - (IOL Haptic) most peripheral point of IOL haptic. These letters were carefully chosen to support systematic simplicity and avoid letter repetition and confusion of terms that are already historically used (such as sulcus-to-sulcus and spur-to-spur) and to generally reflect the first or the last letter of the anatomical structure.

Some letters are repetitively used in general terms, such as L – lens, C – ciliary (also can mean cornea, or chamber), I – iris (also can mean IOL).

Anterior and posterior vertex point of different structures are indexed with number 1 and 2: C1 - anterior corneal vertex, C2 - posterior corneal vertex, L1 - anterior lens vertex, L2 - posterior lens vertex, O1 - anterior IOL optic vertex, O2 - posterior IOL optic vertex.

DISTANCES AND LINES IN THE FRONTAL PLANE 

There are several distances in the frontal plane that are very useful in describing the position of the structures of the anterior segment of the eye.

WTW: White-to-white is the distance between the peripheral white area of the corneal limbus

ATA: Angle-to-angle is the distance between anterior chamber angle recesses.

BTB: Schwalbe’s line -to- Schwalbe’s line  is the distance between both Schwalbe’s lines typically imaged on one scan of the anterior segment.

RTR: Spur-to-spur is the distance between both scleral spurs.

MTM: Margin-to-margin distance, also referred to as the central iris pigment epithelium -to- central iris pigment epithelium distance, is the distance between both pupil margins (M), typically imaged on one scan of the anterior segment. It is generally similar in value to the pupil diameter.

ITI: IPE-to-IPE distance (peripheral iris pigment epithelium - to - peripheral iris pigment epithelium) is the distance between both points I, typically imaged on one scan of the anterior segment.

XTX: Apex-to-apex (the cumulative apex of ciliary body - to - apex of ciliary body) is distance between both apeces of CB typically compiled from images on multiple scans of the anterior segment.

These distances also define corresponding lines, for example the BTB line is defined by the direction of the BTB distance.

CORRESPONDING DISTANCES IN THE AXIAL DIRECTION 

The corresponding distances in the axial direction are:

BD: The Schwalbe’s line depth is the distance between the anterior vertex of the cornea and the LTL line.

RD: The scleral spur depth is the distance between the anterior vertex of the cornea and the RTR line.

MD: The margin depth is the distance between the anterior vertex of the cornea and the MTM line.

ID: The IPE depth is the distance between the anterior vertex of the cornea and the ITI line.

XD: The apex depth or the apex of the ciliary body depth is the distance between the anterior vertex of the cornea and XTX line.

SD: The sulcus depth is the distance between the anterior vertex of the cornea and the STS line.

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