## Interpupillary Distance

Definition The distance between the centers of the pupils of the two eyes.

Landmarks Measure between the centers of both pupils (Fig. 7.39).

Instruments A graduated transparent ruler is most accurate. A tape-measure can be used stretched in a straight line to avoid curvature of the face.

Position The head should be held erect (in the resting position) with the eyes facing straight forward. This is easiest when the patient is reclining, the eye fissures are horizontal, and the eyes are gazing straight upward. A young child may need to be restrained for accurate measurement.

### Alternative The child or infant may lie supine.

Remarks Charts of interpupillary distance are provided in Figs. 7.40 and 7.41. Since the measurement of interpupillary distance requires the eyes to be fixed, this is not an easy task in the infant or young child. Feingold and Bossert, by means of multiple linear regression techniques, have suggested the use of the following formula: IP = 0.17 + 0.59IC + 0.410C, where IP is the interpupillary distance, IC is The inner canthal distance, and OC is the outer canthal distance (Fig. 7.42). The reason for distinguishing increased interpupillary distance from increased inner canthal distance is to distinguish between true hypertelorism and telecanthus or lateral displacement of the lacrimal punta. A rough clinical impression of whether there is lateral displacement of the lacrimal puncta can be obtained by seating the individual directly in front of the examiner and drawing an imaginary vertical line through the inferior lacrimal point. If this line cuts the iris, then there is lateral displacement of the inner canthus. If the inner canthal distance divided by the interpupillary distance is greater than 0.6, then lateral displacement of the inner canthus or dystopia canthorum is present.

Figure 7.39 Measuring interpupillary distance.

Figure 7.39 Measuring interpupillary distance.

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