The interpupillary distance in normal child can be predicted if the inner canthal and outer canthal distances are known. This can help distinguish true hypertelorism (increased separation of the eyes) from telecanthus or lateral displacement of the lacrimal puncta.


interpupillary distance in cm =

= 0.17 + (0.59 * (inner canthal distance in cm)) + (0.41 * (outer canthal distance in cm))



(1) The lines in the figure are parallel, with outer canthal distance the X axis, interpupillary distance the Y axis and the slope of 0.41.

(2) The Y-axis intercept varies for each line and is a function of inner distance. When these are solved for, the line with slope 0.59 and Y axis intercept 0.17 is derived.


NOTE: Feingold and Bossert's paper has a typographical error, with 0.7 printed instead of 0.17


Lateral displacement of the inner canthus can be identified if:

(1) The ratio of the (inner canthal distance in cm) / (interpupillary distance in cm) > 0.6 (from page 146 Hall).

(2) The examiner observes that a vertical line passing through the inferior lacrimal point crosses the iris.

NOTE: I noted that some values from the graph of normal relations gave a ratio > 0.6, so it may be prudent to use both criteria before making the diagnosis of lateral displacement of the inner canthus.



• The patient must hold the eyes perfectly still and looking directly forward. This may be difficult to achieve in an infant or small child.

• I am not sure if the equation is valid for all races.


To read more or access our algorithms and calculators, please log in or register.