The palpebral fissure may be level, mongoloid, or antimongoloid, depending on the relative positions of the medial and lateral canthi. If the outer canthus is higher than the inner canthus, a mongoloid palpebral slant exists (Figure 15). If the outer canthus is lower than the inner canthus, an antimongoloid palpebral slant exists (Figure 16). A straight edge held in front of the palpebral fissure connecting the canthi may be used to compare the relative canthal height. The ‘normal’ relative canthal height depends on race. In whites, the palpebral fissure is usually slightly mongoloid; that is, the lateral canthus is slightly higher than the medial canthus. Careful measurements of the Oriental palpebral fissure indicate less mongoloid slant than would be expected from casual observation. The mongoloid slant illusion in many cases is created by the absence of a skin fold in the upper lid and from a superior epicanthal fold.

Figure 16
‘V’ esotropia in a patient with antimongoloid palpebral fissures.

The palpebral fissure configuration imparts a characteristic appearance to an individual including, at times, a pseudostrabismus. This assumes clinical significance because it is a common cause for referral for suspected esotropia especially in infants. Vertically incomitant strabismus (A and V patterns) in esotropia seems to follow a pattern related to the slant of the fissures. This pattern was first pointed out by Urrets-Zavalia who noted that in esotropia a mongoloid fissure tends to be associated with an A pattern and an antimongoloid fissure with a V pattern. He found no similar correlation in exodeviations. When examining a strabismus patient who has either a mongoloid or an antimongoloid lid fissure, vertical incomitance should be looked for. Patients with myelomeningocele demonstrate a consistent abnormality in the configuration of the palpebral fissure (Figure 17). What appears to be a mongoloid slant in reality may be a straightening of the lower lid margin, which gives the illusion of a mongoloid slant. These patients frequently demonstrate an A pattern usually associated with overaction of the superior oblique muscle, and sometimes with dissociated vertical deviation. The diagnosis of myelomeningocele can be suspected in most cases simply by looking at the lid configuration.

Figure 17
A patient with myelomeningocele and a straight lower lid margin simulating a mongoloid slant. This is a common but unexplained finding in such patients.