What causes the harlequin appearance on frontal skull radiography and CT?
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Answer:
The harlequin eye appearance is asymmetry of the orbits on the frontal view in which the affected superolateral orbital margin is elevated and the greater wing of the sphenoid is displaced upward, so the orbit takes on a raised, quizzical shape resembling a harlequin mask. It is the hallmark of unilateral coronal synostosis, that is, anterior plagiocephaly. The full pattern on CT with three-dimensional reformats is characteristic: premature fusion and sclerosis of one coronal suture, ipsilateral flattening of the frontal bone with compensatory contralateral frontal bossing and ipsilateral occipital flattening, elevation of the ipsilateral sphenoid wing and orbital roof, deviation of the nasal root toward the fused side and of the chin away from it, and an anteriorly displaced ipsilateral ear. It must be distinguished from deformational plagiocephaly, which has patent sutures, a parallelogram head shape with the ipsilateral ear pushed forward and no orbital asymmetry, and from unilateral lambdoid synostosis, which produces a trapezoid shape and an inferiorly displaced mastoid. Syndromic craniosynostosis, particularly Muenke, Saethre-Chotzen, Crouzon and Apert syndromes, must be considered when the synostosis is bilateral or accompanied by limb, facial or hearing anomalies, and it changes counselling and surgical planning.
Why is it called so?
The raised lateral orbital margin gives the eye the elevated, arched appearance of a harlequin’s mask.
Pathophysiology
A cranial suture is a growth site at which bone is deposited perpendicular to the suture as the brain expands. When one coronal suture fuses prematurely, growth is prohibited across that suture and the skull compensates by growing in the directions the remaining patent sutures allow, which is the basis of Virchow’s law. On the fused side the frontal bone cannot advance, so the forehead is flat, and the sphenoid wing and lesser wing are pulled upward and forward, elevating the superolateral orbital rim and shortening the anterior cranial fossa. The contralateral side over-expands to accommodate brain growth. The resulting orbital asymmetry is therefore a direct mechanical consequence of restricted growth at the coronal ring rather than a primary orbital malformation.
Alternative names: Harlequin eye sign; harlequin orbit
Other associated named signs: Copper-beaten skull of raised intracranial pressure in syndromic synostosis and the parallelogram head of deformational plagiocephaly
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