What causes a J-shaped sella on skull radiography and MRI?
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Answer:
A J-shaped sella is a sella turcica whose anterior portion is flattened and elongated by an exaggerated, deepened chiasmatic sulcus, while the dorsum sellae remains rounded, so the floor and anterior wall trace the shape of the letter J. It is also called the omega, shoe or gourd-shaped sella. It is important to know that it is often a normal variant, reported in around 5% of children, so it should not be over-interpreted in isolation. Recognised associations are the mucopolysaccharidoses, especially Hurler and Morquio syndromes; neurofibromatosis type 1, particularly with an optic pathway glioma enlarging the chiasmatic sulcus; achondroplasia; chronic hydrocephalus; and long-standing suprasellar or pituitary masses. Because a chiasmatic mass is the association that matters clinically, a J-shaped sella found in a child should prompt dedicated MRI of the optic pathways rather than reassurance, and the rest of the skeletal survey should be reviewed for dysplastic features.
Why is it called so?
The flattened elongated anterior floor with the rounded posterior curve reproduces the hook of the letter J.
Pathophysiology
The chiasmatic sulcus is the shallow groove on the upper surface of the sphenoid between the tuberculum sellae and the optic canals. Anything that enlarges the tissue lying in it, most importantly a chiasmatic or optic pathway glioma, or that alters bone modelling in the region, remodels the sulcus into a deeper, longer groove. That deepening extends the anterior contour of the sella forwards and flattens it, while the dorsum, which is unaffected, retains its normal rounded profile. In the mucopolysaccharidoses and achondroplasia the mechanism is a primary dysplasia of endochondral ossification at the skull base rather than local pressure, and in hydrocephalus it is chronic transmitted pressure.
Alternative names: Omega sella, shoe-shaped sella, gourd-shaped sella
Other associated named signs: Empty sella, ballooned sella of a pituitary macroadenoma, and the sphenoid wing dysplasia of NF1
References
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