Partially empty sella in pseudotumor cerebri
Patients with Chiari I malformation, especially if obese, may have associated intracranial hypertension, a condition known as the Chiari pseudotumor cerebri syndrome. Surgical decompression of the Chiari malformation without recognizing pseudotumor cerebri can result in persistent headaches and the development of a post-operative pseudomeningocele.
Recognition of partially empty sella prior to posterior fossa decompression is important. The classic appearance is the pituitary gland flattened along the bottom of the sella turcica as in the image on the right. (The condition was initially termed empty sella because the apparently compressed gland was not easily visible on diagnostic studies prior to magnetic resonance imaging. Since MRI allows visualization of the flattened pituitary gland, the term partially empty sella is more accurate.)
While the usual conception is that the gland is compressed to the bottom of the sella due to increased intracranial pressure, a recent study published online in the Journal of Neurosurgery provides evidence that the primary cause of a partially empty sella is enlargement of the bony sella and less so due to a compression of the gland. The authors measured the cross-sectional area of the sella turcica and pituitary on sagittal T1-weighted MR images in "48 patients with pseudotumor cerebri and 48 age-matched controls" and reported:
“Our data showed a mean 38% increase in the sagittal cross-sectional area of the sella turcica in pseudotumor cerebri patients compared with control subjects. This difference in area corresponds to a 63% increase in volume if the sella is modeled as a spherical chamber. The larger the sella, the emptier it appears. This correlation supports Kaufman's hypothesis that the sella becomes enlarged by chronic elevation of intracranial pressure in patients with pseudotumor cerebri. Expansion occurs in all dimensions, but descent of the floor is most prominent.”
“The mean gland area was 42 ± 13 mm2 (95% CI 39–46 mm2) in controls and 34 ± 14 m m2 (95% CI 30–38 mm2) in patients with pseudotumor cerebri. The pituitary gland had a smaller midline cross-sectional area in patients with pseudotumor cerebri (2-tailed t-test, p < 0.05). However… this small reduction in gland cross-sectional area does not necessarily signify a reduction in gland volume.”
Even if the mean gland size is smaller due to “compression or atrophy … the reduction is slight… This explains why tests of pituitary function in patients with pseudotumor cerebri usually show no deficiency.” The authors conclude:
“Our results indicate that chronic elevation of intracranial pressure causes expansion of the pituitary fossa. Little or no reduction occurs in the size of the actual gland. Because the pituitary tissue becomes molded to the walls of a larger container, the sella turcica appears partially empty.” (emphasis added)