Costello syndrome: Analysis of the posterior cranial fossa in children with posterior fossa crowding

Abstract

This study aimed to assess changes in the posterior cranial fossa (PCF) to shed light on the mechanism of cerebellar herniation in children with Costello syndrome (CS) and posterior fossa crowding. We performed a morphovolumetric PCF analysis on brain magnetic resonance imaging (MRI) in seven children with CS (mean age 31 ± 16 months) comparing the MRI scans with those of seven age-matched healthy subjects.PCF volume (PCFV), PCF brain volume (PCFBV) and cerebellar volume (CeV) were assessed on axial T2-weighted MRI. Morphometric parameters (diameters of the foramen magnum, tentorial angle, basiocciput, supraocciput, basisphenoid and exocciput lengths) were measured on sagittal T1-weighted MRI. The volume of the cerebrospinal fluid (CSF) spaces was calculated as PCFV minus PCFBV.Five out of seven CS children showed tonsillar herniation in the upper cervical canal; no child had hydrocephalus but three out of seven children showed ventriculomegaly. In addition, the PCFV/PCFBV ratio, PCFV, CSF spaces volume, basiocciput, basisphenoid and exocciput lengths and latero-lateral and antero-posterior diameters of the foramen magnum were significantly reduced, whereas no significant changes were found in supraocciput length, PCFBV, CeV or hindbrain volume. The volumetric reduction of the PCF due to bony posterior fossa hypoplasia is a predisposing factor for developing cerebellar tonsillar herniation through the foramen magnum in children with CS. The altered anatomy of the foramen magnum and upward expansion of the PCF secondary to an increased tentorial slope serves to explain the possible mechanism of cerebellar herniation in patients with CS.

Keywords: Chiari anomaly; Magnetic resonance imaging; posterior cranial fossa.

Figure 1.

Volumetric measurements of the posterior cranial fossa (PCFV) and cerebellum (PCFBV) in a representative child with Costello syndrome and posterior fossa crowding. T2-weighted sagittal (A, C, E, G) and axial (B, D, F, H) MR images. (A, B) Outlined in white are the boundaries of the posterior fossa which include the entire cerebellum, brain stem and extra-axial spaces. PCFV is shown in green-shaded areas. (C, D) Outlined in white are the boundaries of the neural structures contained within the posterior cranial fossa comprising the entire cerebellum including the parts herniating into the spinal canal and brain stem; extra-axial spaces are not included. PCFBV is shown in red-shaded areas. (E, F) Outlined in white are the boundaries of the cerebellum shown in purple shaded areas. (G, H) CSF spaces around the hindbrain are shown in blue-shaded areas.

Figure 2.

Morphometric parameters of the posterior cranial fossa in a representative child with Costello syndrome. A) Midsagittal T1-weighted MRI; a = length of the basiocciput, between the spheno-occipital synchondrosis (S) and the basion (B); b = length of basisphenoid, between the top of the dorsum sellae (DS) and the spheno-occipital synchondrosis of the clivus (S); a + b = length of clivus; c = length of the supraocciput between the internal occipital protuberance (IOP) and the opisthion (OP); d = distance between the tip of the cerebellar tonsils (*) and McRae’s line (dashed line: B to OP); McRae’s line = antero-posterior diameter of the foramen magnum (AP); α = angle of the cerebellar tentorium (T) to Twining's line (TL: IOP to DS). B) Axial T2-weighted MRI; LL = maximum tranverse diameter of the foramen magnum. C) Coronal T1-weighted MRI; F = length of exocciput extends from the bottom of the occipital condyle (Co) to the top of the jugular tubercle (JT).