Choroidal fissure acts as an overflow device in cerebrospinal fluid drainage: morphological comparison between idiopathic and secondary normal-pressure hydrocephalus

Abstract

To clarify the pathogenesis of two different types of adult-onset normal-pressure hydrocephalus (NPH), we investigated cerebrospinal fluid distribution on the high-field three-dimensional MRI. The subarachnoid spaces in secondary NPH were smaller than those in the controls, whereas those in idiopathic NPH were of similar size to the controls. In idiopathic NPH, however, the basal cistern and Sylvian fissure were enlarged in concurrence with ventricular enlargement towards the z-direction, but the convexity subarachnoid space was severely diminished. In this article, we provide evidence that the key cause of the disproportionate cerebrospinal fluid distribution in idiopathic NPH is the compensatory direct CSF communication between the inferior horn of the lateral ventricles and the ambient cistern at the choroidal fissure. In contrast, all parts of the subarachnoid spaces were equally and severely decreased in secondary NPH. Blockage of CSF drainage from the subarachnoid spaces could cause the omnidirectional ventricular enlargement in secondary NPH.

Figure 1. Three-dimensional views of the ventricles in iNPH, sNPH and normal brain.

Compared to the controls (middle), the ventricles in sNPH patients expanded symmetrically in all directions, whereas the ventricles in iNPH patients expanded disproportionately towards the z-axial direction. The red arrow heads indicate the choroid plexus.

Figure 2. Mean volumes of the segmented intracranial cerebrospinal fluid spaces in iNPH, sNPH and normal brain.

The mean volumes of ventricles (sky blue), basal cistern and Sylvian fissure (pink) and subarachnoid space in the posterior fossa (light green) were the largest in patients with iNPH. The mean volume of the convexity part of the subarachnoid space (light yellow) was equally decreased in patients with iNPH and those with sNPH.

Figure 3. Fine structures of the lateral ventricles and the basal cistern around the inferior choroidal point of the choroidal fissure.

The 3D views (upper) which combine axial (middle) and coronal (lower) sections were at the level of the inferior choroidal point of the choroidal fissure (yellow arrow head) between the ambient cistern (light green circle) and the inferior horn of the lateral ventricles (purple circle). In the normal situation, at the head of the hippocampus (H) and choroid plexus (red arrow head), the ambient cistern is separated from the inferior horn of the lateral ventricles. In patients with iNPH, the ventricles, basal cistern and Sylvian fissure (light blue circle) were enlarged with the concurrent opening of the inferior choroidal point of the choroidal fissure. However, in patients with sNPH, only the ventricle was enlarged and all of the subarachnoid spaces, including the ambient cistern and Sylvian fissure, were severely diminished.

Figure 4. Confirmation of the direct CSF communication between the lateral ventricles and the ambient cistern by shuntography in patients with iNPH with a suspected shunt malfunction.

Contrast medium (yellow arrow head) was observed not only in the left inferior horn of the lateral ventricles but also in the left ambient cistern on CT scan immediately after shuntography (right), compared to that before shuntography (left).

Figure 5. Schematic diagram showing the distribution of CSF in patients with iNPH and sNPH, compared to normal brains.

In iNPH patients, the inferior horn of the lateral ventricles is directly connected with the basal cistern at the inferior choroidal point of the choroidal fissure (yellow circle) inside the choroid plexus.

Figure 6. Fine structures of the subarachnoid spaces around the third ventricle.

Patients with iNPH had the enlarged Sylvian fissure (Syl.) in concurrent with enlargement of the lateral (L) and third (3^rd) ventricles, whereas patients with sNPH had the diminished Sylvian fissure. The border between third ventricle and surrounding subarachnoid spaces (SAS) was thinning in iNPH patients (yellow square). The red arrow heads indicate the trabeculas and partitions in the subarachnoid spaces.