Neuroimaging in normal pressure hydrocephalus

Abstract

Normal pressure hydrocephalus (NPH) is a syndrome characterized by the triad of gait disturbance, mental deterioration and urinary incontinence, associated with ventriculomegaly and normal cerebrospinal fluid (CSF) pressure. The clinical presentation (triad) may be atypical or incomplete, or mimicked by other diseases, hence the need for supplementary tests, particularly to predict postsurgical outcome, such as CSF tap-tests and computed tomography (CT) or magnetic resonance imaging (MRI). The CSF tap-test, especially the 3 to 5 days continuous external lumbar drainage of at least 150 ml/day, is the only procedure that simulates the effect of definitive shunt surgery, with high sensitivity (50-100%) and high positive predictive value (80-100%). According to international guidelines, the following are CT or MRI signs decisive for NPH diagnosis and selection of shunt-responsive patients: ventricular enlargement disproportionate to cerebral atrophy (Evans index >0.3), and associated ballooning of frontal horns; periventricular hyperintensities; corpus callosum thinning and elevation, with callosal angle between 40º and 90º; widening of temporal horns not fully explained by hippocampal atrophy; and aqueductal or fourth ventricular flow void; enlarged Sylvian fissures and basal cistern, and narrowing of sulci and subarachnoid spaces over the high convexity and midline surface of the brain. On the other hand, other imaging methods such as radionuclide cisternography, SPECT, PET, and also DTI or resting-state functional MRI, although suitable for NPH diagnosis, do not yet provide improved accuracy for identifying shunt-responsive cases.

Keywords: cerebrospinal fluid tap test; magnetic resonance; neuroimaging; normal pressure hydrocephalus; shunt surgery.

Figure 1

Axial CT slice of the brain in a patient with NPH. The Evans index can be measured by dividing the maximal width of the frontal horns [B-C] by the maximal width of the inner table of the cranium at the level of the frontal horns [A-D]; or by an equivalent measure, such as by dividing the diameter of the frontal horns [B-C] by the widest brain diameter [E-F].

Figure 2

Coronal head CT (left) and MRI (right) at the level of the posterior commissure: in the left image, the CSF spaces over the convexity near the vertex are narrowed ("tight convexity", red circle), as are the medial cisterns (red circle) - these are typical findings of NPH. On the right image, however, the CSF spaces over the convexity near the vertex (red arrow) and the medial cisterns (green arrow) are widened, a finding consistent with brain atrophy. The blue lines in both images indicate the callosal angle: an angle less than 90º is typical of NPH (left image), while an angle greater than 90º is typical of brain atrophy (right image). The blue arrows indicate periventricular signal alterations. The unilateral occurrence of these alterations (right image) suggests they are probably due to vascular encephalopathy. The abnormalities seen in the left image may well represent transependymal CSF diapedesis due to NPH. (From Kiefer & Unterberg, Dtsch Arztebl Int, 2012, with permission).

Figure 3

Coronal head CT of a 73-year-old man with idiopathic NPH. [A, B and C] show disproportionately enlarged ventricles with periventricular hypointense signal alterations, and expanded sylvian fissure and insular cisterns (thin black arrows), narrowed sulci and subarachnoid spaces at the high convexity near the vertex and midline (white arrow heads), as well as focally dilated sulci over the convexity (curved white arrows) and medial surfaces (straight white arrows).