Changes in aqueductal CSF stroke volume and progression of symptoms in patients with unshunted idiopathic normal pressure hydrocephalus

Abstract

Background and purpose: Idiopathic normal pressure hydrocephalus (iNPH) represents a diagnostic challenge, given its variable presentation and progression. Stroke volume (SV), defined as the mean volume of CSF passing through the aqueduct during both systole and diastole, greater than or equal to 42 muL, serves as a selection criterion for patients with good probabilities of improvement after ventriculoperitoneal shunt surgery (VPS). In this study, we evaluated the changes in SV during the progression of clinical symptoms in patients with suspected NPH.

Materials and methods: Nine patients who presented with clinical and radiologic evidence of NPH, but refused treatment with VPS, were evaluated every 6 months for up to 2 years for progression in their clinical symptoms and changes in their SV, as measured by phase-contrast cine MR imaging (PCCMR).

Results: SV seems to increase between the onset of the symptoms and the following 18 to 20 months, then seems to plateau, followed in the next 18 to 20 months by a slight decline, and finally to a more precipitous drop in the next 12 months. During this time, however, the patient's clinical symptoms progressively worsen.

Conclusion: Patients with a low SV have not necessarily had brain atrophy and can show, in the following months, a progressive increase in SV, which qualifies them as good candidates for VPS. The progressive reduction of the SV in untreated patients with worsening clinical symptoms may be a sign of a progressive cerebral ischemic injury, which renders the NPH irreversible.

Fig 1.

Ventricular enlargement in MR image of a patient with NPH with a positive Evans Index.

Fig 2.

PCCMR imaging CSF dynamic study in a 74-year-old patient with NPH. A, Midline sagittal T1-weighted MR imaging is used to graphically describe the phase-contrast cine series. The section is placed at the level of the inferior colliculus, perpendicular to a line drawn through the distal aqueduct. B, Axial section in which region of interest is drawn as close as possible to the aqueduct border. C, Respective absolute values of CSF during 16 cardiac phases are reported on the graph. The flow plot demonstrates sinusoidal pattern of flow where negative values represent aqueductal systolic CSF volume (microliter) outflow and positive values represent diastolic CSF volume inflow.

Fig 3.

Changes in SV values at 6, 12, 18, and 24 months for each of the 9 patients.

Fig 4.

Changes in SV values (Table 1) standardized for the estimated onset of NPH, as per the reported first symptoms of NPH.

Fig 5.

Proposed model demonstrating SV changes in NPH. A, In healthy subjects, expansion of the cerebral hemispheres occurs both outward and inward. The outward expansion produces venous blood outflow as a result of compression on the cortical veins. Inward expansion produces flow of CSF into the aqueduct as a result of compression of the lateral and third ventricles. B, In communicating hydrocephalus, the brain has already expanded outward during diastole, compressing the cortical veins. However, during systole, with arterial blood entering, the systolic expansion is directed inwards, resulting in a much greater SV in the aqueduct. C, Progressive ischemia and a reduction of arterial inflow results in a decreased “ventricular CSF pump.”