Normal Pressure Hydrocephalus (NPH - Evans Index, DESH Sign & Tap Test)

• Seen in elderly individuals (peak age >65–70 years, equal sex distribution) presenting with the classic clinical triad of Hakim and Adams (present in complete form in up to 50–60%): (1) Gait Disturbance: The earliest, most prominent, and most shunt-responsive symptom; characterized as a 'magnetic', slow, wide-based, short-stepped, shuffling gait with feet appearing glued to the floor; (2) Cognitive Impairment: Subcortical psychomotor slowing, apathy, and impaired executive function; (3) Urinary Incontinence: Urgency and urge incontinence developing late in the course.
• Caused by chronic communicating hydrocephalus characterized by impaired cerebrospinal fluid (CSF) resorption through the arachnoid villi into the superior sagittal sinus:
• (1)
  Pathogenesis: Reduced compliance of the cerebral venous and arterial vascular bed combined with fibrotic thickening of arachnoid granulations (idiopathic, 50%; or secondary to prior subarachnoid hemorrhage, meningitis, or cranial trauma, 50%).
• (2)
  Normal Opening Pressure: CSF opening pressure is characteristically within normal physiological limits (typically 7–18 cm H2O / 70–180 mm H2O) during standard daytime lumbar puncture, but exhibits intermittent nocturnal spikes of elevated intraventricular pulse pressure.

Imaging Findings

• Magnetic Resonance Imaging (Brain MRI - Diagnostic Gold Standard - The DESH Framework):
• –
  Hallmark DESH Sign (Disproportionately Enlarged Subarachnoid-space Hydrocephalus):
• 1.
  Highly specific, pathognomonic morphologic pattern that strongly predicts successful response to surgical ventriculoperitoneal (VP) shunting.
• 2.
  Prominent, disproportionate dilatation of the Sylvian fissures and basal cisterns, accompanied by tight, crowded, compressed gyri and effaced subarachnoid sulci along the high cerebral convexity and parasagittal vertex.
• –
  Ventriculomegaly Indices:
• 1.
  Evans Index: Ratio of the maximum transverse diameter of the frontal horns of the lateral ventricles to the maximum internal diameter of the skull on the same axial slice >0.30.
• 2.
  Callosal Angle: The angle between the bilateral lateral ventricular roofs on a coronal slice through the posterior commissure perpendicular to the AC-PC line measuring strictly <90 degrees (typically 50–80 degrees in NPH, contrasting with >100–120 degrees in ex-vacuo atrophy).
• –
  CSF Flow Void Sign:
• 1.
  Marked, prominent hyperdynamic signal loss (flow void) within the Cerebral Aqueduct of Sylvius and 4th ventricle on T2WI, caused by turbulent, high-velocity pulsatile jet flow of CSF through the narrowed aqueduct.
• –
  Periventricular Changes: Smooth, thin, non-confluent halo of periventricular T2/FLAIR hyperintensity reflecting transependymal CSF resorption.
• –
  DWI / ADC: Shows no acute diffusion restriction; periventricular transependymal CSF seepage demonstrates slightly elevated ADC values.
• –
  **SWI / T2* GRE: Crucial for excluding coexisting Cerebral Amyloid Angiopathy (CAA)** (strictly lobar cortical/subcortical microbleeds on SWI) or hypertensive microangiopathy (deep basal ganglia microbleeds) prior to ventriculoperitoneal shunt placement.
• Computed Tomography (CT Head):
• –
  Symmetrical ventriculomegaly with Evans index >0.30, narrow callosal angle, and tight high-convexity sulci.

Differential Diagnosis

Important Points to Remember

• Diagnostic Pitfall: Confusing NPH with ex-vacuo ventriculomegaly from Alzheimer's disease! This is a critical clinical and imaging discriminator. Both show enlarged ventricles in elderly patients. You MUST evaluate the Callosal Angle (measured on a coronal view perpendicular to the AC-PC line through the posterior commissure): an angle <90 degrees indicates true active NPH (shunt responsive), whereas an angle >100 degrees indicates ex-vacuo atrophy (Alzheimer's, not shunt responsive). Furthermore, NPH shows tight sulci at the vertex with dilated Sylvian fissures (DESH sign), whereas Alzheimer's shows diffuse vertex sulcal widening.
• Crucial Review Areas:
• 1.
  Coronal T1/T2 perpendicular to the AC-PC line through the posterior commissure for Callosal Angle <90 degrees.
• 2.
  High cerebral convexity and vertex sulci: tight, effaced, crowded sulci with enlarged Sylvian fissures (DESH sign).
• 3.
  Evans index on axial CT/MRI: frontal horn ratio >0.30 confirming ventriculomegaly.
• 4.
  Aqueduct of Sylvius on axial/sagittal T2WI: prominent hyperdynamic jet flow void.
• Radiologist Safe Practice & Management:
• 1.
  High-Resolution Volumetric MRI Protocol: Multiplanar 3D T1, T2, and FLAIR imaging to calculate the Evans Index, Callosal Angle, and assess the DESH sign.
• 2.
  Diagnostic High-Volume Lumbar Tap Test: Standardized withdrawal of 30–50 mL of CSF via lumbar puncture; objective quantitative evaluation of gait velocity, stride length, and cadence 2–4 hours post-tap (improvement >20% confirms high shunt responsiveness).
• 3.
  Extended Lumbar Drainage (ELD): Continuous external lumbar drainage (10 mL/hour for 72 hours) if the single tap test is equivocal.
• 4.
  Surgical Ventriculoperitoneal (VP) Shunting: Implantation of an adjustable, programmable valve Ventriculoperitoneal Shunt (or ventriculoatrial shunt); gait disturbance improves in >80% of patients with positive DESH and tap test.
• 5.
  Post-Operative Surveillance: Regular outpatient clinical review and non-invasive transcutaneous programmable valve adjustments to achieve optimal symptom control while avoiding over-drainage complications (subdural hematomas).