NPH and CSF flow studies

Case Details: 64 yr old male with gait abnormality shows on MRI dilated lateral / 3rd ventricles with aqueductal flow kid on T2 with narrow callosal angle / crowding of gyro at vertex,  sylvan fissural dilatation/promient 4th ventricle with CSF flow suggesting average CSF flow of 20cms/sec and  absolute stroke volume of 46 micro litres , suggesting Normal Pressure Hydrocephalus (CSF pressure with in normal range of  less than 18cms of  water or 13 mmHg)

Teaching points by Dr MGK Murthy

Technologist: Mr Aneesh

1. NPH patients  classically exhibit gait (wobbly)(called Magnetic gait) abnormality/ urinary incontinence/ dementia  with spinal tap revealing normal pressure (also acts  as therapeutic trial  for studying improvement before shunting)

2. MRI typically exhibits lateral and third ventricular dilatation (frontal and temporal horns>others) /upward bowing of corpus callosum/ narrow callosal angle / crowding  of gyri at vertex/ transependymal seepage/ cingulate sulcus sign(posterior cingulate narrower than anterior)/dilated sylvan beyond expected exvacuo nature  etc

3. However quantitative CSF flow study  is presently  considered more definitive with increase of aqueductal stroke volume ( average volume of CSF moving through the cerebral aqueduct)(forward stroke volume + reverse stroke volume)/2.

In the later stages of the disease, stroke volume decreases  suggesting ineffectiveness of shunting

4.Flow velocity of  more than  24.5 mL/min or absolute stroke  volume of  more than 42 microlitres  could suggest as well predict good response to VP shunting

5. Etiology of NPH is ill understood .  it could be reduced CSF absorption leading to obstructive variety of hydrocephalus . Or it could be on account of periventricular  ischaemic changes leading tweaked ventricular wall and further reducing the flow of CSF to extracellular spaces

 Quantification of CSF Flow

1.CSF being pulsatile needs TO and FRO measurements rather than bulk amount  measurement including production/ absorption etc

 2.MR Technique is based on location specific sequential application of   a pair of  phase encoding pulses in opposite directions with moving protons experiencing different pulses and producing visible signal

3.Typical CSF flow rate is about 5-8cms/sec (VENC -velocity encoding)., hyper dynamic could make it up to 25cms/sec

4. Three  sets of images are acquired (like in SW )

(a) Rephased image (magnitude of flow compensated signal) where flow signal is high and background structures are visible

(b) Magnitude Image (Magnitude of difference signals)  where  flow signal is  high and  background is suppressed

(c)Phase Image (phase difference signals ) Forward flow  is seen s high signal and reverse flow is seen as low signal  and background is shown as mid Grey

5. Quantification  of CSF flow can be achieved by  defining a region of interest (ROI) and charting velocity VS Time (pulsatile being forward in systole and backwards in diastole). Utility of CSF is commonly used in NPH (aqueductal flow) / Foramen  magnum (CSF flow at this level)abnormalities including post fossa cysts/ chiari malforamtions etc / Arachnoid cysts for evaluating communication to ventricles / VP shunts functioning (where no signal demonstration amounts to no flow ) / spinal cord  syringes