Evaluation (Physicians)
Once an index of suspicion for NPH has been reached in a patient, the appropriate step is to have them evaluated by an neurosurgeon experienced in treating NPH. The aim of this evaluation step is to confirm the probable diagnosis of NPH, but more importantly to assess whether the patient would benefit from placement of a CSF shunt.
The surgical risks of CSF shunting include infection, subdural hematoma, CSF leaks, chronic headaches, shunt-blockage with a need for reoperation, plus the standard risks of general anesthesia and operative procedures in an elderly person. These risks amount to no more than 10-13% in published studies (1,2) depending on the skill of the surgeon. In a recent review of our shunt procedures for NPH at MCV, we observed a less than 8% complication rate. However, these potential risks obviate any possibility of operating on every each and every patient. When operative intervention is recommended for all NPH patients, or a careful diagnosis is not made, the post-operative improvement rate is only 40-60% (3), and in some centres as low as 35% (4). To view recent improvement rates at MCV see "Outcome".
Although centers differ in their approach MCV utilises two well-defined techniques to evaluate patients with suspected NPH.
CSF Fluid Dynamics
In an outpatient setting, a lumbar puncture is performed, and several small boluses of saline are injected into the CSF space at the level of L3-4. A pressure transducer is placed in series with the CSF space, and the transient pressure responses to these boluses are measured. From the resultant waveform several pieces of information can be calculated, namely the pressure-volume index or PVI, and the CSF outflow resistance. This technique of assessment was developed and characterised by Anthony Marmarou Ph.D.
PVI is defined as the calculated volume (in mls) needed to raise intracranial pressure by a factor of 10. A typical curve is shown on the right. The normal range in adults is 25-30 mls. If compliance is reduced by a pathological process, then PVI will tend to diminish, i.e. any given volume added to the CSF will tend to cause a greater rise in ICP. PVI's in patients with NPH range from normal to low (i.e. increased compliance). It is difficult to understand whether this is a feature of the condition, or an age-related phenomenon. Alterations in PVI do not predict potential benefit from surgery.
The second parameter which can be measured is the CSF outflow resistance. The volume of CSF in the brain must be a balance between CSF formation and CSF outflow. Mathematical modelling has defined that ICP equals the rate of CSF production (If) multiplied by the resistance to CSF outflow (Ro) plus the dural sinus pressure (Pd).
In a normal person the CSF formation and outflow component contributes approximately 10% of the total ICP (5). The outflow resistance has been shown to be increased in several studies of NPH (6,7,8). This is difficult to understand given the above equation. Increasing Ro should increase the ICP. Kosteljanetz (8) explained this in two ways, firstly he defined that ICP remains proportional to Ro in patients with NPH as expected. However, it also appears that the CSF component (If x Ro) contributes to a greater proportion of the ICP in patients with NPH. Some studies have found CSF outflow resistance to be a good indicator of patients who will benefit from CSF shunting with a positive predictive value of 80% (9). Other studies are equivocal on this issue and it is possible that the differences relate to the experience of the investigating team.
In summary, therefore, CSF dynamics tests provide supporting evidence for the diagnosis of NPH, especially where CSF outflow resistance is raised, and PVI values are low. Although these values may not predict patients who will benefit from CSF shunts, they do however provide valuable information in determining the pressure setting needed if a shunt is later fitted.
References:
(1) Larsson A. Wikkelso C. Bilting M. Stephensen H. Clinical parameters in 74 consecutive patients shunt operated for normal pressure hydrocephalus. Acta Neurologica Scandinavica. 84(6):475-82, 1991 Dec.
(2) Lund-Johansen M. Svendsen F. Wester K. Shunt failures and complications in adults as related to shunt type, diagnosis, and the experience of the surgeon. Neurosurgery. 35(5):839-44; discussion 844, 1994 Nov.
(3) Black PM. Idiopathic normal-pressure hydrocephalus. Results of shunting in 62 patients. Journal of Neurosurgery. 52(3):371-7, 1980 Mar.
(4) Vanneste J. Augustijn P. Dirven C. Tan WF. Goedhart ZD. Shunting normal-pressure hydrocephalus: do the benefits outweigh the risks? A multicenter study and literature review. [Review] Neurology. 42(1):54-9, 1992 Jan.
(5) Marmarou A, Shulman K, Rosende R: A nonlinear analysis of the cerebrospinal fluid system and intracranial pressure dynamics. J Neurosurg 48: 530-537, 1978.
(6) Borgeson SE: Conductance to outflow of CSF in normal pressure hydrocephalus. Acta Neurochirurgica 71: 1-45, 1994.
(7) Ekstedt J, Friden H: CSF hydrodynamics espcially in the adult hydrocephalus syndrome. In: Beks JWF, Bosch DA, Brock M (eds): Intracranial Pressure III, Springer-Verlag, Berlin, 1976, pp 177-185.
(8) Kosteljanetz M: CSF dynamics and pressure-volume relationships in communiating hydrocephalus. J Neurosurgery 64: 45-52, 1986.
(9) Boon AJ. Tans JT. Delwel EJ. Egeler-Peerdeman SM. Hanlo PW. Wurzer JA. Avezaat CJ. de Jong DA. Gooskens RH. Hermans J. Does CSF outflow resistance predict the response to shunting in patients with normal pressure hydrocephalus?.Acta Neurochirurgica - Supplementum. 71:331-3, 1998.
(10) Williams MA. Razumovsky AY. Hanley DF. Comparison of Pcsf monitoring and controlled CSF drainage diagnose normal pressure hydrocephalus. Acta Neurochirurgica - Supplementum. 71:328-30, 1998.
(11) Chen IH. Huang CI. Liu HC. Chen KK. Effectiveness of shunting in patients with normal pressure hydrocephalus predicted by temporary, controlled-resistance, continuous lumbar drainage: a pilot study. Journal of Neurology, Neurosurgery & Psychiatry. 57(11):1430-2, 1994 Nov.
(12) Haan J. Thomeer RT. Predictive value of temporary external lumbar drainage in normal pressure hydrocephalus. Neurosurgery. 22(2):388-91, 1988 Feb.
(13) Yamamoto T, Marmarou A, Dunbar J, Young H: The evaluation of the prognostic value of a 72 hours prolonged spinal drainage for normal pressure hydrocephalus. ICP X, A Marmarou, R Bullock et al (eds). Springer-Verlag, Wien/New York Acta Neurochir (Suppl) 71: 401, 1998.