27 Natural History and Management Options of Normal-Pressure Hydrocephalus

Bob Homapour and Chris Xenos

Abstract

Normal pressure hydrocephalus is a form of communicating hydrocephalus that generally affects the elderly population. Clinically, it is characterized by the triad of gait apraxia/ataxia, urinary incontinence, and dementia. The natural history is not clear, although limited available data have suggested a progressive clinical decline over time. The results of ventriculoperitoneal shunt placement typically depend on the strength of the clinical evaluation, appropriate work-up, and treatment indication.

Several population-based studies have attempted to determine the prevalence of idiopathic normal-pressure hydrocephalus (iNPH). The most accurate estimate will most likely be represented by surveys carried out on selected patients based on both clinical examination and radiographic assessment.1^{,​ 2} Generally, the prevalence is accepted at a range of 0.5 to 1.5% of the population over the age of 60 years. This figure may be rising for several reasons such as improved diagnostic modalities, disease awareness, and changes in the age distribution pyramid. The incidence increases with age; however, patients between the ages of 70 and 79 years should receive the most attention. They have a higher incidence than those in their 60 s and also have a better risk-to-benefit ratio for surgical measures related to the diagnosis.³

The proposed theory regarding the pathophysiology of iNPH is based on Greitz’s description of the hydrodynamic theory after the appreciation of the Windkessel effect, that is, the elastic reservoir of blood vessels. In the elderly population, especially in the presence of arteriolosclerosis, the Windkessel effect may not occur as the vessel elastin fragments diminish. As the wall of the basilar arteries become more rigid, the systolic pulse waveform is not absorbed by the elasticity of the vessels and is transmitted unimpeded into the capillary bed of the parenchyma. The arterial pulse wave flows unimpeded through the entire parenchyma. This leads to a short-lived increase in parenchymal volume during systole. In the subarachnoid space, this can be compensated for by discharging cerebrospinal fluid (CSF) through the foramen magnum. This does not happen for the parenchyma adjacent to the ventricles and an increased counter pressure occurs. A gradual degeneration of the parenchyma surrounding the ventricle occurs resulting in a passive widening of the caliber of the ventricle without the need for an increase in ventricular pressure.⁴

The cardinal symptoms of the disease include gait disturbance, dementia, and incontinence. This is often referred to as Hakim’s triad. The components may be present concurrently or at different times. Most symptoms, if present, will progress with time. This progression can show variability in its time course. Typically, the condition occurs in patients older than 60 years. All three symptoms will present simultaneously in about one-half of the patients.³ Gait disturbance is almost always present, whereas incontinence or dementia alone caused by iNPH is extremely rare. The gait disturbance is often the first feature noted and is described as a gait apraxia. There is decreased velocity with shorter and more variable strides. Step height is reduced and the dorsiflexion of the foot is insufficient (magnetic gait). The arm swing is preserved distinguishing it from Parkinson’s disease.⁵ Urinary dysfunction starts as frequency and urgency and may progress to incontinence in advanced stages. Fecal incontinence is also a late feature. Due to the age of this cohort, 90% have other significant comorbidities. Except for evidence of gait, balance, and cognitive dysfunction, the neurological examination in iNPH patients is often normal and any lateralizing sign should raise suspicion of an alternative secondary diagnosis.⁶

Magnetic resonance imaging (MRI) is the preferred modality to confirm the absence of a secondary cause and the presence of CSF pathway communication. The Evans ratio is often found to be greater than 0.3. Characteristic morphological features on MRI include frontal and parietal narrowing of the subarachnoid space, upward bowing of the corpus callosum, and marked dilatation of the Sylvian fissure³ (Fig. 27.1). The mismatch between the narrowing of the subarachnoid space at the high convexity with the widening of the Sylvian fissure on fluid-attenuated inversion recovery (FLAIR) sequences seems to have a strong positive predictive value (PPV).⁷ In Japanese papers, this is described as Disproportionately enlarged subarachnoid space hydrocephalus (DESH). DESH may have a high predictive value in demonstrating shunt responsiveness, but its negative predictive value was not found to be as useful in excluding patients from surgery.⁸ Periventricular white mater lesions on FLAIR are common, but if positioned more peripherally, may represent chronic ischemic changes.⁹

Gait disturbances react most extensively to treatment; therefore, gait tests are of high clinical significance. Some symptoms are subtle and subjective, so it is important to consider patient and relative reports regarding everyday life.¹⁰ Gait speed is generally assessed by the time taken to cover a 10-m distance. In addition, a 3-m timed up and go (TUG) test is performed. The TUG test has been shown to be a reliable quantitative test for predicting improvements in gait in patients 12 months after surgery.¹¹ This assessment also includes recording step length and the quality of the gait.

After the initial recording of the gait parameters, the test is repeated the same day after a tap test. The lumbar puncture is performed with an 18- to 20-gauge spinal needle. CSF is taken for analysis and the opening pressure is measured. The average opening pressures in NPH studies range from 8.8 ± 1.3 to 14.62 ± 1.5 mm Hg. In the cases where the opening pressure is greater than 18 mm Hg, further investigations are needed to exclude secondary causes.¹² Approximately 30 to 50 mL of CSF is allowed to drain off and after a brief period of rest on the short stay ward, the gait assessment is repeated by the same assessor. If the tap test is negative, the patient is given the option of repeating the gait test or insertion of a lumbar drain for 48 to 72 hours. The drainage system is monitored to produce an effusion rate of CSF of about 5 to 10 mL per hour and ideally a total drainage of greater than 300 mL.¹³ The gait tests are recorded before, during, and after the drainage period.

Other invasive diagnostic modalities include intracranial pressure (ICP) monitoring and lumbar infusion test. During ICP monitoring, the presence of B waves has been noted to be associated with NPH due to presumed lower compliance. This involves ICP oscillations with a frequency of 0.3 to 3 per minute and amplitudes of up to 50 mm Hg. False positives are relatively common. Increased pulse pressure amplitude during sleep has a much stronger correlation with NPH. This correlation with shunt responsiveness varies from 50 to 90%. Unfortunately, these studies are limited by a paucity of aged-matched control data.14 This phenomenon has also been demonstrated in lumbar infusion tests. As iNPH is communicating, the ICP monitoring can be done through a lumbar drain, needle, or intracranial device.¹⁵ In the infusion test, the ICP is measured during an infusion of artificial CSF at a constant rate between 0.76 and 2 mL/min.¹⁶ This typically involves infusion through one spinal needle and pressure measurement through a second needle. The increased CSF pressure generated by the infusion is registered and once a new steady state for pressure is achieved, the infusion is discontinued. At the point when the newly set plateau is achieved, it is assumed that the reabsorption rate is equal to the infusion rate. The infusion usually takes about 40 to 60 minutes to achieve the plateau in pressure. Outflow resistance R _out (Torr/mL/min) is a measure of the pressure increase following a bolus injection. By calculating outflow resistance, the quantitative extent of a disruption of reabsorption or disruption of passage of CSF can be determined. ICP increases with increasing outflow resistance in a nonlinear fashion.¹⁷ Outflow resistance is calculated with the following formula and considered abnormal above a value of 13 (Fig. 27.2):

R _out = P _SS – P ₀ /I _INF.

The PPV of the tap test is reported to be between 73 and 100%, with sensitivity at 26 to 62% and specificity of 33 to 100%. The PPV of external lumbar drainage is reported as 80 to 100%, with sensitivity of 50 to 100% and specificity of 60 to 100%. The infusion test has a higher sensitivity than the tap test (57–100%), but a similar predictive value of 75 to 92%. Some authors advocate combining the tap test with other modalities such as those mentioned earlier to increase the PPV.^{18,​ 19} Trial of lumbar drainage for a number of days has the highest negative predictive value and PPV.¹²

27.2 Selected Papers on the Natural History of Idiopathic Normal Pressure Hydrocephalus

●Scollato A, Tenenbaum R, Bahl G, Celerini M, Salani B, Di Lorenzo N. Changes in aqueductal CSF stroke volume and progression of symptoms in patients with unshunted idiopathic normal pressure hydrocephalus. AJNR Am J Neuroradiol 2008;29(1):192–197

The natural history of iNPH is not clear and the evidence to support the treatment is not of high quality. There is no gold standard test for diagnosis. If shunt placement were offered solely on the basis of history, examination, and presence of ventriculomegaly on imaging, only 46 to 61% of patients would benefit from surgery. Only very few studies include iNPH patients who did not undergo shunting and include the criteria of diagnosis and an objective method of outcome assessment.20 Six studies are listed in Table 27.1, which include the inclusion criteria and outcome of 102 patients who were diagnosed iNPH and did not undergo shunt insertion surgery. The follow-up period ranges from 3 months to 7.2 years.^{18,​ 21,​ 22,​ 23,​ 24,​ 25} Table 27.2 ^{21,​ 22} shows two studies that looked into the outcome of unshunted probable iNPH patients with no added confounding factor. Both show that unshunted patients progressively worsen over time. Table 27.3 ^{24,​ 25} shows the results of patients thought to benefit from shunting based on selection with a prognostic test. Since none of these tests are 100% specific or sensitive, the unshunted group might include some true iNPH patients. The rest might represent patients with other diagnoses or advanced iNPH. The natural history of patients thought to be unlikely to benefit from shunt surgery is unknown. Some patients did improve, demonstrating iNPH is not a universally progressive condition. However, the contribution of improvement from the temporary lumbar drainage of CSF is not clear. A significant number of patients showed evidence of clinical stabilization of one or more components of the triad. All studies demonstrate an improvement in either clinical assessment or outcomes score in the shunted patients. The patients who did not undergo shunting were excluded because either they did not meet the criteria set by the authors or the patients refused surgery. Each study has a different diagnostic assessment and inclusion criteria for shunting. Follow-up periods vary and each study presents outcome data in a different way. The percentage of patients who either do not change, or worsened clinically, is significantly higher in the unshunted group. No trial has yet compared the placement of a shunt versus conservative management in a randomized controlled manner. Although the level of evidence for these studies is classed at 4, collectively it is possible to draw reasonable conclusions. The majority of unshunted patients deteriorate without surgery and shunt insertion does improve outcome. The deterioration in the iNPH can occur as early as 3 months after initial assessment. In a study in which shunting was delayed due to hospital-related administrative issues, the magnitude of improvement after surgery was not affected by the length of preoperative delay, which shows reversibility even when treated late. The final outcome of surgery is often linked to the severity of preoperative symptoms. However, the delay in treatment resulted in progression of comorbidities, and the proportion of patients able to live independently was significantly lower in the delayed group.²⁶ From a health economic perspective, shunt surgery is cost-effective and reduces caregiver burden.²⁷

Table 27.1 Summary of studies looking at outcomes of shunted versus unshunted patients

Table 27.2 Studies that looked at outcome of unshunted patients who were diagnosed based on radiology and clinical findings

Table 27.3 Studies that looked at outcomes in unshunted patients where the diagnosis was based on a prognostic test

27.4 Selected Papers on the Treatment Options of Idiopathic Normal Pressure Hydrocephalus

●Malm J, Kristensen B, Stegmayr B, Fagerlund M, Koskinen LO. Three-year survival and functional outcome of patients with idiopathic adult hydrocephalus syndrome. Neurology 2000;55(4):576–578

Although there have been studies to suggest lumboperitoneal shunting is a safe and beneficial option, because of their higher failure rates, the ventriculoperitoneal (VP) shunt is usually considered the treatment of choice.28^{,​ 29} There is some evidence to suggest that clinical improvement is significantly better with a low opening pressure of the shunt. However, this has to be balanced with the increased risk of overdrainage. Therefore, the addition of an antisiphon device is usually recommended. The condition of patients typically deteriorates even with a patent shunt; therefore, adjustability of the opening pressure of the shunt may offer some advantage at this stage. Initial valve adjustments are not necessary for at least 6 weeks to 3 months after the placement of a shunt. This is because of the chronic nature of the disease and the slow pace at which clinical symptoms can vary.³⁰ Although there are no specific data showing the superiority of one valve over another in this condition, our surgeons use almost exclusively the Miethke ProGav 2.0 programmable shunt with reservoir and a patient appropriate shuntassistant antisiphon unit. The other commonly used system is the Codman Hakim programmable valve with Siphonguard as the antisiphon device. To select the valve opening pressure, the following equation can be used to achieve normal ICPs:

ICP = GD – HPD + VOP + IAP,

where HPD is the hydrostatic pressure difference between the abdomen and the head, VOP is the valve opening pressure, IAP is the intra-abdominal pressure, and GD is the setting for the gravitational unit. The average ICP in the lying position has been found to be approximately 13 cm H₂O and the average IAP is between 7 and 10 cm H₂O. HPD is generally taken to be zero in these calculations. Variations can exist when patients tend toward obesity or extremes of height. As mentioned earlier, it is advantageous to be able to adjust VOP to match deterioration in symptoms with time or changes in body habitus resulting in changes in IAP.³¹

However, downregulating the pressure improves drainage in the horizontal position but increases risks associated with overdraining. In this situation, adjustability of the gravitational device may be more advantageous. All shunt configurations used for NPH in our unit contain an adjustable valve, an antisiphon device either integrated in the valve or as an in line add-on gravitational valve, a reservoir, and antibiotic impregnated catheters (Fig. 27.3). Adjustable gravitational units (ProSA, Aesculap) are used only in special circumstances or following complications. The Dutch Normal-Pressure Hydrocephalus study in 1999 demonstrated better outcomes in the course of the disease with lower set differential pressure valves. Unfortunately, the lower valve settings had a 73% overdrainage rates compared to 34% for the intermediate-pressure range.³²

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