TIPS AND TRICKS

Head circumference above the 90th percentilefor age and height is associated with a three-fold increased risk of adult hydrocephalus[4].

Several lines of evidence indicate that CSF pulsatility is disturbed in NPH. High CSF pulse pressures (despite normal or near normal static ICP) could physically distort the ventricles and alter the normal pathways for the clearance of cerebrospinal fluid. CSF is produced intraventricularly by arterial pulsations in the choroid plexus. It then circulates down through the cerebrospinal cavity before returning to the superior sagittal sinus (SSS) where it is cleared via absorption by arachnoid villi. In NPH, clearance through the SSS may be impeded as a result of compression of its tributary veins during the cardiac cycle[6]. It has been noted that some patients with NPH have enlarged subarachnoid spaces (SAS) toward the ventral and middle portions of the brain but reduced spaces around the superior sagittal sinus (see Neuroimaging section, below). This is thought to indicate an altered pathway of CSF circulation in NPH in which increased bulk reflux of CSF occurs into ventricles during diastole, fostering reverse transependymal CSF flow and clearance through lateral venous sinuses.

SCIENCE REVISITED

The Monro–Kelli Doctrine, hyperdynamic CSF flow and venous pulsations in NPH
The adult skull is a closed, non-distensible space. Each time the heart beats, arterial blood flows into the cranial cavity under pressure. According to the Monro–Kelli Doctrine, the volume of the arterial inflow must be equal to the volume of CSF and venous blood that leaves the cranial cavity. Net input must equal net output.
Normally, the brain is compliant enough that the energy of each cardiac pulsation is partially transferred to brain tissue, resulting in outward radial expansion of the ventricles. In NPH, the ventricles become maximally enlarged. As a result, most of the energy of arterial pulsation is transferred to the CSF and venous system. This can result in rapid “hyperdynamic” CSF flow through the aqueduct of Silvius, third ventricle and fourth ventricle that can be detected by phase contrast MRI, as well as pulsatile venous flow.

It is not entirely known how disturbances in the CSF compartment translate into brain dysfunction and the clinical symptoms in NPH. Static ICP is inadequately elevated in NPH to cause cerebral dysfunction in itself. However, ventricular distension and widened dynamic CSF pulse pressures may be sufficient to compress capacitance vessels in the brain, reducing their availability to increase cerebral perfusion during periods of peak demand. Reduced cortical cerebral blood flow can occur in NPH, but overall brain perfusion is not universally compromised nor is it consistently improved after treatment. Small vessel ischemic cerebrovascular disease is nevertheless linked to progression of NPH. As the burden of cerebrovascular disease increases, NPH generally becomes more refractory to treatment. In chronically untreated cases, small vessel infarction occurs throughout the periventricular region, giving rise to a condition that is virtually indistinguishable from Binswanger’s disease.

Ventricular expansion, transependymal fluid movement and age-associated reductions in cerebral compliance may make the brain more susceptible to the repeated impact of the CSF pulsations. Physical distortion of neurons and their periventricular processes caused by ventricular enlargement has been hypothesized to delay or disrupt neuronal transmission in NPH.

Pathological studies have failed to identify lesions at the gross or molecular levels that are universally diagnostic of NPH or unequivocally explain its etiology Not surprisingly, the most consistent finding in NPH patients at autopsy is simply enlargement of the cerebral ventricles.

Symptoms

Normal pressure hydrocephalus is associated with gait ataxia, urinary incontinence, and dementia. Symptoms fall on a continuum from very mild to quite severe and are not limited to those of the classic triad. Symptoms are stage dependent, and may be minimal early in the disease and/or confined to just one or two domains of the triad. It is important for clinicians to become familiar with the full spectrum of presentations and stages of NPH.

Gait and balance

Impairments of walking and balance are the most readily observed symptoms of NPH and the most reliably reversed by treatment.

The characteristic gait disturbance in NPH is variably described as “magnetic,” “shuffling” or glue-footed.”
Patients with NPH typically show a reduced foot–floor clearance and a widened base, walking in short steps with their toes pointed outward. The number of steps required to cover a given distance is often increased as a result.
There is reduced counterrotation of the hips and shoulders while walking. Accelerometer studies show an increased tendency to sway while walking as well as standing in place.
Tandem gait is frequently disturbed, although this is observed in many elderly patients without NPH.
It often takes symptomatic NPH patients longer than normal to rise from a chair and to walk a short distance.
Patients with NPH frequently fall backward (retropulse) either spontaneously or as a consequence of being pulled backwards on the “Pull Test.”
Turning in place may require multiple small steps, so-called “en bloc” turning.
NPH patients often fall directly forward or backward when bending or on uneven terrain, but can fall in any direction.
There may be a prolonged latency when starting ambulation or stopping, a symptom overlapping that of idiopathic Parkinson’s disease (PD). Parkinsonism may be present in NPH patients either as a co-morbid illness or as a consequence of NPH itself. Parkinsonism secondary to NPH tends to be less responsive to treatment with dopamine precursors or agonists than in uncomplicated PD.

A timed walking test over a set distance is an inexpensive and sensitive method for identifying and following the gait disturbances in NPH. Standardized clinical gait assessment is useful for rating the full range of associated gait and balance disturbances.

Control of urination

The most common urinary symptoms in NPH are frequency, urgency, and nocturia. These early symptoms may progress to urinary incontinence as the disease progresses. In most cases, incontinence is confined to micturition but in advanced stages defecation may be involved as well. NPH patients are initially aware of their urinary symptoms and embarrassed when incontinence develops. With progression of the disease and particularly with advancing dementia, they may develop indifference to incontinence.

Asking subjects or spouses to keep a bladder diary indicating frequency/urgency of urination and incidents of incontinence can provide useful diagnostic information. Urological evaluation is recommended to rule out other causes of urinary dysfunction. Urodynamic studies in NPH patients tend to show a neurogenic-type pattern and may reveal an increased postvoid residual. Persons with untreated NPH may be at increased risk of urinary tract infections (UTI) owing to incomplete voiding. Those with recurrent UTIs may benefit from antimicrobial prophylaxis.

Cognition

The profile of cognitive impairments in NPH is typically “subcortical” with frontally weighted deficits and relative sparing of language function. Not infrequently, NPH occurs in combination with diseases such as Alzheimer’s which may add elements of cortical, limbic, and paralimbic disturbances to the profile of cognitive dysfunction. Cognitive impairment in NPH often manifests as a disturbance of executive function, with difficulties carrying out multistep tasks, multitasking, formulating abstractions, and dividing attention. Memory loss is often secondary to impaired information retrieval. Recognition memory is relatively preserved as evidenced by performance improving with cues or multiple choice. This contrasts with Alzheimer’s disease in which the information is rapidly lost from memory and may neither be recalled or recognized. Language ability usually remains intact, although phonemic (letter) fluency and confrontational naming are decreased in conjunction with frontal systems deficits. Ideomotor praxis may be preserved but some patients with NPH have difficulty transitioning from a standing to a recumbent position such as on an examining table.

Screening tests such as the Folstein Mini Mental State Examination may not be sufficiently sensitive to detect subtle frontal systems deficits in NPH. Timed performance-based tasks and tasks with frontal weightings are recommended to assess impairments in suspected cases of NPH. Tests such as Trails B and the Symbol-Digit Test tend to be sensitive to NPH-related deficits as well as to improvement with treatment. Certain tests of upper extremity function (maze drawing and serial dotting) have recently been found sensitive to impairments in NPH and are responsive to CSF drainage[7]. Neuropsychological testing can be useful in documenting subtle cognitive dysfunction in mild stages of NPH and for tracking response to treatment.

Other findings

A variety of psychiatric disturbances ranging from psychosis and agitation to depression and anxiety disorders have been reported in association with NPH, either as exacerbation of preexisting conditions or arising de novo. In some cases, psychiatric symptoms are responsive to treatment of hydrocephalus. Recent onset of hypertension has been reported in an unexpected fraction of patients with newly diagnosed NPH, leading to the speculation of a possible causal relationship. Decreased hearing and frank deafness have been rarely associated with NPH, but primarily in the aftermath of shunt placement rather than as presenting symptoms. The same is true of epilepsy, which may occur in as many as 10% of shunted NH patients.

Diagnostic criteria

International consensus criteria for the diagnosis of idiopathic NPH were published in 2005[8]. These evidence-based guidelines divide NPH into probable and possible subcategories to reflect the level of certainty about the diagnosis. The guidelines also identify “shunt-responsive NPH” as the subset of cases that have a positive outcome from treatment. The criteria for probable and possible INPH are listed in Box 4.1.

Differential diagnosis

The symptoms of NPH overlap those of several conditions that are common in elderly individuals. Alzheimer’s disease, Parkinson’s disease and other neurodegenerative conditions can manifest similar symptoms and may occur coincidentally with NPH. Spinal stenosis, arthritic conditions, and orthopedic disorders can cause gait and balance disturbances resembling those of NPH. Prostatic enlargement and a number of other urological conditions can give rise to the urgency, frequency, and incontinence that are also associated with NPH. Differential diagnosis of NPH therefore requires careful exclusion of other conditions, while appreciating that some cases of NPH occur in the setting of co-morbidities. The challenge when such conditions are found becomes determination of the extent to which symptoms are attributable to NPH versus the co-morbidities.

Box 4.1 International consensus criteria for idiopathic normal pressure hydrocephalus

Probable idiopathic NPH

The diagnosis of probable idiopathic NPH is based on clinical history, brain imaging, physical findings, and physiological criteria

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