Normal Pressure Hydrocephalus

What’s Normal Pressure Hydrocephalus?

Normal pressure hydrocephalus (NPH) refers to a syndrome which results from the build-up of cerebrospinal fluid (CSF). Characteristic symptoms of NPH include ventriculomegaly (cerebral ventricles), gait apraxia (abnormal gait), cognitive impairment, and urinary incontinence. Initial publications on NPH reported that adult patients presented with hydrocephalus (build-up of fluid in the brain) without a concomitant increase in intracranial pressure (i.e. “normal pressure”) (Oliveira, Nitrini and Román, 2019).  “Magnetic” or “glue-footed” gait and frontal ataxia remain other common names of the gait dysfunction seen in NPH. Gait abnormality may manifest in different forms. This depends on the nature of the specific cortical–basal ganglia–thalamocortical circuitry disruption. Diagnosis of idiopathic NPH should focus on discriminating the gait abnormality of other possible causes. Patients with idiopathic NPH-related gait abnormality often have small steps, difficulty turning, wide base, and postural instability with a positive pull test. Idiopathic NPH cognitive impairment, often called frontosubcortical dementia, comes with different severity levels depending on the patient’s pattern of cortical–basal ganglia–thalamocortical circuitry disruption, related to the distribution of increased CSF content in the cranial vault or comorbid pathology. Psychomotor slowing, decreased attention and concentration, apathy, and impaired executive functions form common cognitive features. If a patient experiences anomia, a comorbid cortical neurodegenerative disease may seem likely. Urinary symptoms include frequency, urgency, and over time, urgent incontinence. Urinary urgency often comes as the first urinary sign of dysfunction and then grows into urgent urinary incontinence. Slow gait can aggravate symptoms. A successful CSF diversion procedure leads to significant improvement in these urinary symptoms (Graff-Radford and Jones, 2019).

Nomenclature

The term hydrocephalus combines the Greek words for water (hýdōr) and head (kephalos). Thus, hydrocephalus generally depicts the increase in the CSF content within the cranial vault- caused by different circumstances. The term, however, also depicts the increase in CSF content in the extraventricular cranial subarachnoid space.

What Causes Normal Pressure Hydrocephalus?

Congenital factors

Experts have suggested that congenital factors may play a role in NPH. Under a large umbra, hydrocephalus has two categories: obstructive and communicating. Obstructive (non-communicating) hydrocephalus results as a consequence of a blockage of normal CSF flow through the subarachnoid and ventricular spaces, due to the patient developing a brain lesion that exerts obstructive mass effect or a congenital condition. Stenosis of the cerebral aqueduct in young people commonly leads to hydrocephalus. From the results of research findings, authors have associated large head size with NPH, providing further evidence to support the role of congenital factors in NPH.  In contrast, communicating hydrocephalus leads to an increase in intracranial CSF content without a gross anatomic lesion obstructing flow. NPH falls under communicating hydrocephalus that occurs insidiously without substantially increasing CSF pressure (Graff-Radford and Jones, 2019).

NPH has two classifications; primary and secondary, with most cases classified as primary. Primary/idiopathic normal pressure hydrocephalus (iNPH) occurs without other identifiable causes while secondary NPH results from meningitis, subarachnoid hemorrhage, head trauma, or

brain tumor. NPH remains prevalent in persons above 65 years of age and affects about 0.3% up to 3% of persons in this demographic group. The prevalence rate can turn out higher as NPH closely resembles other neurodegenerative diseases, leaving room for some misdiagnosis (Nassar and Lippa, 2016). 

Vascular Risk Factors

Studies done in humans and with animal models have shown that vascular diseases that alter CSF dynamics may lead patients to develop NPH (Graff-Radford and Jones, 2019).

Cerebrospinal Fluid Absorption Impairment

Early medical scientists thought idiopathic NPH resulted from decreased CSF absorption which causes increased CSF volume over time, with a concomitant compensatory rise in ventricular volume in order to maintain normal intracranial pressure. Recent studies have provided more evidence in support of this reasoning by showing a strong association between poor CSF absorption and good outcomes after shunt surgery in idiopathic NPH patients (Graff-Radford and Jones, 2019).

How Do Specialists Diagnose Normal Pressure Hydrocephalus?

Excellent clinical skills, neuropsychological examination, and diagnostic/imaging modalities can help distinguish between differential diagnoses and NPH. (Nassar and Lippa, 2016).

International guidelines have indicated some imaging features that can guide the diagnosis of NPH. These include

• Ventricular enlargement with Evans’s index greater than 0.3
• The absence of macroscopic obstruction to CSF flow
• Enlarged temporal horns of the lateral ventricles not entirely due to hippocampus atrophy
• Callosal angle of 40° or greater
• Periventricular signal changes on CT and MRI due to altered brain water content and not entirely attributable to microvascular ischemic changes or demyelination. 
• Flow void in the Sylvian aqueduct or fourth ventricle on MRI.

Differential diagnoses of NPH include Alzheimer’s disease (AD), dementia with Lewy bodies (DLB), atypical Parkinsonism, vascular dementia (VaD), and progressive supranuclear palsy (PSP). iNPH has no particular clinical, imaging, or pathological features. It has no definitive tests capable of accurately diagnosing the condition. Thus confirmation can prove quite complex and requires the involvement of dementia experts (Oliveira, Nitrini and Román, 2019).

How Do Specialists Treat Normal Pressure Hydrocephalus? 

Ventriculoperitoneal shunting remains the treatment of choice for iNPH and has a success rate of around 50% to 80%. A majority of patients improve with shunt surgery while a minority undergo surgical treatment with a CSF shunt. According to surgeons, secondary NPH patients often see better outcomes. This may result from swift intervention with adequate VPS placement, which remains the first-line of treatment (Oliveira, Nitrini and Román, 2019). 

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References 

Graff-Radford, N.R. and Jones, D.T. (2019). Normal Pressure Hydrocephalus. CONTINUUM: Lifelong Learning in Neurology, [online] 25(1), pp.165–186. Available at: https://pubmed.ncbi.nlm.nih.gov/30707192/ [Accessed 7 Feb. 2022].

Nassar, B.R. and Lippa, C.F. (2016). Idiopathic Normal Pressure Hydrocephalus. Gerontology and Geriatric Medicine, [online] 2, p.233372141664370. Available at: https://pubmed.ncbi.nlm.nih.gov/28138494/ [Accessed 4 Feb. 2022].

Oliveira, L.M., Nitrini, R. and Román, G.C. (2019). Normal-pressure hydrocephalus: A critical review. Dementia & Neuropsychologia, [online] 13(2), pp.133–143. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6601311/ [Accessed 4 Feb. 2022].