Idiopathic Normal Pressure Hydrocephalus
Idiopathic normal pressure hydrocephalus or iNPH is a condition of aging characterised by a triad of clinical features namely – dementia, gait disturbance and urinary incontinence. Patients are typically in their sixties to eighties and have characteristic CT or MRI scan findings.
It is an under recognised condition and in many cases a patient’s symptoms may simply be accepted as normal aging or be attributed to another condition for which there may be no effective treatment. Estimates of incidence or prevalence vary but it is probably found in about 1.5% of people over 60 years of age meaning that only about 20% of people with iNPH receive treatment.
The importance of recognising the condition is underlined by the fact that it is one of the few reversible causes for cognitive decline or dementia. In some cases, the effects can be dramatic, in other cases the effects of treatment are less dramatic but can still be life changing.
What are the symptoms of NPH?
Clinically, there is a gradual, progressive decline in short term memory with loss of executive functions, e.g., ability to plan and strategise. Patients may become apathetic and even somnolent.
The gait is broad based due to poor balance. Foot steps are short and shuffling with occasional freezing. Some have described it as lower body Parkinson’s disease.
Urinary symptoms initially consist of urgency but will progress to more frequent episodes of incontinence. Not all of the clinical features need to be present. Prominence of a gait disorder is said to be a good prognostic sign.
How is NPH diagnosed?
The diagnosis can be difficult and there is no one test that will ‘clinch’ the diagnosis. Diagnosis is based on the usual combination of the history, physical findings and imaging (CT or MR) but may be assisted by supplementary investigations such as CSF tap tests.
CT or MRI scans demonstrate large lateral ventricles out of proportion to sulcal dilation. In iNPH it is a communicating form of hydrocephalus so generous subarachnoid spaces may still be seen. Dilation of the Sylvian fissures is common. Periventricular and deep white matter hyper-intensities do not preclude the diagnosis.
Ancillary tests such as CSF infusion studies, tap tests and lumbar drains attempt determine the extent of CSF circulation disruption by measuring values such as the resistance to CSF absorption, or by trying to replicate the effects of CSF shunting by draining CSF temporarily.
Prof Owler’s preference is to use these tests more sparingly. The CSF tap test is a lumbar puncture where about 20mls of CSF is drained. Although this should be replaced relatively quickly in theory, in practice a temporary but significant improvement in symptoms may be seen in some patients. A positive result to a CSF tap test is said to have a high positive predictive value, but a negative test has a low negative predictive value. Hence, this test is reserved for patients whose presentation is less typical.
How is NPH treated?
The treatment of iNPH is surgical in the form of a CSF shunt. The shunt is simply a tube that drains CSF from the ventricle in the brain to a distant site for absorption – this is almost always the peritoneal cavity. A valve regulates the flow of CSF through the shunt. Prof Owler uses a programmable valve set at a medium-high pressure initially which can be gradually reduced as needed. Adjustment takes a few seconds in the office using a magnet.
While CSF shunting is a relatively simple neurosurgical procedure, the elderly nature of patients with the condition and added complexities of multiple co-morbidities make this patient group fragile and prone to complications. Serious consideration to the potential risks must be given before proceeding with treatment. Please refer to the separate section of CSF shunts for hydrocephalus on this website.
To learn more about CSF shunts for hydrcephalus, click on the link below:
How is outcome measured and what are the results?
Our patients with presumed iNPH who are considered for CSF shunting are normally assessed using the Addenbrooke’s Cognitive Examination. The gait is assessed by various timed tests, measures of balance and through video. We also incorporate other measures or urinary symptoms and independence. These objective measurements are repeated post-operatively at 3, 6 and 12 months.
We have published our results using this approach. Approximately 85% of patients demonstrate significant improvement after CSF shunting Furthermore, amongst those that do improve, improvement, particularly in gait, continues over a 12-month period presumably due to improvement in muscle condition and overall fitness. Patients who do not improve as expected on these tests can have their shunt tested and/or adjusted. Patients and their relatives generally find these assessments reassuring and educational.
While patients with iNPH represent a small proportion of patients with dementia and deteriorating gait, it is worthwhile considering the diagnosis especially in patients who exhibit ventricular enlargement on imaging in the absence or significant atrophy. Successful treatment can significantly improve the quality of life not only of patients but also of their families.