Normal pressure hydrocephalus (NPH) remains a controversial entity with often ambiguous imaging findings. It is classically characterised by the triad of gait apraxia, urinary incontinence, and dementia, although not all patients with NPH have all three.
On imaging, it can be characterised both on CT and MRI by enlarged lateral and third ventricles out of proportion to the cortical sulcal enlargement.
This article is focused on idiopathic NPH. For a discussion of secondary causes of communicating hydrocephalus, please refer to the article hydrocephalus.
It is important to note that there are many causes of communicating hydrocephalus without elevated opening CSF pressures, such as trauma, prior subarachnoid haemorrhage, and meningitis. This is sometimes confusingly referred to as secondary NPH 3. Most clinicians will assume that one is referring to idiopathic NPH if no qualifier is used.
The majority of cases of normal pressure hydrocephalus are idiopathic. The incidence is much higher in elderly populations (although exact numbers are difficult to establish, as the diagnosis is often not made in nursing home patients) for the following reasons:
- approximately 20% of patients in nursing homes have gait impairment
- approximately 10% of these patients also have dementia
- 15% have incontinence 19
A large study in Norway found a striking increase in incidence with increasing age 3:
- 3.3 per 100,000 for people 50 to 59 years of age
- 49.3 per 100,000 for people 60 to 69 years of age
- 181.7 per 100,000 for people 70 to 79 years of age
The classical clinical findings of normal pressure hydrocephalus are 1-3
- urinary incontinence
- deterioration in cognition (dementia)
- gait disturbances
These can be remembered with the unkind mnemonic Wet, Wacky, and Wobbly.
As the name suggests, mean CSF opening pressure in patients with NPH is within the normal range (<18 cmH2O or 13 mmHg) 3.
A classic neurological sign in NPH is magnetic gait, where the patients' feet appear to be magnetically attached to the floor.
The underlying cause remains controversial. One theory is that NPH is an obstructive type of communicating hydrocephalus due to reduced of CSF resorption. A second theory suggests that NPH results from weakening of the ventricular wall due to periventricular white matter ischemic damage 3. The periventricular white matter ischemic change has also been hypothesised to slow the flow of CSF through the extracellular spaces, resulting in a "back-pressure" effect, leading to ventricular enlargement.
Shearing forces are exerted on the periventricular white matter as the ventricles enlarge. As the corticospinal tracts to the legs run medially, these tangential shearing forces cause gait disturbance.
Although CT is able to visualise the anatomical changes of NPH, it is inferior to MRI.
- ventriculomegaly 1-3
- changes in sulcal size
- crowding of the gyri at the vertex (with small sulci)
- cingulate sulcus sign: posterior part of cingulate sulcus is narrower than the anterior part, the divider between the two being a line drawn parallel to the floor of the 4th ventricle17
- Sylvian fissures out of proportion to sulcal enlargement, which is minimal, and hippocampal and mesial temporal lobe volumes, which are near normal
CSF flow changes
- aqueductal flow void due to increased CSF velocity across the aqueduct in patients with NPH
- best seen on T2 and PD spin echo sequences 14 but it is no longer useful sign, as in modern high strength MRI machines aqueductal CSF flow void is present in most of the normal subjects. There is a well-established correlation between higher flow velocities and the favourable outcome after ventriculoperitoneal (VP) shunting; hence, quantitative (rather than qualitative) methods have been developed which can be useful not only in patient selection for shunting but also in diagnosis and predicting prognosis 12-15.
CSF flow studies 3
- increased aqueductal CSF stroke volume
- aqueductal CSF stroke volume is the average volume of CSF flowing through the cerebral aqueduct during both systole and diastole
- increased aqueductal peak velocity
- various publications have set various normal and abnormal ranges
- flow rate of > 24.5mL/min 95% specific for NPH 9,11
- stroke volume of ≥ 42μL shown in one paper to predict a good response to shunting 10
- stroke volume upper limit is now suggested to be variable between institutions due to intrinsic scanner differences; thus each centre should obtain their own "normal values", with the upper limit being suggested as two times the normal value 16
- studies have shown that aqueductal stroke volume decreases later in the disease process despite clinical progression
- this has been theorised to be caused by cerebral atrophy, which indicates that the patient is unlikely to respond to shunt surgery 18
- a recent paper has cast serious doubt on the usefulness of aqueduct stroke volume in patient selection for VP shunting 21
- increased aqueductal CSF stroke volume
MRS has not been shown to have any added value for differentiating idiopathic NPH from other types of dementia, nor does it help in patient selection for VP shunting 22.
Nuclear medicine is less important in diagnosing NPH. Some of the features described are 13
- early detection of the radiotracer into lateral ventricles giving a heart-shaped appearance of lateral ventricles rather than a normal trident pattern
- persistence of radiopharmaceutical beyond 24-48 hours due to impaired absorption
- radiotracer does not extend to the superior aspect of convexities of lateral ventricles
- retrograde CSF flow into lateral ventricles
Treatment and prognosis
Treatment of normal pressure hydrocephalus, once the diagnosis is established, is with CSF shunting, usually a ventriculoperitoneal shunt (VP shunt). The challenge is identifying those patients which will benefit from shunting. Favourable prognostic factors include 3
- short duration of presurgical symptoms (less than 6 months)
- onset of gait disturbance before dementia
- temporary symptom relief from a CSF tap test (removal of 40ml of CSF via lumbar puncture)
- absence of significant cerebral vascular disease
- presence of an aqueductal flow void on T2 imaging 10,14
History and etymology
It is thought to have been initially described by Colombian neurosurgeon Salomón Hakim and R D Adams in 1965, although it may actually have been described earlier under a different name by McHugh 4,6-7.
The possible imaging differential spectrum includes:
- normal ageing brain
- Alzheimer dementia: may show greater dilatation of perihippocampal fissures 2
- obstructive hydrocephalus: due to mass lesion (e.g. pineal region, tectal plate, midbrain)
- Lewy body dementia: visual hallucinations and delusions are more prominent
- Parkinson disease: unilateral symptoms are important
- AIDS-dementia complex: positive HIV serology
- 2. Holodny AI, Waxman R, George AE et-al. MR differential diagnosis of normal-pressure hydrocephalus and Alzheimer disease: significance of perihippocampal fissures. AJNR Am J Neuroradiol. 1998;19 (5): 813-9. AJNR Am J Neuroradiol (abstract) - Pubmed citation
- 3. Hurley RA, Bradley WG, Latifi HT et-al. Normal pressure hydrocephalus: significance of MRI in a potentially treatable dementia. J Neuropsychiatry Clin Neurosci. 1999;11 (3): 297-300. J Neuropsychiatry Clin Neurosci (full text) - Pubmed citation
- 4. Bradley WG. Normal pressure hydrocephalus: new concepts on etiology and diagnosis. AJNR Am J Neuroradiol. 2000;21 (9): 1586-90. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 5. Bradley WG, Scalzo D, Queralt J et-al. Normal-pressure hydrocephalus: evaluation with cerebrospinal fluid flow measurements at MR imaging. Radiology. 1996;198 (2): 523-9. Radiology (abstract) - Pubmed citation
- 6. Conn HO, Lobo FM. What do physicians know about normal pressure hydrocephalus and when did they know it? A survey of 284 physicians. Yale J Biol Med. 2008;81 (1): 19-29. Free text at pubmed - Pubmed citation
- 7. Hakim S, Adams RD. The special clinical problem of symptomatic hydrocephalus with normal cerebrospinal fluid pressure. Observations on cerebrospinal fluid hydrodynamics. J. Neurol. Sci. 1968;2 (4): 307-27. Pubmed citation
- 8. Ivkovic M, Liu B, Ahmed F et-al. Differential diagnosis of normal pressure hydrocephalus by MRI mean diffusivity histogram analysis. AJNR Am J Neuroradiol. 2013;34 (6): 1168-74. AJNR Am J Neuroradiol (full text) - doi:10.3174/ajnr.A3368 - Pubmed citation
- 9. Al-Zain FT, Rademacher G, Lemcke J et-al. [Idiopathic normal-pressure hydrocephalus. Flow measurement of cerebrospinal fluid using phase contrast MRI and its diagnostics importance]. Nervenarzt. 2007;78 (2): 181-7. doi:10.1007/s00115-006-2231-7 - Pubmed citation
- 10. Bradley WG, Scalzo D, Queralt J et-al. Normal-pressure hydrocephalus: evaluation with cerebrospinal fluid flow measurements at MR imaging. Radiology. 1996;198 (2): 523-9. Radiology (abstract) - Pubmed citation
- 11. Manley GT, Hemphill C, Stiver S. Intracranial Pressure and Brain Monitoring XIII. Springer. (2009) ISBN:3211855785. Read it at Google Books - Find it at Amazon
- 12. Krauss JK, Regel JP, Vach W et-al. Flow void of cerebrospinal fluid in idiopathic normal pressure hydrocephalus of the elderly: can it predict outcome after shunting?. Neurosurgery. 1997;40 (1): 67-73. Pubmed citation
- 13. Sandler MP, Coleman RE, Patton JA et-al. Diagnostic Nuclear Medicine. Lippincott Williams & Wilkins. (2003) ISBN:0781732522. Read it at Google Books - Find it at Amazon
- 14. Hayhow B, Begic F, Evans A et-al. Communicating hydrocephalus with reversible cognitive impairment. Aust N Z J Psychiatry. . doi:10.1177/0004867413511547 - Pubmed citation
- 15. Bradley WG. Cerebrospinal fluid dynamics and shunt responsiveness in patients with normal-pressure hydrocephalus. Mayo Clin. Proc. 2002;77 (6): 507-8. doi:10.4065/77.6.507 - Pubmed citation
- 16. Bradley WG. Idiopathic normal pressure hydrocephalus: new findings and thoughts on etiology. AJNR Am J Neuroradiol. 2008;29 (1): 1-3. doi:10.3174/ajnr.A0867 - Pubmed citation
- 17. Adachi M, Kawanami T, Ohshima F et-al. Upper midbrain profile sign and cingulate sulcus sign: MRI findings on sagittal images in idiopathic normal-pressure hydrocephalus, Alzheimer's disease, and progressive supranuclear palsy. Radiat Med. 2006;24 (8): 568-72. doi:10.1007/s11604-006-0074-6 - Pubmed citation
- 18. Scollato A, Tenenbaum R, Bahl G et-al. 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): 192-7. doi:10.3174/ajnr.A0785 - Pubmed citation
- 19. Shprecher D, Schwalb J, Kurlan R. Normal pressure hydrocephalus: diagnosis and treatment. Curr Neurol Neurosci Rep. 2008;8 (5): 371-6. Free text at pubmed - Pubmed citation
- 20. Brean A, Eide PK. Prevalence of probable idiopathic normal pressure hydrocephalus in a Norwegian population. Acta Neurol. Scand. 2008;118 (1): 48-53. doi:10.1111/j.1600-0404.2007.00982.x - Pubmed citation
- 21. Ringstad G, Emblem KE, Geier O, Alperin N, Eide PK. Aqueductal Stroke Volume: Comparisons with Intracranial Pressure Scores in Idiopathic Normal Pressure Hydrocephalus. AJNR. American journal of neuroradiology. 36 (9): 1623-30. doi:10.3174/ajnr.A4340 - Pubmed
- 22. Algin O, Hakyemez B, Parlak M. Proton MR spectroscopy and white matter hyperintensities in idiopathic normal pressure hydrocephalus and other dementias. The British journal of radiology. 83 (993): 747-52. doi:10.1259/bjr/43131041 - Pubmed