**7. Diagnostic guidelines**

thological evidence demonstrating the involvement of white matter. A diversity of patholog‐ ical observations, such as direct mechanical compression of the periventricular white matter, ischaemic demyelination and infarction, have been noted in INPH [12, 13, 52]. Indeed, DWMC and subcortical infarctions are commonly seen in patients with Binswanger's disease, and these patients often have similar symptoms to those with INPH. Tullberg et al. [51] evaluated the diagnostic features of DWMC and PVH using MRI, and found that no MRI variable could reliably differentiate NPH from BD. One explanation for this result, put forward by the authors, is that NPH and BD are two disorders with similar pathophysiological mechanisms or that

As mentioned earlier, CSF flows back and forth the aqueduct during the cardiac cycle in response to arterial blood flow to the brain. This was initially observed as a flow void, consisting of a decreased MRI signal, mainly in the aqueduct on T2‐weighted images of early MR scans in patients with communicating hydrocephalus [53–55]. CSF flow void can be observed in normal individuals, but it is more prominent in INPH [56]. Increased aqueductal CSF flow initially appeared to be predictive of a good response with shunting [55, 57, 58], but further studies have found poor correlation between the extent of CSF flow void and surgical

Patients with suspected INPH, based on clinical features and neuroimaging, should undergo a high‐volume CSF tap to predict response with shunting. The rationale for a CSF tap is that it simulates the physiological effect of a shunt [45]. The patient is assessed pre‐ and post‐CSF tap for gait and cognitive improvements. About 40–50 ml of CSF is usually removed. Gait is most likely to ameliorate following CSF tap; therefore, it is the best indicator of response. In our centre, we use the 10‐m timed‐walk test and Tinetti test to assess gait and balance before and after CSF removal. Our patients are also consented for video recordings as these can be useful to retrospectively assess patients especially when the improvement following CSF tap is not clear. It is common that some patients or their carers only notice an improvement a couple of days down the line. We therefore carry out follow‐up telephone assessments in all our patients 3 days after they have undergone a CSF tap. This is quite a subjective measure for improvement. Nevertheless, it reduces the chances of missing potentially suitable candidates for shunt surgery. Unfortunately, although the CSF tap test has a high positive‐predictive value for shunt success, it has a low sensitivity [60] and should not be used to exclude patients from shunt surgery. There are patients who do respond to shunt surgery after a negative tap test. The first edition of the Japanese guidelines advocated repeating the tap test if initially negative [61], but the more recent edition suggests that further investigation may be required [62]. Recently, Yamada et al. have shown that the timing of the CSF tap affects the accuracy of the test [63]. It should be carried out as soon as symptoms appear [63]. An external lumbar drain, which provides continuous drainage, has a similar predictive value to the CSF tap test, a higher

they form a pathophysiological continuum of increasing microangiopathy [51].

*6.1.4. CSF flow void*

536 Update on Dementia

outcome [56, 59].

**6.2. CSF tap test**

Different centres use different criteria for diagnosing INPH and recommending shunt surgery. The decision to shunt a patient with predominantly gait disturbance, typical imaging features and a positive CSF tap test is straightforward. However, the difficulty arises when patients do not show characteristic clinical features or do not show a definite improvement to CSF removal. The purpose of diagnostic guidelines in INPH is to identify those patients who are most likely to benefit from shunt surgery. Recently, the American Academy of Neurology has published its own practice guidelines [66]. However, the international [49] and Japanese [62] guidelines are probably the two most widely used and will be given further consideration in this chapter. Both have some similarities but also a few notable differences. The terms 'possible' and


\* These features are supportive but not essential for a diagnosis of possible INPH.

**Table 1.** Imaging features of INPH: comparing the international and Japanese guidelines.

'probable' INPH are employed in each, with diagnostic criteria based on clinical and imaging features. However, the Japanese guidelines use the term 'probable INPH' in those who improve following the removal of CSF. They also label those who respond to shunt surgery as having 'definite' INPH. The international guidelines make no mention of response to CSF removal or shunting in their diagnostic criteria. The neuroimaging criteria also differ. **Table 1** shows a comparison of the diagnostic neuroimaging features used in these guidelines.
