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High-Convexity Tightness Predicts the Shunt Response in Idiopathic Normal Pressure Hydrocephalus - AJNR News Digest
July-August 2018
ADULT BRAIN

High-Convexity Tightness Predicts the Shunt Response in Idiopathic Normal Pressure Hydrocephalus

Yoshiyuki Nishio

Wataru Narita

Idiopathic normal pressure hydrocephalus (iNPH) is a neurologic disorder clinically characterized by gait disturbance, cognitive impairment, and an overactive bladder. Ventriculomegaly has been considered a neuroimaging hallmark of iNPH, which reflects the compression of the brain parenchyma caused by abnormal CSF dynamics. However, differentiating iNPH from neurodegenerative diseases, such as Alzheimer disease and progressive nuclear palsy, is difficult solely based on the existence of ventriculomegaly because this morphologic feature is widely observed in the diseases that cause atrophy of the brain.

In 1998, Kitagaki and colleagues1 first demonstrated that decreased CSF space at the midline high convexity (“high-convexity tightness”) is a distinctive neuroimaging feature of iNPH. Contrary to the widely held view of that time, Sylvian fissure dilation was observed in all patients with iNPH in their patient sample. Following Kitagaki and colleagues’ article, the diagnostic value of these neuroimaging features, which are coined “disproportionately enlarged subarachnoid space hydrocephalus (DESH),” has been confirmed by several multicenter prospective cohort studies.2 To date, a variety of additional neuroimaging features have been proposed for iNPH, including focal enlargement of cortical sulci and bumps in the lateral ventricular roof.

CSF shunt surgery is the criterion standard treatment for iNPH. Unfortunately, the effect of this treatment varies from patient to patient. The presurgical prediction of the effectiveness of shunt surgery is a big issue in the management of iNPH. However, the predictive markers of shunt response are currently poorly defined. To address this issue, we investigated the predictive values of presurgical neuroimaging features, including ventriculomegaly, high-convexity tightness, Sylvian fissure dilation, white matter T2 hyperintensities, focal enlargement of cortical sulci, and bumps in the lateral ventricular roof, for 1-year changes in clinical symptoms after CSF shunt surgery in 60 consecutive patients with iNPH.3

Our study demonstrated that high-convexity tightness was the most useful neuroimaging marker to predict a good response to CSF shunt surgery for iNPH. Our results are interpretable from 2 viewpoints. First, high-convexity tightness may correlate with the reversibility of the brain tissue. At the early stage of iNPH, functional impairment presumably results from the compression of the brain parenchyma. As the disease evolves, irreversible pathologic changes, such as axonal injuries and/or neuronal loss, occur as consequences of prolonged brain compression and/or microcirculatory failure. High-convexity tightness may be associated with relatively preserved brain elasticity and other factors that contribute to the reversibility of the brain tissue. Second, high-convexity tightness may represent the morphologic typicality of iNPH. Patients with iNPH frequently have comorbid neurologic conditions, such as neurodegenerative and cerebrovascular diseases, which are associated with poor treatment outcomes. In patients with “pure” iNPH, the brain probably has morphologic features close to the typical pattern or the “mean.” In contrast, the brain morphology of patients with comorbid neurodegenerative diseases should deviate from the typical pattern of iNPH.

The clinical entity of iNPH has faced skepticism and criticism since the original proposal of NPH by Hakim and Adams. A recent review article by Espay and colleagues4 attests that the topic is still a matter of huge controversy. The authors focus on the lack of diagnostic pathologic features/specific diagnostic markers and the scarcity of randomized trials of CSF shunt surgery in iNPH, both of which originate from the circularity of diagnosis by response to treatment.4 We suggest that breaking the circularity and defining the “core” iNPH patients who highly respond to CSF shunt surgery are necessary steps to plan meaningful randomized trials. We believe that our study contributes to that effort by providing evidence for a possible neuroimaging marker that is predictive of a good treatment outcome in iNPH.

References

  1. Kitagaki H, Mori E, Ishii K, et al. CSF spaces in idiopathic normal pressure hydrocephalus: morphology and volumetry. AJNR Am J Neuroradiol 1998;19:1277–84.
  2. Hashimoto M, Ishikawa M, Mori E, et al. Diagnosis of idiopathic normal pressure hydrocephalus is supported by MRI-based scheme: a prospective cohort study. Cerebrospinal Fluid Res 2010;7:18, 10.1186/1743-8454-7-18.
  3. Narita W, Nishio Y, Baba T, et al. High-convexity tightness predicts the shunt response in idiopathic normal pressure hydrocephalus. AJNR Am J Neuroradiol 2016;37:1831–37, 10.3174/ajnr.A4838.
  4. Espay AJ, Da Prat GA, Dwivedi AK, et al. Deconstructing normal pressure hydrocephalus: ventriculomegaly as early sign of neurodegeneration. Ann Neurol 2017;82:503–13, 10.1002/ana.25046.

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