The aetiologies of lower urinary tract (LUT) dysfunctions (e.g. overactive bladder syndrome and dysfunctional voiding) is not well understood. One of the potential causes is peripheral neuropathy. The LUT is innervated by three peripheral nerves: the hypogastric, pelvic and pudendal nerves. The pelvic and pudendal nerves arise from the sacral plexus, specifically the motor neurones of S2, S3 and S4. The hypogastric nerve is part of the sympathetic nervous system, arising from T10-L2. 

Peripheral nerves can be directly visualised with standard magnetic resonance imaging (MRI) or ultrasound (US) techniques. However, these methods assess nerves qualitatively. Diffusion tensor imaging (DTI) is an MRI technique which can assess axonal integrity and nerve function (1, 2). DTI measures the Brownian motion of water molecules within the nervous system to assess axonal organisation. In fluid-filled spaces, waters diffusivity is described as isotropic, as it diffuses equally in all directions. Cell membranes can hinder the diffusion of water resulting in anisotropy (directionally dependent motion). Peripheral nerves have high anisotropy due to their well-organised structure and axon bundles that run longitudinally along the proximal to distal course of the nerve. Pathological changes to the peripheral nervous system can modify the axonal tracts integrity, leading to changes in its diffusivity, which is quantifiable using DTI.

DTI characterises diffusivity mathematically in a 3D space with a symmetric 3 x 3 matrix (measured in 6 or more different directions). DTI can be described by its eigenvectors and eigenvalues (λ1, λ2 and λ3), which are used to derive the quantitative parameters: fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD).

The study aims to systematically review the use of DTI in the lumbosacral plexus (LSP) to evaluate its potential application clinically and as a research tool in neuro-urology.

A comprehensive literature review was conducted following the PRISMA guidelines (3) using the PubMed/Medline, Embase (with online Ovid interface) and Cochrane databases. A flowchart of the selection process is depicted in Figure 1. 

The search query used ‘(((Diffusion tensor imaging) OR (fiber tractography) OR (DTI) OR (magnetic resonance tractography)) AND ((lumbosacral plexus) OR (sacral plexus) OR (sacral nerves)) AND (human))’, as this resulted in more results than utilising Medical Subject Heading (Mesh) diffusion tensor imaging AND lumbosacral plexus. The last search date was the 30th March 2023.  

Inclusion criteria included living human studies written in English with DTI used to visualise and analyse a minimum of S1 and S2 nerves.

As seen in Figure 1, 101 studies were identified. After removal of duplicates, 71 were assessed for eligibility. 20 studies were selected for full text assessment, with 12 being deemed appropriate for inclusion. 

Outcomes of the studies included for review are displayed in Figure 2. Parameters included: study size, study population, nerves analysed and clinical findings.  

12 studies analysed the lumbosacral plexus using DTI. 6/12 included patients with lower urinary tract symptoms or endometriosis. 

5/12 studies compared more than one patient with a known pathology with asymptomatic controls. 4/5 studies found statistically significant differences in either FA or MD DTI parameters between patient cohorts and asymptomatic controls.

12 studies demonstrated DTI can successfully quantify peripheral nerve axonal tracts integrity of LSP. 9 studies have reported DTI parameters are significantly different between controls and patient cohorts with pathology known to result in neuronal dysfunction. The role of DTI in “idiopathic” LUT tract dysfunction does not appear to have been directly investigated. Furthermore, future DTI studies should not exclusively focus on the LSP. The hypogastric nerve, arising from T10-L2, can also contribute to lower urinary tract dysfunction and therefore should also be assessed. As the hypogastric nerves have a larger diameter, DTI analysis should be less challenging than at the LSP.

Numerous studies have convincingly demonstrated DTI can quantify peripheral nerve axonal tracts integrity, a proxy for nerve function, of the LSP. “Idiopathic” LUT dysfunction may arise secondary to pathology at the central or peripheral nervous system and/or myogenic/urethral level. DTI parameters may be a key piece in the puzzle to help identify the root cause in select patients to allow for better targeted/more effective management.

Basser PJ, Mattiello J, LeBihan D. MR diffusion tensor spectroscopy and imaging. Biophys J. 1994;66(1):259-267. doi:10.1016/S0006-3495(94)80775-1
Chiou SY, Hellyer PJ, Sharp DJ, Newbould RD, Patel MC, Strutton PH. Relationships between the integrity and function of lumbar nerve roots as assessed by diffusion tensor imaging and neurophysiology. Neuroradiology. 2017;59(9):893-903. doi:10.1007/s00234-017-1869-0
Page MJ, McKenzie JE, Bossuyt PM, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021;372:n71. Published 2021 Mar 29. doi:10.1136/bmj.n71

Continence 7S1 (2023) 100927
DOI: 10.1016/j.cont.2023.100927