Hypothesis / aims of study
Neurophysiological studies (PNTML and EMG) have demonstrated re-innervation activity, considered a marker of pudendal nerve injury, following vaginal delivery (1). Partial denervation of the pelvic floor has been shown to contribute to the development of stress urinary incontinence, prolapse and fecal incontinence (2).
MR Neurography (MRN) detects changes in signal from nerves and has diagnostic potential to visualise pudendal nerve inflammation, compression and entrapment alongside associated anatomical trauma such as levator avulsion and anal sphincter injury after childbirth (3). There are no published studies evaluating MRN of the pudendal nerve in women after childbirth.
Aims:
1. To develop a MRN protocol to visualise the pudendal nerve and its branches after childbirth.
2. To determine if any abnormalities of the nerve can be visualised after vaginal delivery associated with obstetric anal sphincter injury (OASI).
Study design, materials and methods
Five nulliparous women and 20 primiparous women >18 years who had a first delivery with OASI were recruited. Images were obtained using a high-resolution 3T-MR scanner using a range of protocols that aimed to optimise nerve visualisation. Images from five nulliparous controls were assessed to evaluate and optimise the different protocols.
Analysis of nerve intensity to assess for neuropathy was performed based on previously published data looking at segmental T2 hyperintensity, nerve calibre change, irregularity, loss of fascicular pattern, and denervation oedema/atrophy in target muscles (obturator internus, levator ani and anal sphincter complex). Common entrapment sites were reviewed (between sacrospinous and sacrotuberous ligaments, ischial spine Alcock’s canal). All images were reviewed by two urogynaecoloists and a radiologist and agreement over findings made in tandem.
Results
24 of 25 recruited women completed MRN, (one withdrawal due to claustrophobia), using fourteen different protocols. The mean age was 32.0 (23-40), interval after delivery was 168 days (79-282), BMI 27.3 (20-40) and obstructive defecation score 5.12 (0-14). Nine women had a forceps and three had a ventouse delivery.
The course of the pudendal nerve was traced from the lumbosacral plexus to its terminal branches in the control images (Figure 1). The pudendal neurovascular structures were visible in all study patients and controls. The best protocol includes a water-ideal T2 sequence in the axial plane. Water only sequencing allowed dampening of water flow to help distinguish nerves from blood vessels. A second T2 sequence in the axial plane is recommended to corroborate the findings. Diffusion-weighted imaging (DWI) in the axial plane demonstrates enhancement in nerve signalling indicative of neuropathy. Cube views facilitate tracing neurovascular structures between imaging slices.
Of the 19 study women who completed MRN, 17 had enhancements in signalling overlying one of the key terminal branches (Figure 2). These changes were identified most commonly in the perineal branch (14 bilateral, 1 unilateral), followed by the inferior rectal branch (5 bilateral, 3 unilateral) and the dorsal nerve of the clitoris (6 bilateral, 1 unilateral). Two women demonstrated normal pudendal nerve signalling without changes.
Interpretation of results
This study describes an optimised MRN protocol for the pudendal nerve after childbirth. Vaginal deliveries with OASI are more traumatic than vaginal deliveries with lesser perineal trauma and are more likely to have associated nerve injuries which explains the high proportion of abnormal nerve signalling found in the study group. These were most commonly bilateral and multi-site, and occurred in tandem with other pelvic floor injuries to the levator ani, anal sphincter and soft tissues.