Midline sacral meningeal cysts (MSMC) are cerebrospinal fluid (CSF)-filled dural diverticula that communicate with the terminal thecal sac (1). Although widely considered to be asymptomatic, cases with voiding difficulties or radicular pain have been anecdotally reported. The aim of this study is firstly, to describe the clinical presentation of patients with symptomatic MSMC; secondly, to assess the impact of the cyst on sacral nerve root function; and thirdly, to assess whether nerve root injury is more frequent in patients with MSMC than in those with Tarlov cysts, the most frequent type of sacral nerve cysts.

Consecutive patients with MSMC, referred for pelvic neurological assessment between January 2017 and July 2021 and presenting with at least one symptom relative to the pelvic area, participated in a cross-sectional review of symptoms using validated questionnaires: the Urinary Symptom Profile (USP) to assess stress urinary incontinence, overactive bladder and low stream symptoms, the Constipation Scoring System and the Arizona Sexual Experience Scale to identify sexual dysfunction. Findings of pelvic neurophysiology (pudendal sensory evoked potentials, sacral dermatomal sensory evoked potentials, external anal sphincter electromyography) and urodynamics testing were collected retrospectively. The relationship between neurophysiology, MRI findings and patients’ symptoms were assessed using Fisher and ANOVA tests. Neurophysiology findings were compared with those of Tarlov cyst patients from a previous study performed in the department during the same period of inclusion (2).

Eleven female patients were included (mean age 42.3±12.4y). All patients reported urinary symptoms. Back pain (91%), radicular leg pain (91%), bowel symptoms (45%) and sexual dysfunction (75%) were also frequently reported. 
Nine patients (82%) had abnormal findings on neurophysiology; three (27%) patients had one abnormal test, and six (55%) had two abnormal tests. Seven patients had abnormal dermatomal SEPs (64%); five patients had abnormalities in one dermatome, two patients in three dermatomes. Seven patients had abnormal pudendal SEPs (64%), and six patients had unilateral abnormality, while one patient had bilateral pudendal SEP abnormalities. Only one patient (11%) had abnormal EAS electromyography.
MRI showed compression of a median of 6 (range: 3-8) sacral nerve roots. No association was observed between abnormal dermatomal SEPs and compression of the related sacral nerve roots. Abnormal neurophysiology results did not relate to the dimensional extent of the cyst, as neither the number of sacral vertebral levels (p=0.94) nor the greatest dimension in the mediolateral axis (p=0.58) correlated with neurophysiology findings.

Ten patients underwent a uroflowmetry, four of which had an abnormal curve, and three had a post-void residual > 100ml.
Clinical presentation and results of the different tests are summarized in Table 1.

Sixty-five female patients were included in a simultaneously running study assessing nerve root injury in patients with sacral Tarlov cysts, using the same protocol. The proportion of abnormal neurophysiology tests was not significantly different between patients with TC and those with MSMC (57% versus 82%, respectively, p=0.19). However the number of abnormal tests was greater in patients with MSMC (p=0.046), and the proportion of patients with at least two abnormal tests was significantly higher in patients with MSMC than in those with sacral TC (55% versus 18%, respectively, p=0.018).

Whilst TC are dilatations of the perineurial space within nerve root sheaths, MSMC are true dural diverticula. They arise from the terminal thecal sac, usually in the midline region, expand within the sacral canal and, depending on whether they arise from the ventral or the dorsal aspect of the dural sac, will compress the sacral nerve roots against the dorsal or ventral surface of the sacral canal, respectively. MSMC and TC interactions with the sacral nerve roots are therefore very different: in TC, the nerve root fibres are either splayed out in the cyst’s wall or run through the cyst’s lumen; with MSMC, the sacral nerve roots are extrinsically compressed. This difference may therefore account for the higher rate of neurological damage found in MSMC.
Similar to our findings in TC patients, the neurophysiology abnormalities in MSMC patients preferentially involved sensory over motor pathways. We hypothesise that this is related to nerve fibre size, sensory nerves being thinner and less myelinated than motor nerve fibres, and thereby more sensitive to compression-induced injury. 
Almost all patients described bladder symptoms and a large proportion reported constipation or sexual dysfunction. 
It is plausible that sacral nerve root injury may impact genito-urinary functions seeing that the sacral somatic and splanchnic innervation travel together in the sacral nerve roots. Voiding difficulties would represent the most likely symptom, and a significant post-void residual or abnormal uroflowmetry would be expected findings. Five patients had abnormal uroflowmetry, which may reflect the impact of nerve injury on bladder function, but other factors such as medication – and in particular pain-killers – may also play a role.
Despite its small patient number, to our knowledge this study represents the largest studied cohort of patients specifically suffering with symptomatic MSMC. Seeing the prevalence of abnormal neurophysiology findings in this patient group, it is conceivable that pelvic neurophysiology may advance the interpretation of symptoms in patients in whom the potential involvement of their MSMC is still widely excluded on the arbitrary assumption that these lesions are innocuous.

In contrast with their generally presumed innocuousness, our results indicate that bladder, bowel and sexual symptoms are highly prevalent in patients with symptomatic MSMC.  Furthermore, injury to the sacral somatic innervation is seen in most patients. Lastly, MSMC are more likely to cause sacral nerve root damage than sacral Tarlov cysts.

Nabors MW, Pait TG, Byrd EB, et al. Updated assessment and current classification of spinal meningeal cysts. J Neurosurg. Mar 1988;68(3):366-77. doi:10.3171/jns.1988.68.3.0366
Hentzen C, Cabrilo I, Malladi P, et al. Sacral Tarlov cysts: Neurophysiology abnormalities and correlation with pelvic sensory and visceral symptoms. Eur J Neurol. Sep 2023;30(9):2838-2848. doi:10.1111/ene.15869