Considering the advantage of high contrast resolution, ability to depict the entire spinal subarachnoid space including fluid collections and leakages, and non-invasive nature (i.e., no LP and no radiation exposures), MRM may be regarded as the first-line examination in the diagnosis of CSF leakages. Its high contrast resolution contributes to detect indirect findings such as epidural-paraspinal fluid collections, especially small amounts of leakages along bone structures [9–11]. Furthermore, MRM with intrathecal gadolinium injection can provide both physiologic and morphologic information, which enables the detection of direct CSF leakages with higher sensitivity than any other technique including computed tomography myelography . In the present study, similar to the previous study evaluating ICSFL on MRM , CSF leakage was distributed around nerve roots and paraspinal area at the lumbosacral level. Predominant lumbosacral distribution was not surprising because the thecal punctures were performed at this location. It is expected that these characteristic paraspinal fluid collections are the result of CSF escaping from the epidural space into the paraspinal loose connective tissues, similar to the retrospinal C1–2 fluid collection reported in patients with PDPH and SIH [12, 13]. In addition to these anatomical factors, the low resolution of the 2D MRM sequence depicts CSF leakages in the paraspinal area more definitely than those around nerve roots in this study.
Although the association between PDPH and CSF abnormalities (i.e., between CSF loss and a reduction in intracranial pressure) is not disputed, the exact pathophysiology of PDPH remains unclear. PDPH is considered to be caused by the hole left in the dura after the lumbar puncture needle has been withdrawn, which can allow CSF leakage from the subarachnoid space . Among the various risk factors for PDPH including puncture procedure variables, patient characteristics, and a past history of chronic headaches, the size and design of the needle used for the lumbar puncture are the most significant determinants of PDPH [2, 3, 15]. As a result, its incidence can vary widely, depending on the population involved and the needles and techniques used [3, 16, 17]. The incidence of PDPH in this study (14%) was comparable to that described in a previous report in which 20G cutting spinal needles were used .
It is worth noting that the incidence of PDPH did not differ significantly between the radiologically CSF leakage-positive and -negative patients in our study. On the surface, these findings do not seem to support the hypothesis that CSF leakage through dural holes causes PDPH. However, several previous studies have indicated that the volume of CSF lost via leakage and CSF hypotension are not associated with PDPH and have also questioned the dural hole hypothesis [16, 19]. In addition, neuroimaging studies performed with MRI or RICG after lumbar puncture have revealed that some patients with postpuncture CSF leakage are asymptomatic [4–6]. However, such leakages can still cause clinical problems, especially with the diagnosis of disorders such as SIH . Together, these findings indicate that the existence and volume of CSF leakage are not necessarily associated with PDPH and suggest that the underlying mechanisms of PDPH are more complex.
A simple older explanatory model for PDPH is that the reduction in intracranial pressure induced by persistent CSF leakage causes traction between pain-sensitive structures such as meningeal membranes, blood vessels, and nerves [3, 20]. On the other hand, hypersensitivity to substance P and the Monro-Kellie doctrine, which suggests that compensatory intracranial vasodilatation is induced by CSF leakage, have recently been recognized as viable hypotheses regarding the cause of PDPH [1, 21]. Considering these new hypotheses, it is not surprising that some of the patients in this study without definite CSF leakage on MRM images complained of PDPH.
A number of limitations of the present study need to be addressed. The main limitation is the relatively small study population. The lack of available clinical data due to the study’s retrospective nature is also problematic point. These made it difficult to evaluate various factors that affect the incidence of PDPH, such as the number of lumbar punctures, body mass index, and the patients’ medical histories. Furthermore, the incidence of CSF leakages on MRM might also be affected by the number of lumbar punctures. Another limitation is our use of the 2D MRM sequence; i.e., the lower resolution made this study qualitative rather than quantitative evaluation. To perform the precise measurement of CSF leakage, it is necessary to use the 3D MRM sequence, which achieves higher spatial resolution. Additionally, the interval between lumbar punctures and MRM may be too long in some cases. Therefore, there is a chance of resorption of CSF leak in radiologically-negative patients. In spite of these limitations, the fact remains that most of the patients with definite CSF leakage after lumbar puncture with the relatively large 21G Quincke spinal needle were asymptomatic.