Nicholas L Zalewski, Alejandro A Rabinstein, Waleed Brinjikji, Timothy J Kaufmann, Deena Nasr, Michael W Ruff, Eoin P Flanagan
9 min
Spinal dural arteriovenous fistulas (sDAVF) are rare vascular anomalies that, if undiagnosed, lead to permanent neurological damage. Because they frequently present with symptoms and MRI findings similar to inflammatory or neoplastic conditions, they are often misdiagnosed. This study investigated whether a specific, previously observed pattern of contrast enhancement on MRI could serve as a reliable diagnostic indicator for sDAVF.
The researchers performed a retrospective review of 80 patients with confirmed sDAVF. They specifically analyzed pretreatment gadolinium-enhanced MRI scans for the presence of the "missing-piece sign," defined as at least one focal geographic nonenhancing area surrounded by a long segment of intense holocord enhancement. This group was compared against a control group of 144 patients with various alternative myelopathy diagnoses—including sarcoidosis, multiple sclerosis, neuromyelitis optica, and spinal cord tumors—to determine if this sign is unique to sDAVF.
Of the 51 patients with sDAVF who received gadolinium contrast, 44 showed intraparenchymal enhancement, and 19 (43%) exhibited the missing-piece sign. Importantly, this sign was absent in all 144 patients in the control group. The researchers suggest that the nonenhancing "missing pieces" may represent areas where the spinal cord's intrinsic venous system provides better drainage, despite the surrounding venous hypertension that causes the adjacent cord to enhance. Over half of the sDAVF patients (58%) had received an incorrect diagnosis before the fistula was correctly identified, highlighting the clinical significance of recognizing this radiological feature to facilitate prompt angiography and intervention.
Recognizing the missing-piece sign can significantly improve diagnostic accuracy for clinicians encountering patients with chronic, progressive myelopathy. By pointing toward an underlying vascular cause, this sign helps avoid common pitfalls like misdiagnosis as transverse myelitis, which may result in harmful steroid therapy, or misdiagnosis as a tumor, which might lead to unnecessary spinal cord biopsies. Early identification is crucial, as the neurological deficits caused by sDAVF are often reversible with timely endovascular or surgical occlusion.
Alex: Previously, that enhancement was generally described as nonspecific and hazy. The new observation is an abrupt, geographically defined gap within otherwise intense enhancement. Instead of asking only whether the cord enhances, the authors ask whether its pattern contains a distinctive interruption.
Sam: Describe that interruption for someone who cannot see the scan. What would qualify, rather than just ordinary patchiness?
Alex: Imagine a long stretch of spinal cord that brightens strongly after contrast. The enhancement involves the cord’s whole cross-section, but a sharply defined area within it does not brighten. The authors required the surrounding stretch to extend beyond two vertebral segments.
Sam: So the gap matters because of what surrounds it. It is not simply any area that fails to enhance.
Alex: That surrounding pattern is central to the definition. A neurologist and a neuroradiologist verified the findings. The authors then looked for the same pattern in patients with confirmed alternative causes of enhancing spinal cord lesions.
Sam: Let’s separate the full patient group from the scans they could actually evaluate. How much material went into that comparison?
Alex: The researchers reviewed a referral group of eighty patients with confirmed fistulas. Only fifty-one had an available pretreatment MRI performed with gadolinium. So the enhancement analysis did not include every patient in the original group.
Sam: That distinction matters because the sign requires a contrast scan. Within the evaluable group, how often did it appear?
Alex: Forty-four patients had enhancement within the spinal cord. Nineteen of those showed the missing-piece sign. That is just under half of the patients whose cords enhanced, not half of all patients with a fistula.
Sam: So even among enhancing fistulas, most did not show it. That already limits its usefulness as a way to rule the disorder out.
Alex: Absence of the sign cannot exclude a fistula on these results. The more encouraging finding is the comparison group: none of the one hundred forty-four patients with other confirmed diagnoses showed the sign. Those controls also had enhancing spinal cord lesions, making the comparison relevant to this diagnostic problem.
Sam: Were those alternatives the diseases clinicians would genuinely confuse with a fistula?
Alex: They included inflammatory conditions such as multiple sclerosis and spinal cord sarcoidosis, as well as several tumors. There were also patients with spinal cord compression. The comparison therefore went beyond healthy scans or lesions that never enhanced.
Sam: No examples in that comparison sounds promising. But does it justify calling the sign specific to fistulas?
Alex: The authors say it suggests specificity. My reading is that it is a promising distinguishing clue, not proof that no other disease can produce it. The controls cover several relevant alternatives, but they do not establish performance across every patient a clinician might encounter.
Sam: Give me a concrete case where this additional clue could matter, especially when the familiar vessel signs are missing.
Alex: One patient with the missing-piece sign had no flow voids on the accompanying scan. Angiograms had also failed to identify the fistula. It was discovered during surgery undertaken for a planned spinal cord biopsy.
Sam: That puts the diagnostic stakes in practical terms. A vascular disorder was still being approached as something that might require sampling the cord.
Alex: More than half of the patients with the sign had received an alternative diagnosis before the fistula was confirmed. Suspected spinal cord tumor was one of those diagnoses. The paper also warns about biopsy risk and notes that corticosteroid treatment can worsen this disorder.
Sam: If the sign raises suspicion, what investigation are the authors hoping it would prompt?
Alex: Angiography, meaning imaging of the blood vessels. Digital subtraction angiography uses contrast and subtracts background structures to make vessels visible. Some patients in this series needed repeat angiograms, so an initially negative examination did not always end the diagnostic search.
Sam: Does the missing piece tell us anything about the underlying tissue, or is it purely a visual label?
Alex: The authors propose a mechanism, but do not demonstrate it. They suggest that areas without enhancement may have better routes for venous drainage than neighboring cord. In their explanation, uneven drainage could produce uneven leakage of contrast into the tissue.
Sam: Then the biggest uncertainty is whether that useful-looking pattern survives testing outside this retrospective series.
Alex: The study’s stated limitations are its retrospective design, relatively small numbers, and variation in imaging protocols and timing. It also did not test whether teaching clinicians the sign speeds diagnosis or improves outcomes. Those are potential benefits, not observed results of this study.
Sam: With those limits, who should read the full article, and where should they start?
Alex: Neurologists and neuroradiologists evaluating unexplained myelopathy should read it, as should researchers studying diagnostic imaging. Start with the Methods definition of the sign, then the Results comparison with alternative diagnoses. The Discussion is useful for separating the proposed drainage mechanism from what the scans actually establish.
Sam: And what should someone outside that specialty remember?
Alex: An enhancing spinal cord lesion does not automatically mean inflammation or a tumor. A sharply defined gap within otherwise intense enhancement may be a reason to investigate a vascular cause—not a diagnosis by itself.
Sam: Thanks for listening.