The methods used to diagnose PACNS primarily include CSF examination, imaging, and pathological investigation. Pathology plays a vital role in the diagnosis of PACNS, whereas imaging plays an auxiliary role to the pathological examination. If no pathological examination is made, PACNS can only be an exclusionary diagnosis 1,6,15.
Imaging is extremely valuable in the diagnosis of PACNS. As well as the traditional technologies of CT, MRI, and cerebrovascular imaging, such as computed tomography angiography (CTA), MRA, and digital subtraction angiography (DSA), the diagnosis of PACNS with various other imaging techniques has been reported, including new molecular imaging procedures 16–18. Most PACNS patients have various MRI abnormalities 19, but some PACNS patients have normal angiograms 20. MRA and DSA examinations of PACNS patients can detect stenosis or dilation of multisegment blood vessels in ischemic lesions, but rarely find long, complete vascular occlusions or aneurysm-like changes 1,6. Until recently, DSA was the most widely used technique to diagnose PACNS 6,18,21. The most typical manifestations of PACNS on MRI are multiple asymmetric ischemic lesions involving the bilateral cerebral hemispheric cortex, subcortical structure, and deep white matter, accompanied by intracerebral or subarachnoid hemorrhage. Other rare manifestations include hemorrhage-like, mass-like, or cyst-like lesions in the brain parenchyma, which are challenging to distinguish from other cerebrovascular diseases, demyelination diseases, and brain tumors 6,22. Patients with a solitary mass-like PACNS are relatively rare (< 5%), and the differential diagnosis is more difficult 1. The literature also reports that DWI and DTI examinations can detect PACNS patients with negative MRI results 6,23, and DWI sequences can detect acute ischemic lesions 21. In study a 16 patients, 15 showed DWI hyperintensity and nine of these also showed hypointensity in the middle of the lesion. All 16 patients underwent a brain biopsy (a stereotactic procedure in 10 and open-wedge surgery in six), which showed lymphocytic angiitis in the majority of patients (15/16) and necrotizing angiitis in one patient 24. In one patient with PACNS with a tumor-like mass lesion, there was no evidence of acute infarction on DWI 25.
Tissue biopsy is the key to diagnosing PACNS 26, and the role of pathological biopsy in the diagnosis of PACNS is irreplaceable. Brain biopsy is now considered the ‘gold standard’ strategy for the definitive diagnosis of PACNS 19 and should be pursued not only because it provides information that establishes the diagnosis but also effectively excludes other similar diseases 7,20,21,27. Imaging methods cannot detect some cases of PACNS involving peripheral blood vessels. However, even when imaging reveals typical PACNS manifestations, the biopsy results may not support PACNS diagnose 6. Pathology results confirm that the inflammation associated with PACNS usually manifests as transmural damage to the involved vessel wall and immune cell infiltration. Thickening of the vessel wall can cause vascular stenosis, disturbing the microcirculation. Simultaneous rupture of the vessel wall and secondary intracranial hemorrhage can occur due to the fragility of the vessel wall 6.
Areas of CT or MR enhancement, or areas of abnormal MRS are often been used to guide stereoscopic biopsy and localize intracranial lesions. A framed stereotactic device biopsy, positioning by CT-enhanced, MR-enhanced T1, T2-weighted imaging and MRA-enhanced imaging, was used to diagnose a case of PACNS 15. The diagnosis rate can reach 80% when imaging is used to assist pathological biopsy 6,22. In one study, the classic angiographic features of angiitis were associated with biopsy or postmortem confirmation in only 4.6% of patients 28 (32/701), and positive angiography was not correlated with positive biopsy results in a study of 34 PACNS patients 29. The diagnosis rate of PACNS achieved with surgical biopsy ranged from 36–83% in previous studies 13,15.
The surgical biopsy complication rate is 16%, which includes serious complications such as cerebral hemorrhage and epilepsy 13,15, whereas stereotactic biopsy is less traumatic. It can be used in critically ill patients, and its complication rate is < 13%, with patients experiencing only transient or minor complications 15. Therefore, the diagnosis rate of pathological biopsy should be increased as possible for the accurate diagnosis and appropriate treatment of patients with PACNS. Because the number of cases is small and clinical experience of this disease is limited, the process of PACNS biopsy has not been standardized. The leptomeninges, cortex, and deep white matter must be sampled simultaneously to improve the diagnostic biopsy rate 13,15.
The low incidence of PACNS and its high heterogeneity means that there is a lack of randomized controlled trials of its treatment 6. Glucocorticoids alone or in combination with cyclophosphamide can achieve satisfactory results 22. However, it was reported that the administration of azathioprine, methotrexate, or mycophenolate mofetil as maintenance therapies can achieve more satisfactory results 27.
Our patient had a small intracranial lesion (with a volume of 3.1 cm3). MRI and MRA examinations showed that the left vertebral artery was narrow, consistent with the imaging features reported in the literature 23. Limaye et al. reported that when two patients (20%) underwent MRS, the Cho/NAA ratios in both patients were approximately 0.9 19. The result of MRS in this patient (Cho/NAA = 1.22) does not have a noticeable prompting effect on the judgment of the lesion's nature. We considered that, because the MR T2 and FLAIR features of PACNS patients can be either normal or abnormal, they could have abnormal DWI and ADC images. Therefore, we performed comprehensive imaging examinations before performing stereotactic biopsy in our patient. The imaging examination allowed areas with different characteristics on the DWI and ADC maps to be selected as multiple targets for stereotactic biopsy, which allowed us to make a precise pathological diagnosis. Follow-up for 3 years confirmed that the patient recovered well after treatment (Glasgow Outcome Scale of 5).