Diagnostic Tests and the 'Great Mimics'

Diagnosing Primary Angiitis of the Central Nervous System (PACNS) is rarely a straightforward process. Because PACNS is so rare and mimics many other conditions, doctors cannot rely on a single test to confirm it ^[1][2]. Instead, they use a “multimodal” approach—combining your clinical history with imaging and laboratory tests—to piece together the puzzle ^[1]. No single test provides absolute certainty.

The Role of Brain Imaging (MRI)

An MRI is usually the first step in the diagnostic journey. While it is excellent at showing the damage the disease has caused, it is not “diagnostic” on its own because the patterns it reveals can be seen in many other diseases ^[1][3].

• Heterogeneous Findings: PACNS can look different on every scan. Common findings include small strokes in multiple areas of the brain, bleeding (hemorrhage), or “bright spots” that indicate inflammation in the brain tissue or the leptomeninges (the brain’s protective lining) ^[3][1]. FLAIR or T2 “bright spots” are nonspecific and do not by themselves indicate inflammation.
• Vessel-Wall MRI (VW-MRI): This is a specialized, high-resolution scan that looks directly at the walls of the blood vessels rather than just the blood flowing through them ^[4]. It can show concentric enhancement—where the entire wall of the vessel lights up, suggesting inflammation ^[4][5]. However, this test has limits; it may be negative in patients who only have disease in very small vessels, and vessel-wall enhancement can also occur with RCVS, atherosclerosis, infection, and other vasculopathies ^[6].

Looking at the “Plumbing”: Angiography

A catheter cerebral angiography (also called DSA) is a procedure where a thin tube is used to inject dye into your blood vessels to get a highly detailed map of your brain’s circulation ^[4]. It carries some risks, including bleeding or stroke.

• Beading and Narrowing: In medium and large-vessel PACNS, this test may show “beading,” where a vessel alternates between narrowed and widened sections ^[7][6]. Angiographic beading is supportive, but not specific for PACNS.
• Small-Vessel Disease: It is important to know that a “normal” angiogram does not rule out PACNS. If the inflammation is only in the smallest vessels, they are too tiny for the angiogram to see ^[4][8].

Analyzing the Spinal Fluid (CSF)

A lumbar puncture (spinal tap) allows doctors to check the cerebrospinal fluid (CSF) that bathes your brain.

• Inflammatory Markers: In most PACNS cases, the CSF shows signs of inflammation, such as an increased number of white blood cells (pleocytosis) or elevated protein levels ^[1][4]. CSF abnormalities are common but nonspecific, and normal CSF does not exclude PACNS, particularly in some large-vessel presentations.
• Excluding Infection: The main job of the CSF analysis is to investigate whether your symptoms are being caused by a hidden infection or a type of cancer, both of which can look exactly like vasculitis on an MRI ^[1][9]. This involves cell counts, protein/glucose, cultures or PCR, and cytology/flow cytometry. Blood tests and evaluations for systemic autoimmune, infectious, and malignant causes are also critical.

The “Great Mimics”: What Else Could It Be?

Before confirming PACNS, your care team must rule out several other conditions that look nearly identical under a microscope or on a scan:

• RCVS (Reversible Cerebral Vasoconstriction Syndrome): This is a critical mimic. Unlike PACNS, RCVS is often triggered by medications or stress and usually involves a sudden, “thunderclap” headache. RCVS can cause ischemic stroke, hemorrhage, and seizures and requires urgent evaluation. While the vessel narrowing usually reverses on its own within 12 weeks, early imaging may be normal, and reversal by 12 weeks is a retrospective feature rather than a safe initial rule-out test. Distinguishing RCVS from PACNS matters because treatment decisions differ and corticosteroids may be harmful in RCVS ^[10][11].
• Systemic Vasculitis: Diseases like lupus or ANCA-associated vasculitis can attack the brain, but they also attack other organs like the kidneys or lungs ^[9][12].
• Cerebral Amyloid Angiopathy (CAA): Often seen in older adults, this condition involves a buildup of proteins in the vessel walls that can cause bleeding, mimicking the appearance of vasculitis ^[9][12].

Because of these complexities, the European Stroke Organisation (ESO) and other experts recommend that a final diagnosis should ideally be reviewed by a multidisciplinary team at a specialized center ^[1][13]. In many cases, a brain biopsy can provide the strongest tissue confirmation in selected cases, helping establish a “definite” or “probable” diagnosis ^[9][8].

What to Bring to Your Visit:
Bring your medication and supplement lists, prior imaging, symptom onset dates, and any questions about contrast or catheter procedures. Being prepared helps your team accurately interpret these complex tests.