The Role of Brain Biopsy in Confirming PACNS

While imaging and spinal fluid tests provide important clues, a brain biopsy is the most definitive way to demonstrate vasculitis in tissue for confirming a diagnosis of PACNS ^[1][2]. Because the treatment for PACNS often involves powerful drugs that suppress your immune system, your doctors want to be as certain as possible that they are treating the right condition—and ruling out mimics like infection or cancer ^[3][4].

The Procedure and Its Risks

A biopsy is especially considered if your doctors suspect small-vessel PACNS. In this version of the disease, the inflammation occurs in vessels so tiny that even the most advanced angiograms cannot see them ^[5][6].

The procedure typically involves a neurosurgeon removing a small sample of tissue. To get the most accurate result, the surgeon usually targets an area that looked abnormal on your MRI, specifically sampling the leptomeninges (the brain’s lining), the cortex (the outer layer of the brain), and the underlying white matter ^[7][8]. Including these different layers of tissue increases the diagnostic yield, though success rates vary significantly ^[9][7].

However, biopsy is not automatically required, and it carries material risks that depend on the target and location. Patients should be told that risks can include bleeding, infection, seizures, anesthesia complications, a new neurologic deficit, and a nondiagnostic sample. Biopsy is ideally obtained before prolonged immunosuppression when clinically safe, but urgent treatment should not be delayed when the treating team believes delay is dangerous.

The Three Patterns of Inflammation

When a pathologist looks at your brain tissue under a microscope, they look for specific patterns of damage to the blood vessel walls. There are three main ways PACNS shows up, though they often overlap ^[10][11]:

1. Granulomatous: This is a common pattern. It involves a specific type of chronic inflammation where immune cells form tiny clumps called granulomas. While sometimes associated with amyloid-beta, amyloid-beta may indicate an amyloid-related angiitis/CAA spectrum rather than simply supporting one routine PACNS subtype ^[11][10].
2. Lymphocytic: In this pattern, the vessel walls are crowded with lymphocytes (a type of white blood cell). This is often seen in cases that look like “tumors” on an MRI ^[11][6].
3. Necrotizing: In this pattern, the inflammation causes “necrosis,” or the death of the vessel wall tissue. While often severe, “necrotizing” is not universally the most severe prognostic category ^[11][12].

Importantly, patterns can overlap, and pathology findings do not independently determine prognosis or treatment.

Understanding a “Negative” Biopsy

It is vital to know that a negative biopsy result does not automatically mean you do not have PACNS ^[13]. PACNS is a “patchy” disease, meaning it might attack one small area of brain tissue while the vessel just a few millimeters away looks perfectly healthy ^[14][13].

Because the surgeon may sample a healthy patch by chance, false negatives occur ^[13][14]. If your biopsy is negative or nondiagnostic but your symptoms and other scans strongly suggest PACNS, a carefully established “probable PACNS” diagnosis may be made after appropriate exclusion of mimics, and your doctor may still proceed with treatment ^[3].

Your Pathology Report: Questions for Your Team

When your biopsy is evaluated, you can ask your care team to ensure appropriate studies were performed and interpreted by an experienced neuropathologist ^[10][15][16]:

• Tissue Compartments: Was the sample able to include the meninges (lining), cortex (gray matter), and white matter?
• Vessel Wall Description: Was there evidence of transmural inflammation (inflammation through the entire wall)?
• Exclusion Stains: Were appropriate stains, immunohistochemistry, PCR, or cultures used to evaluate for bacterial, fungal, or viral infections? (No single stain rules out all infections).
• Amyloid Testing: Was the tissue checked for amyloid-beta, and does it help distinguish PACNS from cerebral amyloid angiopathy (CAA)?