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Brain Fog After CAR T-Cell Therapy: What New Research Shows

Some people who receive CAR T-cell therapy describe a frustrating mental “fogginess” afterward—forgetfulness, trouble concentrating, or feeling slower to think through tasks. For cancer survivors who have fought hard to get to the other side of treatment, these cognitive changes can be discouraging, even when the cancer response is excellent.

New research from Stanford Medicine helps explain why this can happen and points toward possible treatment strategies being explored.¹

Read the research report published by Stanford Medicine>

Read publication in Cell from May 12, 2025>

What is CAR T-cell therapy?

CAR T-cell therapy is a type of immunotherapy that uses a patient’s own T cells (a kind of immune cell). Doctors collect T cells, “reprogram” them to recognize specific targets on cancer cells, and then infuse them back into the body so they can seek out and destroy cancer.²

CAR T-cell therapy was first approved in 2017 for certain leukemias and is now used for several blood cancers (and is being tested in clinical trials for additional cancers).³

What does “brain fog” mean in this context?

“Brain fog” isn’t a formal medical diagnosis, but it’s a common way patients describe symptoms such as:

• Difficulty concentrating
• Slower thinking or processing speed
• Short-term memory lapses
• Trouble multitasking
• Feeling mentally “tired” even after rest

In the Stanford-led work, the cognitive effects observed were described as typically mild—not the kind of decline associated with dementia—but still meaningful because they impact day-to-day quality of life and may not always resolve quickly on their own.⁴

What the new research found

1) CAR T-cell therapy alone may contribute to cognitive symptoms

One of the key questions researchers asked was whether cognitive changes could happen even when you set aside other potential contributors (like chemotherapy, radiation, or the cancer itself).

In mouse models, CAR T-cell therapy was associated with measurable changes in cognitive performance (including attention and short-term memory), even when cancers were located outside the brain.⁵

2) A common pathway may link “chemo brain,” illness-related brain fog, and CAR T-related brain fog

The researchers reported that the underlying biology they observed resembles mechanisms seen in other brain fog settings—such as after chemotherapy and after some respiratory infections (like flu and COVID-19).⁶

3) Microglia (brain immune cells) appear to play a central role

The brain has its own immune “first responders,” called microglia. In the study, microglia appeared to shift into a reactive, inflammatory state after CAR T-cell therapy, producing immune signaling molecules (cytokines and chemokines).⁷

A helpful way to visualize the proposed chain reaction:

• Immune activation → microglia become “revved up”
• Microglia release inflammatory signals
• Those signals disrupt cells that support healthy nerve signaling

This is the same idea referenced in the image caption used with the Stanford report: in therapy- or illness-induced brain fog, microglia can produce inflammatory chemicals that reduce oligodendroglial cells and their myelin.⁸

4) Oligodendrocytes and myelin may be affected

Oligodendrocytes are cells that make myelin, a fatty insulating layer around nerve fibers that helps signals travel efficiently through the brain.

In the study, inflammatory changes were associated with disruption of oligodendroglial health and myelin-related processes—changes that can plausibly contribute to feeling mentally slower or less sharp.⁹

The paper also described broader nervous-system impacts in mice, including signs of a persistent CNS immune response and disruptions involving hippocampal neurogenesis (a process connected with learning and memory).¹⁰

What about evidence in humans?

This is an important point: much of the experimental work was done in mice, but the researchers also examined human brain tissue from participants in a CAR T clinical trial for brainstem tumors. Those analyses supported the idea that microglia and oligodendrocytes can show reactive/dysregulated states after CAR T therapy.¹¹

Can CAR T-related brain fog be treated?

Researchers are not saying there is an approved, standard medication today specifically for CAR T-related cognitive symptoms—but the study is encouraging because it tested strategies that reversed cognitive effects in mice:

• Temporary microglial depletion (in mice, a short period depleted microglia; when they returned, they came back in a more normal, non-reactive state and cognitive performance improved).¹²
• Blocking a chemokine receptor pathway (CCR3), which improved cognitive performance and rescued oligodendroglial deficits in the mouse model.¹³

The Stanford report also notes that the team identified compounds similar to existing medications or those in clinical development, raising hope that treatments could be translated more quickly than if development had to start from scratch.¹⁴

What this means for patients and caregivers right now

If you or a loved one notices brain fog after CAR T therapy, you’re not imagining it—and you’re not alone. This area is getting increased research attention because survivorship and quality of life matter.

Here are practical next steps that may help (and are reasonable to discuss with your care team):

1) Tell your oncology team what you’re experiencing

Be specific about:

• When symptoms started
• Whether they’re improving, stable, or worsening
• How they’re affecting work, driving, medication management, or daily tasks

2) Ask about screening and supportive care

Depending on your situation, your team may suggest:

• Neurocognitive screening or referral to neuropsychology
• Occupational therapy strategies for memory and organization
• Cognitive rehab programs (often used in chemo-related cognitive impairment)

3) Check for other treatable contributors

Brain fog can be worsened by factors that are sometimes addressable, such as:

• Sleep disruption
• Anxiety/depression
• Anemia, thyroid issues, vitamin deficiencies
• Medication side effects
• Ongoing inflammation or infection recovery

4) Know when to seek urgent care

Seek urgent medical attention if there is sudden or severe confusion, new weakness, slurred speech, severe headache, seizures, or rapidly worsening symptoms—especially in the setting of recent cancer therapy.

Questions you can bring to your next appointment

• “Is brain fog something you see after CAR T therapy in your patients?”
• “What symptoms would you want me to report right away?”
• “Can you refer me for cognitive screening or supportive therapy?”
• “Are there clinical trials studying cognitive effects after CAR T therapy?”
• “Could any of my current medications be contributing to these symptoms?”

Stay connected

At Stage4Hope, we share research updates like this because informed patients and caregivers are empowered patients and caregivers. If you’re navigating late-stage cancer treatment or survivorship challenges, we invite you to explore more resources and join our community for continued support.

References:

• 1, 2, 3, 4, 5, 6, 7, 9, 11, 12, 14: Stanford Medicine
• 8: EurekAlert
• 10, 13: PubMed