Watching the person you love become slower, quieter, or more confused during glioblastoma treatment is frightening. Cognitive changes - shifts in memory, attention, language, or personality - are among the most distressing symptoms caregivers face. And one of the most common questions they ask is: is this the tumor getting worse, a drug side effect, or something the treatment itself is doing?
The honest answer is: it is often impossible to know without clinical evaluation. But understanding the three main causes of cognitive change during glioblastoma treatment can help you ask better questions, notice patterns earlier, and push for the right tests or dose adjustments at the next visit.
What Does "Cognitive Change" Mean in This Context?
Cognitive changes refer to shifts in how a person thinks, remembers, communicates, or processes information. In glioblastoma, these may include:
- Short-term memory loss - forgetting conversations, appointments, or recent events
- Word-finding difficulty - pausing mid-sentence or substituting the wrong word
- Slowed processing - taking longer to respond or follow instructions
- Reduced attention or difficulty concentrating on tasks
- Personality shifts - irritability, emotional flatness, or social withdrawal
- Difficulty planning or carrying out multi-step tasks
These symptoms can appear before treatment starts, worsen during it, or emerge months after radiotherapy ends. Tracking exactly when they appeared and what else changed around the same time is your most useful starting point.
What Are the Three Main Causes of Cognitive Change in Glioblastoma?
Cognitive changes during glioblastoma treatment fall into three overlapping categories: the tumor's direct effect on brain tissue, the neuropsychiatric and metabolic effects of corticosteroids like dexamethasone, and the neurotoxic effects of radiation and chemotherapy (often called chemo brain). They can look similar and happen at the same time.
Cause 1: The Tumor Itself
Glioblastoma grows inside the brain and displaces or destroys functioning tissue. Where the tumor sits determines which cognitive functions are most affected. A tumor in the left frontal lobe is closely linked to language production and executive function - planning, organizing, and decision-making. A right-temporal lesion may affect memory or the ability to read social cues. An insular tumor can alter personality and emotional regulation in ways that look like depression or apathy.
Beyond the tumor mass itself, the surrounding brain swelling - called cerebral edema - adds pressure on healthy tissue. That pressure can impair function in areas far from the tumor. This is why cognitive symptoms can suddenly worsen even when imaging looks stable, a topic covered in our guide to glioblastoma brain edema: causes and treatment.
Tumor-related cognitive changes tend to be focal - meaning they affect a specific function tied to the tumor's location. They also tend to be progressive if the tumor is actively growing, though they may fluctuate with shifts in edema level.
Cause 2: Dexamethasone and the Cognitive Cost of Steroids
Dexamethasone is a corticosteroid - an anti-inflammatory drug - prescribed to most glioblastoma patients at some point in treatment. Its job is to reduce brain swelling, and many patients feel noticeably better within hours of their first dose. But dexamethasone carries a significant neuropsychiatric burden that caregivers are often not warned about.
Research from the EORTC trial 26101, which tracked neurocognitive functioning in patients with recurrent glioblastoma, found that higher corticosteroid use was significantly associated with worse performance in working memory, language, and executive functioning - even after accounting for other clinical factors. The relationship was dose-dependent: higher doses over longer periods caused greater cognitive impairment.
A systematic review and meta-analysis of dexamethasone use in glioblastoma found that systemic adverse effects occurred in up to 50% of patients, with psychiatric symptoms among the most frequently reported. Neuropsychiatric effects of prolonged corticosteroid use can include memory disturbances, mood swings, irritability, insomnia, and depression. In some patients, more severe symptoms such as agitation or delirium may occur. The same review found that baseline corticosteroid use was independently associated with shorter overall survival - which is why oncologists aim to keep the dose as low as effective for as short a time as possible.
If cognitive changes appeared shortly after starting or increasing dexamethasone, and they include insomnia, mood shifts, and word-retrieval problems alongside general mental fogginess, steroids are a likely contributor.
Sleep disruption is one of the most overlooked and correctable contributors to cognitive impairment during treatment - particularly when dexamethasone is involved. Poor sleep directly impairs memory consolidation, attention, and processing speed. In addition to the sleep strategies your clinical team may recommend, some caregivers explore Ayurvedic sleep options alongside medical management.
Dexamethasone-related cognitive effects are often reversible when the dose is tapered under supervision. You can ask the oncologist directly whether a supervised taper is appropriate given the current imaging and clinical status.
Cause 3: Radiation Effect and Chemo Brain
The term "chemo brain" refers to cognitive difficulties - fogginess, memory lapses, slowed thinking - that can occur during or after cancer treatment. In glioblastoma, temozolomide (TMZ), the standard oral chemotherapy, is given concurrently with radiotherapy and then as maintenance cycles afterward. Research in lower-grade glioma suggests that temozolomide alone does not dramatically worsen neurocognitive function over the short term. But the combination of chemotherapy with cranial radiation changes the picture substantially.
Cranial radiation damages multiple neural structures that support cognition. The hippocampus - a small curved structure deep in the brain's temporal lobe that is critical for forming new memories - is particularly vulnerable. Radiation can also damage white matter (the long-range fibers connecting brain regions) and reduce blood supply to neural tissue. According to research on the medical management of brain tumors and the sequelae of treatment, radiation-induced neurocognitive decline can involve progressive white matter changes and loss of neuronal progenitor cells in the hippocampus - the stem-like cells that normally support memory and learning.
Radiation-related cognitive changes tend to follow a recognizable time course:
- Acute phase (during or just after radiation): fatigue and mild fogginess - usually temporary
- Early delayed phase (a few weeks to 3 months post-radiation): transient worsening, often linked to short-term demyelination (damage to nerve insulation) that typically stabilizes
- Late delayed phase (months to years after radiation): more persistent memory and processing-speed deficits related to white matter changes and vascular injury
If cognitive problems emerged 2 to 4 weeks after finishing radiotherapy, or are gradually worsening in the months that follow, radiation effect is a strong candidate. Note that at the 2-to-3-month post-chemoradiation mark, an MRI may show what looks like tumor growth but is actually a treatment reaction - a phenomenon called pseudoprogression. Our article on pseudoprogression in glioblastoma after chemoradiation explains how this is diagnosed and what it means for ongoing management decisions.
How Do the Three Causes Look Different on a Timeline?
| Feature | Tumor Effect | Dexamethasone | Radiation - Chemo Brain |
|---|---|---|---|
| When it appears | Any time; often gradual or tied to tumor growth | Days after starting or increasing the dose | Weeks to months after radiotherapy ends |
| Symptoms affected | Specific to the tumor's brain location | Memory, mood, sleep, language | Memory, processing speed, sustained attention |
| MRI pattern | Tumor growth or increased mass effect | May show reduced edema on lower dose | White matter changes; may mimic tumor progression |
| Reversibility | Partial, if edema is controlled | Often reverses with supervised taper | Partial; late effects may persist |
| Key clinical question | Has the scan changed? | When did the steroid dose change? | When did radiotherapy finish? |
This framework is a guide, not a diagnostic tool. All three causes can overlap in the same patient at the same time. A formal clinical evaluation - including repeat MRI and neuropsychological assessment where indicated - is needed to determine which factor is dominant and whether treatment adjustment is warranted.
What to Tell the Care Team When You Report These Changes
When you bring cognitive changes to the oncology team, specific information speeds up evaluation. Try to document:
- Exactly when the change first appeared - before, during, or after which treatment phase
- What changed in the treatment regimen around the same time - a steroid dose adjustment, a new medication, or the end of radiotherapy
- Whether sleep quality worsened at the same time as the cognitive changes
- Whether changes are focal (one specific function affected) or global (everything seems slower)
- Whether changes are stable, getting worse, or fluctuating day to day
The team may order repeat MRI with advanced imaging sequences, refer the patient to a neuropsychologist for formal cognitive testing, adjust the dexamethasone dose, or evaluate for contributing factors such as uncontrolled seizure activity, medication interactions, or mood disorders. Depression and anxiety during glioblastoma treatment independently impair memory and attention, and treating them often improves cognitive symptoms in parallel - worth raising if mood has also shifted.
If you are unsure whether you are getting a complete picture from the current team, you can arrange a remote second opinion through Art of Healing Cancer to have a specialist review the imaging, medication list, and clinical history together - which is often the fastest way to identify which cause is dominant and whether anything actionable has been missed.
What May Help Cognitive Function During Treatment?
No drug has proven benefits specifically for reversing radiation-related cognitive decline in glioblastoma. But several approaches are associated with managing contributing factors and preserving cognitive reserve:
- Dexamethasone dose minimization: a supervised taper to the lowest effective dose may meaningfully reduce steroid-related cognitive and mood side effects
- Seizure control review: uncontrolled subclinical seizures and the sedating effects of some antiepileptic drugs can worsen cognitive function - the oncology team can assess whether the current regimen is contributing
- Mood disorder management: depression and anxiety independently impair memory and attention; treating them often improves cognitive performance in parallel
- Light aerobic exercise: where physically safe given the patient's neurological status, exercise has been studied for potential cognitive benefit during cancer treatment
- Neuropsychological rehabilitation: structured cognitive training and compensatory strategies, available through rehabilitation services at larger cancer centers, can help patients and caregivers develop practical daily coping approaches
When to Talk to Your Doctor
Contact the neuro-oncology team promptly if cognitive changes are sudden rather than gradual, if there is a new focal deficit (abrupt loss of speech, new arm or leg weakness on one side, or sudden confusion about familiar surroundings), or if disorientation appears. Sudden changes can indicate tumor progression, a seizure, a blood clot, or another cause that needs urgent evaluation - do not wait for the next scheduled appointment.
For slower changes, raise them at the next visit. Cognitive symptoms are often undertreated because caregivers assume they are an unavoidable and fixed feature of the diagnosis. Dose adjustments, seizure medication review, mood assessment, or a neuropsychological referral may each help substantially. You are not overreacting by bringing them up.
If you want to map the cognitive symptom timeline against imaging and treatment history before the next clinic visit, you can upload MRI scans and reports at the Glioblastoma Center patient journey page to request a remote case review by the team.
This article is for general information and is not a substitute for medical advice. Always consult your oncologist or care team about your specific situation.