Brain Fog in Aspergillosis: Why It Happens and What Might Help

Brain fog can make familiar tasks unexpectedly difficult, sometimes threatening a person’s ability to work or manage everyday life. What causes it, why can corticosteroids sometimes make it better or worse, and what can patients do?

When thinking becomes difficult

During a recent patient discussion, a woman described how difficult it had become to continue working in a demanding environment while experiencing brain fog.

Her job requires her to remain on her feet, concentrate on proceedings and respond to what is happening around her. These are abilities she has relied upon throughout her working life. Yet she described how much harder they had become.

She believes her brain fog is associated with the oral corticosteroids she needs to take.

Her experience raises an important question: what happens when an illness, or the treatment needed to control it, interferes with the mental abilities someone depends upon to earn a living?

For many patients, brain fog is much more than occasional forgetfulness. It can undermine confidence, make ordinary conversations exhausting and turn familiar tasks into unexpected challenges.

And perhaps most frustratingly, it is a symptom that other people cannot easily see.

But what exactly is brain fog? Do we know what causes it? And, most importantly, can anything be done?

What is brain fog?

Brain fog is an informal term used to describe a collection of cognitive symptoms.

People commonly report:

• Difficulty concentrating or maintaining attention.
• Forgetting information they would normally remember.
• Losing track of conversations or what they were doing.
• Struggling to find familiar words.
• Taking longer to understand or process information.
• Feeling mentally exhausted after relatively simple tasks.
• Difficulty planning, organising or making decisions.

Some patients describe it as trying to think through thick fog. Others say that their brain simply will not work at its usual speed.

Importantly, brain fog is not a diagnosis in itself. It describes an experience that may have several different underlying causes.

It is also not necessarily a sign of dementia or permanent brain damage.

Many people experience fluctuating symptoms, with periods of relatively clear thinking interrupted by episodes of cognitive difficulty.

That variability is scientifically interesting because it suggests that, in some cases, brain function is being temporarily disrupted rather than permanently lost.

Why might aspergillosis cause brain fog?

Brain fog is not usually included among the defining diagnostic symptoms of aspergillosis. Nevertheless, people with chronic pulmonary aspergillosis (CPA), allergic bronchopulmonary aspergillosis (ABPA) and other chronic respiratory illnesses may experience cognitive difficulties.

There are several plausible explanations, and more than one may be operating at the same time.

1. Inflammation and the immune system

When the body responds to infection or inflammation, immune cells release chemical messengers called cytokines.

These substances help coordinate the immune response, but inflammatory signalling can also affect the nervous system.

We are familiar with some of these effects during influenza or other infections: exhaustion, reduced concentration, loss of motivation and a strong desire to rest.

Researchers sometimes refer to this coordinated response as sickness behaviour.

It may be a useful short-term adaptation during illness, but persistent inflammatory activity could contribute to ongoing symptoms.

In CPA, chronic infection and inflammation may continue over long periods. In ABPA, allergic inflammation can fluctuate, sometimes becoming particularly active during exacerbations.

It is therefore biologically plausible that inflammation contributes to cognitive symptoms in some patients.

However, we do not yet have strong evidence identifying a specific inflammatory mechanism responsible for brain fog in aspergillosis.

2. Fatigue and reduced mental energy

Fatigue is a major problem for many people with chronic respiratory disease.

But fatigue is not always simply a feeling of sleepiness.

Patients may experience profound physical and mental exhaustion, even after relatively modest activity.

Thinking itself requires sustained effort. Concentrating, remembering information, making decisions and following conversations all depend on networks of brain activity.

When someone is already exhausted by illness, ordinary cognitive tasks may become much more demanding.

This can create a frustrating cycle: the more someone tries to concentrate, the more mentally exhausted they become.

3. Oxygen and carbon dioxide levels

The brain requires a continuous supply of oxygen.

Significant reductions in blood oxygen levels can affect concentration, alertness and cognitive performance.

In people with advanced respiratory disease, inadequate ventilation may also allow carbon dioxide to accumulate in the blood.

Raised carbon dioxide can cause headaches, drowsiness and confusion.

These problems are particularly important during respiratory deterioration.

However, it would be incorrect to assume that every patient experiencing brain fog has low oxygen levels. Many people report cognitive difficulties despite having satisfactory oxygen measurements.

A normal spot oxygen saturation reading also does not assess carbon dioxide levels or explain every possible cause of cognitive symptoms.

4. Poor sleep

Sleep disturbance is another potentially important contributor.

Persistent coughing, breathlessness, discomfort, anxiety and medication effects can all interfere with sleep.

Poor sleep affects attention, memory and the ability to process information.

A patient may therefore experience substantial brain fog even when their respiratory disease appears relatively stable.

Sleep apnoea and other sleep disorders can also contribute and may be worth investigating when symptoms suggest them.

The corticosteroid paradox: why can steroids make brain fog better or worse?

This is one of the most interesting questions raised by patients.

Some describe developing brain fog while taking oral corticosteroids.

Others report that their thinking becomes noticeably clearer after an increase in corticosteroid treatment.

How can the same type of medicine apparently produce opposite effects?

The answer is that corticosteroids influence several biological systems simultaneously.

Corticosteroids can reduce inflammation

Medicines such as prednisolone and methylprednisolone suppress inflammatory activity.

For someone experiencing an inflammatory exacerbation, effective treatment may improve symptoms, sleep and general wellbeing.

If inflammation or illness-related exhaustion is contributing to cognitive difficulties, improved disease control might also improve mental clarity.

However, an improvement after taking corticosteroids does not prove that brain inflammation was present.

Corticosteroids can also affect the brain directly

Glucocorticoid hormones influence brain regions involved in memory, attention, emotion and stress responses.

Excessive glucocorticoid exposure can produce insomnia, agitation, anxiety, mood changes and difficulties with concentration.

The effects depend on factors including dose, duration, individual susceptibility and the person’s underlying health.

This means that a patient may experience worse cognitive symptoms even while their respiratory inflammation is improving.

What about adrenal insufficiency?

Cortisol is an essential hormone produced by the adrenal glands.

It helps regulate metabolism, blood pressure, stress responses and many other bodily functions.

Long-term corticosteroid treatment can suppress the body’s own cortisol production. Some patients develop secondary or tertiary adrenal insufficiency.

When cortisol availability is inadequate, symptoms can include severe fatigue, weakness, dizziness, nausea and difficulty concentrating.

During infection or other physiological stress, the body’s normal requirement for cortisol increases.

For someone with adrenal insufficiency, an appropriate increase in replacement treatment under their established sick-day plan may improve symptoms caused by inadequate cortisol availability.

This provides one possible explanation for reports of brain fog improving after a corticosteroid dose increase.

But it is important not to jump to conclusions.

An improvement following extra corticosteroid does not establish adrenal insufficiency, and increasing the dose is not a reliable treatment for brain fog in general.

Both insufficient and excessive glucocorticoid exposure can cause problems.

Patients should not change prescribed corticosteroid treatment independently, except according to an agreed clinical plan.

People with adrenal insufficiency should follow their personalised sick-day guidance and seek urgent medical help when symptoms suggest adrenal crisis.

Further information: NHS guidance on adrenal insufficiency and Addison’s disease and NHS information about corticosteroids.

Why does brain fog fluctuate?

One of the most striking features of brain fog is that it can change considerably.

Someone may struggle to concentrate in the morning but feel clearer later in the day. Symptoms may worsen during infection, after poor sleep or during periods of increased physical activity.

Some people describe a marked improvement after treatment changes.

There are several possible explanations:

• Inflammatory activity may fluctuate.
• Sleep quality and fatigue vary from day to day.
• Medication concentrations and effects change over time.
• Cortisol availability and physiological requirements vary.
• Pain, anxiety and physical exertion can affect concentration.
• Blood oxygenation and ventilation may change during respiratory deterioration.

These factors can interact.

For example, an infection may increase inflammation, disrupt sleep, worsen fatigue and alter cortisol requirements simultaneously.

This makes it difficult to identify one cause simply by observing when brain fog appears.

Nevertheless, recognising patterns can be clinically useful.

What can patients do to help themselves?

There is currently no single established treatment for brain fog associated with aspergillosis.

However, that does not mean nothing can be done.

The most useful approach is often to look systematically for contributing factors that might be treatable.

1. Describe exactly what has changed

The phrase brain fog is useful, but clinicians may need more detail.

Try to describe specific difficulties:

• Are you forgetting conversations?
• Do you lose track of what people are saying?
• Is reading or following instructions unusually difficult?
• Do you struggle to find words?
• Does thinking become harder as the day progresses?
• Are you making mistakes in tasks you previously performed easily?

Explaining how symptoms affect daily functioning can be particularly powerful.

For example:

“I used to be able to follow complex conversations at work. Now I lose track halfway through and cannot always respond quickly enough.”

This communicates much more than simply saying that concentration is poor.

2. Look for patterns

A short symptom diary may help identify changes associated with:

• Respiratory exacerbations or infections.
• Changes in corticosteroid dose or timing.
• Antibiotic or antifungal treatment.
• Poor sleep.
• Increased physical or mental activity.
• Episodes of breathlessness.
• Changes in fatigue or general wellbeing.

A diary does not need to be complicated.

Recording symptom severity, medication changes and one or two relevant observations may be enough to reveal a useful pattern.

It can also provide a clearer picture for the clinical team than trying to remember several weeks of fluctuating symptoms during a short appointment.

3. Ask whether there are treatable medical contributors

Depending on the patient’s history and symptoms, clinicians may consider:

• Whether respiratory infection or inflammation is adequately controlled.
• Whether oxygenation or ventilation needs assessment.
• Whether medication adverse effects are contributing.
• Whether corticosteroid treatment or adrenal function needs review.
• Whether anaemia, thyroid dysfunction, vitamin deficiencies or metabolic disturbances might be involved.
• Whether sleep disorders, pain, anxiety or depression are contributing.

Not every patient needs every investigation.

The aim is to avoid assuming that cognitive symptoms are simply an unavoidable part of chronic illness.

4. Manage mental energy

Even when the underlying cause cannot immediately be corrected, practical strategies may reduce the impact of brain fog.

These include:

• Scheduling demanding tasks for times when concentration is usually better.
• Breaking complex activities into shorter stages.
• Taking regular breaks before mental exhaustion becomes overwhelming.
• Using written reminders, calendars and checklists.
• Reducing distractions when concentration is important.
• Allowing extra time for decisions and unfamiliar tasks.
• Avoiding unnecessary multitasking.

These approaches do not cure brain fog, but they may make everyday life more manageable.

Patients whose symptoms worsen substantially after exertion may need more individualised pacing advice rather than simply being encouraged to increase activity.

5. Consider support at work

For someone whose employment depends on concentration, memory or rapid decision-making, brain fog can be particularly distressing.

Occupational health services may help assess whether temporary or longer-term adjustments would be useful.

Depending on the role, adjustments might include additional breaks, altered working hours, reduced interruptions, written instructions or changes to particularly demanding duties.

In the UK, some people with long-term health conditions may be entitled to reasonable workplace adjustments under the Equality Act 2010.

Brain fog should not have to become severe enough to threaten someone’s employment before its impact is taken seriously.

Further information: Acas guidance on reasonable adjustments at work.

When is brain fog something more urgent?

There is an important distinction between persistent cognitive difficulties and sudden confusion.

Brain fog often develops gradually or fluctuates over time.

Sudden confusion, disorientation, unusual drowsiness or a marked change in consciousness may indicate an acute medical problem.

Possible causes include severe infection, low oxygen levels, raised carbon dioxide, metabolic disturbances, medication effects or adrenal crisis.

New, sudden confusion requires urgent medical assessment. In the UK, call 999 if someone suddenly becomes confused, particularly if they are severely unwell.

New weakness affecting one side of the body, facial drooping or speech disturbance may indicate a stroke and also requires an immediate 999 call.

These symptoms should not be dismissed as ordinary brain fog.

Further information: NHS advice on sudden confusion and delirium.

What is research beginning to reveal?

Although brain fog remains poorly defined, scientific understanding is improving.

Researchers are increasingly recognising that cognitive symptoms occur across many different conditions, including chronic inflammatory illnesses, endocrine disorders and post-infectious syndromes.

Several areas of research are particularly promising.

How inflammation affects brain function

Scientists are investigating how immune signals influence the brain, including changes in neural communication, metabolism and the activity of immune cells within the nervous system.

This work may help explain why some people experience cognitive symptoms during or after illness.

However, findings from one condition cannot automatically be applied to aspergillosis.

The importance of cortisol rhythms

Cortisol normally follows a daily rhythm, with concentrations changing throughout the day and in response to physiological stress.

Conventional oral corticosteroid replacement does not always reproduce this pattern precisely.

Researchers are investigating whether more physiological replacement strategies can improve outcomes in people with adrenal insufficiency, including aspects of wellbeing and cognition.

This is relevant to patients who develop adrenal suppression after prolonged corticosteroid treatment.

Better ways to measure cognitive symptoms

Another important development is the increasing use of structured cognitive assessments and patient-reported outcome measures.

Instead of treating brain fog as a vague complaint, researchers are attempting to measure changes in attention, processing speed, memory and mental fatigue.

Better measurement may help identify different types of cognitive difficulty and determine whether treatments genuinely improve them.

Could treatments eventually become more targeted?

If researchers can distinguish brain fog caused predominantly by inflammation, hormonal disturbance, sleep disruption or other mechanisms, treatment may become more individualised.

A patient whose symptoms are driven by inadequate cortisol replacement would need a different approach from someone whose difficulties are associated with sleep disruption or persistent inflammatory disease.

This is a plausible direction for future research, rather than an established treatment strategy.

There is still considerable work to do, particularly in understanding cognitive symptoms associated specifically with CPA and ABPA.

Brain fog deserves to be taken seriously

For someone living with aspergillosis, brain fog can be among the most frustrating symptoms.

It affects abilities that are fundamental to everyday life: remembering, concentrating, communicating, working and making decisions.

Yet because it is invisible, its impact can easily be underestimated.

The woman who described struggling to continue her courtroom work illustrates this particularly clearly. Her difficulty was not simply feeling tired or occasionally forgetting something. It was the possibility that her illness or its treatment was interfering with the abilities her livelihood depended upon.

We should also remember that brain fog may have several causes operating together. The fact that a clinician cannot immediately identify one explanation does not mean the symptoms are imaginary.

There are reasons for cautious optimism.

Some contributing factors are already identifiable and treatable. Others are becoming better understood as researchers investigate the relationships between inflammation, hormones, sleep and brain function.

And the fact that brain fog can sometimes improve substantially, even after prolonged periods of difficulty, suggests that at least some of the underlying changes can be reversible.

For patients, the most important message is this:

Brain fog is real. It deserves proper attention. And although we do not yet have all the answers, there may be practical ways to reduce its impact today and better treatments in the future.

Further reading and useful resources

• NHS Your COVID Recovery – Information about post-infectious symptoms, including cognitive difficulties and fatigue.
• NHS: Addison’s disease – Information about adrenal insufficiency, treatment and adrenal crisis.
• NHS: Steroids – Corticosteroid treatment and potential adverse effects.
• NHS: Sudden confusion (delirium) – Recognising when confusion requires urgent assessment.
• Acas: Reasonable adjustments – Guidance on workplace adjustments for people with disabilities and long-term health conditions.

This article is intended for general information and does not replace individual medical advice. Patients should not alter prescribed corticosteroid doses without guidance from their clinical team or an established sick-day plan.

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