Tinnitus: An Unrecognised Symptom of Functional Neurological Disorder

This study demonstrates that tinnitus is significantly more prevalent in individuals with Functional Neurological Disorder (FND) than in controls and exhibits similar attention-dependent characteristics, suggesting it should be recognized as a potential symptom of FND arising from shared underlying mechanisms.

The Gist

The Big Idea: The Brain's "Stuck" Alarm

Imagine your brain is like a highly sophisticated smart home security system. Its job is to constantly scan the world, compare what it expects to happen with what it actually senses, and then update its settings accordingly.

• Normal Function: If the system hears a noise (a sensor trigger), it checks if there's an intruder. If there isn't, it resets the alarm. The noise stops, and the system goes back to sleep.
• The Problem (FND): In Functional Neurological Disorder (FND), the brain's security system gets "stuck." Even after the initial trigger (like a bump or a scare) is gone, the alarm keeps blaring. The brain refuses to update its settings, so it keeps thinking there is a threat when there isn't one.

This paper suggests that Tinnitus (hearing ringing or buzzing when there is no sound) is actually just another version of this same "stuck alarm," but happening in the ears instead of the muscles or vision.

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The Mystery: Why Didn't We Notice Before?

For a long time, doctors thought FND only affected how people moved (like shaking or paralysis) or how they felt touch (like numbness). They assumed the "auditory system" (hearing) was off-limits for FND.

The authors of this paper asked a simple question: "If FND is a glitch in how the brain processes any sensation, why wouldn't it also cause phantom hearing?"

They hypothesized that tinnitus is a "phantom" sensation—just like the phantom limb pain some people feel after losing an arm. It's a noise the brain creates because it failed to turn off the alarm after a minor ear injury or noise exposure.

The Experiment: Checking the Evidence

To test this, the researchers looked at a massive international database of people with FND and compared them to people without FND.

1. The "Headcount" (Database Study)
They asked thousands of people: "Have you had ringing in your ears in the last month?"

• The Result: It was a landslide. 54% of people with FND said "Yes." Only 17% of the control group (people without FND) said "Yes."
• The Analogy: Imagine walking into a room of 100 people with FND, and 54 of them are holding a "Ringing Ears" sign. In a room of 100 regular people, only 17 are holding that sign. That's a huge difference.

2. The "Honesty Check" (Symptom Experiment)
The researchers worried: "Maybe people with FND just say 'yes' to everything because they want to report symptoms. Maybe they aren't actually hearing the ringing; they just think they should."

To test this, they ran a small experiment where they asked people to list symptoms they had in the last month.

• Group A: Symptoms related to FND (e.g., dizziness, weakness).
• Group B: Random, unrelated symptoms (e.g., sore throat, swollen ankles, cough).

The Result:

• People with FND reported way more FND symptoms than the control group (as expected).
• BUT, when it came to the random symptoms (sore throats, etc.), both groups reported the exact same amount.
• The Takeaway: People with FND aren't just "making things up" or saying yes to everything. They are specifically reporting the symptoms that match their condition. This proves the tinnitus is real and specific to the disorder, not just a reporting bias.

The "Why": A Failure to Reset

The paper argues that tinnitus in FND isn't about the ear being broken; it's about the brain failing to reset.

• The Metaphor: Think of a door that was slammed shut by a gust of wind.
  • Normal Brain: The door slams, the brain notices the noise, realizes the wind stopped, and the door settles. Silence returns.
  • FND Brain: The door slams, but the brain gets confused. It keeps thinking, "The wind is still blowing!" even though the air is still. So, the brain keeps "hearing" the wind (the ringing) even though the door is actually closed.

The study found that people with FND who had tinnitus were actually less likely to have permanent hearing loss than the control group. This supports the idea that their ears were fine, but their brain's "reset button" was broken.

The Conclusion: A New Perspective

The authors conclude that Tinnitus should be considered a symptom of FND.

This changes how we look at the disorder:

1. It's not just about muscles: FND isn't limited to movement or touch; it affects how we hear too.
2. It's a "Domain-General" Glitch: The brain's error isn't specific to one part of the body; it's a general problem with how the brain updates its expectations based on reality.
3. Hope for Treatment: If tinnitus is caused by the brain failing to "unlearn" a symptom, then therapies that help the brain relearn how to ignore the noise (like attention training or cognitive behavioral therapy) might work for tinnitus in the same way they work for FND movement disorders.

In short: This paper suggests that for many people with FND, the ringing in their ears isn't a separate ear problem—it's the brain's security system refusing to turn off the alarm. Recognizing this could lead to better treatments for both tinnitus and FND.

Technical Summary

1. Problem Statement

Functional Neurological Disorder (FND) is a prevalent neurological condition characterized by symptoms that fluctuate with attention and often mimic organic neurological diseases. Current theoretical frameworks, specifically the Bayesian brain model, conceptualize FND as a failure of perceptual inference where abnormal "priors" (expectations) fail to update in response to sensory evidence. This leads to a failure of symptom resolution after an inciting peripheral stimulus.

While FND is known to affect motor, somatosensory, and visual domains, auditory symptoms are traditionally excluded from its diagnostic criteria. However, tinnitus (the phantom perception of sound without an external source) shares critical phenomenological and mechanistic features with FND:

• It is a "phantom" perception.
• It is strongly modulated by attention.
• It is often triggered by peripheral insults (e.g., noise exposure) but persists due to central processing failures.

The Hypothesis: The authors propose that tinnitus reflects the same underlying pathophysiological process as FND (a domain-general disorder of perceptual inference). Therefore, tinnitus should be significantly more prevalent in people with FND (pwFND) than in the general population, and its presence would support the view that FND is not limited to specific sensorimotor systems.

2. Methodology

The study employed a two-pronged approach using data from FND Research Connect, an international database for pwFND and controls.

Study 1: Prevalence Analysis (Database Mining)

• Data Source: Longitudinal data from 791 registered participants (732 pwFND, 59 controls).
• Selection Criteria: Only data regarding symptom experience in the past month was analyzed to ensure recency. Participants in remission were excluded.
• Measurement: Participants were asked if they experienced "Tinnitus (ringing or buzzing in the ears)."
• Statistical Analysis: Bayesian logistic regression was performed using the Python Bambi package.
  • Metrics: Posterior means with 95% Highest Density Intervals (HDIs).
  • Region of Practical Equivalence (ROPE): Defined as +/- 0.1 standardized mean differences to determine if group differences were practically significant.
  • Odds Ratios: Calculated to quantify the strength of the association.

Study 2: Agreement Bias Control (Nested Experiment)

• Objective: To rule out "agreement bias" (the tendency of pwFND to report any symptom regardless of relevance).
• Design: A sub-study involving 38 pwFND and 38 controls.
• Task: Participants reported symptoms experienced in the last month from a list of:
  • 14 FND-related symptoms.
  • 7 non-FND related symptoms (e.g., sore throat, swollen ankles).
• Statistical Analysis: Bayesian Poisson regression comparing the count of FND vs. non-FND symptoms reported by each group.

3. Key Results

Tinnitus Prevalence

• FND Group: 54% (HDI 50–57%) of pwFND reported tinnitus in the preceding month.
• Control Group: 17% (HDI 8.5–25%) of controls reported tinnitus.
• Statistical Significance: The posterior odds for a between-group difference were >4000, indicating overwhelming evidence that tinnitus is more common in pwFND. The odds ratio was approximately 5.7.
• Hearing Loss Correlation: Among those with tinnitus, pwFND were significantly less likely to report hearing loss (27%) compared to controls (50%), suggesting the tinnitus in the FND group is less likely driven by permanent peripheral damage and more likely by central perceptual failure.

Symptom Reporting Bias

• FND Symptoms: As expected, the pwFND group reported significantly more FND-related symptoms than controls (Odds > 4000).
• Non-FND Symptoms: There was no significant difference between groups in the reporting of non-FND symptoms (e.g., sore throat). Both groups reported a mean of ~2 non-FND symptoms.
• Conclusion: The high rate of tinnitus in the FND group is specific to the condition and not a result of a general tendency to over-report symptoms.

4. Key Contributions

1. Identification of a New Symptom Domain: The study provides empirical evidence that tinnitus is a common symptom of FND, challenging the conventional exclusion of auditory symptoms from the FND spectrum.
2. Validation of Domain-General Mechanisms: The findings support the hypothesis that FND is a domain-general disorder of perceptual inference. The mechanisms driving tinnitus (abnormal priors, failure to update expectations) appear to operate across auditory, somatosensory, and motor domains.
3. Methodological Rigor: By utilizing a large international cohort and an internal control for agreement bias, the study robustly isolates tinnitus as a specific correlate of FND rather than a reporting artifact.
4. Theoretical Refinement: The authors propose a unified mechanism where both tinnitus and functional sensory symptoms arise from a failure of symptom resolution following a transient peripheral insult, rather than de novo symptom generation.

5. Significance and Implications

• Clinical Practice: Clinicians should consider tinnitus as a potential manifestation of FND, particularly in patients with a history of transient auditory insults who do not have permanent hearing loss. This could lead to earlier diagnosis and more targeted treatments.
• Therapeutic Cross-Fertilization: Recognizing the shared mechanisms suggests that therapeutic approaches developed for FND (e.g., attentional retraining, cognitive behavioral therapy targeting perceptual inference) could be adapted for chronic tinnitus, and vice versa.
• Research Direction: The paper calls for longitudinal studies to track the course of tinnitus in pwFND and experimental paradigms to directly test auditory prediction-error processing and precision weighting in this population.

Limitations Noted:

• The control group was smaller than the FND group (though prevalence rates matched general population estimates).
• The study is cross-sectional, limiting causal inference regarding onset and persistence.
• The database may underrepresent individuals with mild or short-lived FND symptoms.

Conclusion:
The authors conclude that tinnitus should be formally recognized as a symptom of Functional Neurological Disorder. The convergence of high prevalence, shared phenomenology, and overlapping mechanistic theories suggests that FND represents a fundamental breakdown in the brain's ability to resolve sensory symptoms, extending well beyond the motor and visual systems into the auditory domain.