Functional Neurological Disorder (FND) is increasingly recognised as a condition involving disruptions in attention, prediction, and motor control across large-scale brain networks. While the diagnosis is primarily clinical, there is growing interest in objective markers that support a positive diagnostic approach.

A new multicentre study using video-oculography (VOG)—a precise method of recording eye movements—has provided the largest dataset to date examining saccades, smooth pursuit, antisaccades, and abnormal eye movement patterns in FND. The findings give us fresh insight into how FND affects cognitive–motor control and may help clinicians differentiate FND from organic neurological diseases in complex presentations.

1. Why Eye Movements Matter in FND

Eye movements are tightly linked to networks involved in:

attention
motor preparation
sensory integration
prediction and inhibition
cognitive control

Because these systems are directly implicated in FND pathophysiology, ocular motor tasks can reveal subtle abnormalities that may not be detectable in routine clinical examination.

VOG allows clinicians to objectively measure latency, variability, accuracy (gain) and error patterns, offering a window into the “effortful”, inconsistent, and attention-sensitive performance often characteristic of FND.

2. Key Findings From the New Study

A. Increased Saccadic Latency

One of the most robust findings was prolonged reaction times (latency) in FND patients across both horizontal and vertical saccades.

FND patients had significantly longer latencies than healthy controls.
Latencies were similar to, or slightly higher than, patients with multiple sclerosis (MS).
Lateralised symptoms did not lead to lateralised saccadic delays.

Clinical meaning:
Delayed initiation of eye movements reflects the increased cognitive load, heightened self-monitoring, and impaired attentional shifting that characterise FND. This aligns with the broader concept of effortful voluntary control replacing automatic motor processes.

B. Marked Variability – A Signature of FND

Perhaps the most important discriminator was variability.

FND patients showed dramatically increased variability in saccade latency compared with both controls and MS patients.
Variability strongly correlated with longer latencies—when patients slowed down, they also became more inconsistent.
Upward vertical saccades were especially variable.

Why this matters:
Variability is a core positive diagnostic feature in FND—seen across motor, gait, sensory, and speech presentations.
Here, VOG quantifies that inconsistency objectively, supporting the clinical picture.

C. Vertical Saccades: Hypometria and Variability

Vertical saccades provided another differentiating pattern:

Upward and downward saccades in FND were hypometric (undershooting the target).
Increased variability of upward saccade gain was characteristic.
MS patients also showed abnormalities, but of different patterns (e.g., more downward variability).

This suggests subtle differences in how fronto-parietal and brainstem networks behave in functional vs. structural disorders.

D. Antisaccade Performance – Impaired but Non-Specific

The antisaccade task measures response inhibition and voluntary control.

Findings:

Both FND and MS groups had higher error rates than controls.
Latency of corrected responses was also significantly prolonged.
In FND, antisaccade errors increased slightly in patients with psychiatric comorbidities or neurotropic medication use.
When medicated patients were removed, antisaccade errors resembled those of controls.

Interpretation:
Antisaccade deficits reflect impaired inhibition and increased cognitive load—common in many neuropsychiatric conditions—so this is not specific to FND.
However, in complex cases it contributes to the overall pattern of attentional and executive dysfunction.

E. Smooth Pursuit – Often Jerky in FND

Smooth pursuit abnormalities were notable:

34% of FND patients showed saccadic (jerky) pursuit
Compared with 18% of controls
Less severe than MS (60%), but still a meaningful difference

Smooth pursuit requires sustained attention and continuous sensorimotor integration—functions often impaired in FND.

F. Abnormal Eye Movements – Only in 13% but Highly Characteristic

Although not frequent, certain qualitative eye movement disorders were seen in FND:

Convergence spasm: 7%
Pseudo-flutter: 3%
Functional gaze restriction: 5%
Functional ptosis/blepharospasm: ~3%
Importantly: no nystagmus, differentiating FND from cerebellar or vestibular pathology.

These findings mirror previous clinical reports and emphasise that while many FND patients have normal baseline eye movements, a minority show distinctive, task-triggered abnormalities.

3. How Do Ocular Abnormalities Vary Across FND Subtypes?

The study included patients with diverse FND presentations—functional weakness, gait disorders, tremor, dystonia, sensory loss, dissociative seizures, and visual symptoms. Although subgroup analysis was limited, several patterns are noteworthy:

Functional Motor Symptoms (weakness, gait problems)

More likely to show increased latency and variability, reflecting global motor planning disruption.

Hyperkinetic FND (tremor, dystonia, jerks)

Higher likelihood of smooth pursuit saccadic intrusions, reflecting impaired sensorimotor gating.

Sensory FND / Visual Symptoms

Not clearly associated with distinct VOG findings—eye movement abnormalities were similar across subgroups.

Dissociative Seizures

Small numbers, but antisaccade performance tended to be worse, consistent with attentional difficulties.

FND with Psychiatric Comorbidity

Slight increase in antisaccade errors
No effect on latency or variability

Overall, inconsistency and excessive cognitive effort appear common across all phenotypes.

4. What This Means for Clinical Practice

A. VOG is not diagnostic—but highly supportive

The authors emphasise that eye-tracking abnormalities should not be used in isolation. Instead, they:

strengthen a positive FND diagnosis
help differentiate FND from structural disorders in ambiguous cases
prevent misdiagnosis (e.g., suspected cerebellar or brainstem disease)
provide objective data to support discussions with patients

B. Variability is the most valuable marker

If one feature stands out as “characteristic” of FND, it is this:

Marked variability in saccade latency and gain that exceeds what is seen in both healthy controls and MS.

C. VOG may have therapeutic value

Objective visualisation of inconsistent or task-dependent abnormalities can help patients understand their condition, enhancing engagement in treatment.

5. Final Thoughts

This large, rigorous VOG study strengthens the evidence that FND affects eye movements through disrupted attentional, predictive, and cognitive control mechanisms, not structural pathology.

The most meaningful ocular motor signatures of FND include:

Prolonged saccade latency
Marked latency variability (a core positive sign)
Vertical hypometria with variability
Jerky smooth pursuit
Task-triggered functional eye movement disorders (e.g., convergence spasm)

These findings add valuable, objective markers to the clinician’s toolkit—particularly in diagnostically complex cases.