Introduction

Optic neuritis (ON) affects roughly half of patients with multiple sclerosis (MS) over the course of disease [1] and typically presents with subacute, unilateral vision loss, driven by inflammation-induced retinal nerve fiber layer (RNFL) thinning and axonal loss [2–4]. However, RNFL thinning can also occur in the absence of clinically apparent ON, plausibly due to damage within the optic radiations (geniculocalcarine tract) or inflammation involving other vulnerable segments of the afferent visual pathway, such as the myelinated optic nerves and chiasma [2–5].

Previous studies have reported significantly reduced optic nerve diameter (OND) and optic nerve sheath diameter (ONSD) in MS [6–8]. Although advanced modalities, including optical coherence tomography (OCT), visual evoked potentials (VEP), and magnetic resonance imaging (MRI), are widely used to detect visual dysfunction and degeneration of the retina and optic nerve, there is growing interest in bedside, non-invasive, and cost-effective alternatives [5,9,10]. Trans‑orbital sonography (TOS) has emerged as a potential surrogate marker of neurodegeneration, as it allows for direct measurement of OND and ONSD, typically at 3 [7,9,11–15] and 5 mm [7,13–15] posterior to the globe [16], and has shown promise for detecting both acute ON and chronic atrophy. Nevertheless, conclusive evidence regarding its validity and reliability remains limited.

Given this background, we measured OND and ONSD in patients with MS, with and without prior ON, and in healthy controls, to assess the diagnostic potential of TOS while disentangling ON-related injury from ON-independent neurodegeneration.

Materials and methods

Results and discussion

A total of 56 patients with MS (32 females (57.1%); mean age: 40.32 ± 9.12 years) (Table 1) and 60 healthy controls (35 females (58.3%); mean age: 31.68 ± 8.47 years) were included. Sex distribution did not differ between groups (p = 0.90), but patients with MS were significantly older than controls (p < 0.001).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 1. Comparison of the baseline characteristics of the participants.

https://doi.org/10.1371/journal.pone.0336564.t001

Across the MS cohort, OND, ONSD, and the OND/ONSD ratio were not significantly associated with sex, age, MS type, EDSS score, disease duration, family history of MS, DMDs, number of lesions, or cervical/thoracic lesions. Infratentorial lesions were associated with smaller right ONSD (p = 0.017). A history of right-sided ON was associated with smaller right OND and ONSD (both p < 0.001), and a history of left-sided ON was associated with smaller left OND (p < 0.001) and ONSD (p = 0.002) (Table 2).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 2. Relationships between OND, ONSD, and OND/ONSD ratio, and demographic, clinical and paraclinical characteristics of patients with MS (p-value).

https://doi.org/10.1371/journal.pone.0336564.t002

OND, ONSD, and the OND/ONSD ratio on both sides were significantly smaller in patients with MS, compared to healthy controls, even after adjusting for age (p < 0.001). The reductions in OND and ONSD were particularly pronounced in those with prior ON; however, OND/ONSD ratio did not significantly differ between patients with MS with and without prior ON (Table 3).

Download:

• PPT
  PowerPoint slide
• PNG
  larger image
• TIFF
  original image

Table 3. Comparison of OND, ONSD, and OND/ONSD ratio between patients with MS with and without a history of ON, and healthy controls.

https://doi.org/10.1371/journal.pone.0336564.t003

Our findings indicate that relapse-free patients with MS exhibit significantly reduced OND and ONSD compared with healthy controls. Several studies likewise report significantly smaller OND [6,8,9,15,23], ONSD [7,8,13–15,23], and OND/ONSD ratio [8] in MS. One study also found a smaller ONSD (5 mm posterior to the globe) in SPMS compared with those with RRMS [7], suggesting that optic nerve atrophy may progress with disease severity. In contrast, two studies observed no significant differences [12,24]. Furthermore, in a study including patients with MS and neuromyelitis optica spectrum disorder (NMOSD)—AQP4-positive/negative and MOG-positive)—OND and ONSD did not significantly differ between the groups [25].

Consistent with previous reports [11,13,23,25], we found that a history of ON is associated with smaller OND and ONSD, a pattern compatible with subclinical inflammation and/or diffuse axonal degeneration leading to structural optic nerve injury in MS. However, multiple investigations did not observe a consistent association between ON history and reductions in OND or ONSD [6,8,9,12,24]. Timing likely contributes to these discrepancies: in patients with MS with first-episode retrobulbar neuritis (n = 20), the OND of the affected eye was 0.6 mm greater than that of the unaffected eye [21], suggesting acute swelling may mask the chronic atrophy seen later.

We found that infratentorial lesions were associated with a smaller right ONSD, whereas lesions in the cervical and thoracic spinal cord showed no such relationship. This localization-specific pattern may reflect broader neuroanatomical connectivity: lesions near the visual pathways could influence CSF dynamics [26], thereby altering the thickness of the optic nerve subarachnoid space [27]. This interpretation is consistent with prior evidence linking ONSD to intracranial CSF pressure [27,28]. Moreover, infratentorial involvement has been associated with greater disability [29] and autonomic dysregulation [30–32], which can affect venous outflow and CSF flow, potentially contributing to ONSD alterations. Finally, subtle anatomical asymmetries in the optic canal or nerve [33,34] may accentuate side-specific effects in regions with altered CSF compliance.

In our cohort, TOS-derived measurements did not vary by sex, age, MS type, EDSS score, disease duration, family history of MS, or use of DMDs. By contrast, several studies have linked smaller OND [6,8,12] and ONSD [6,8,13,14] to higher EDSS scores, reflecting greater disability. ONSD has also been reported to correlate with disease duration [13], MRI-detected lesions [13,21], P100 amplitude [13], and P100 latency [8,13]; similarly, OND has been associated with disease duration [6], normalized total brain volume, gray matter volume, white matter volume, and ventricular CSF volume [9]. These discrepancies likely reflect heterogeneity in sample sizes and participant characteristics, disease duration, sonographic protocols, diagnostic thresholds, and imaging timing.

Despite promising findings, several limitations warrant caution. Convenience sampling in a tertiary-referral setting likely contributed to an atypical female-to-male ratio, MS type mix, and a high proportion on rituximab (> 75%); while reflective of local practice, the small single-center sample, and variability in DMD regimens constrain generalizability. Patients were significantly older than healthy controls; although age was adjusted for, residual confounding may have persisted. Controls were drawn from medical students and staff, which may not represent the general population. Normative OND and ONSD values were unavailable, limiting comparisons to established standards. The cross-sectional design prevents causal inference or assessment of temporal change. TOS captures structural changes; integration with functional measures (e.g., VEPs) would strengthen structure-function correlations. All ultrasounds were performed by a single trained neurologist, which reduces inter-observer variability yet underscores operator dependence and potential reproducibility challenges. Nearly all patients had cervical cord lesions, suggesting possible selection bias toward more severe cases, and exclusion of comorbidities further narrows applicability. Future longitudinal, multicenter studies with larger, more diverse cohorts and multimodal assessments are needed to validate and extend these results.

Conclusions

TOS detected optic nerve alterations in MS, particularly among individuals with prior ON. Although these findings support TOS as a potentially informative adjunct for assessing disease burden, the cross-sectional design and modest sample size limit causal inference and immediate clinical translation. Larger prospective studies are needed to validate these observations and to define the role of TOS as a complementary tool in MS assessments.

Supporting information