We found that orbital schwannomas and CVMs have overlapping clinical features, with a mean age of 45–50 years and predominantly affecting females (54–62%). The most common ophthalmic symptom was gradual proptosis in schwannoma and CVM groups (prevalence, 54% and 46%; and proptosis, 3 mm and 2.5 mm, respectively), without statistical difference. In this regard, radiologic findings proved valuable for differential diagnosis. Heterogeneous appearance on T2WI, diffuse early-phase enhancement, and the presence of a tail sign and target sign, were all indicative of a potential diagnosis of schwannoma. Conversely, a linear T2 hypointensity and nodular early-phase enhancement suggested a diagnosis of CVM. CT imaging revealed significantly lower median CT value in schwannomas. En bloc excision without bony marginotomy proved effective in most cases of pathologies and subtotal resection of anatomically-challenging schwannomas did not lead to recurrence.
Radiologic studies, especially MRI, play a crucial role in evaluating orbital tumors. The lesion characteristics noted in our study align with previous research. Schwannomas, characterized by smooth margins, predominantly manifest in the superior to superomedial regions of the orbit. This is linked to their frequent origin from supratrochlear and supraorbital branches of the frontal nerve.3,4,13,18 In contrast, CVMs typically appear as well-circumscribed lesions with lobulated margins.5,6,18 CVMs commonly manifest in the lateral intraconal aspect of the mid-orbit.5,6 Although anatomic patterns offer some guidance in differentiating schwannomas and CVMs, both lesions may potentially develop from various locations within the orbit.
Although non-contrast MRI characteristics may be non-specific, certain features may offer advantages in differentiating between orbital schwannoma and CVM. Two distinct cellular patterns are observed in orbital schwannoma, known as Antoni A and Antoni B. Antoni A tissue is made up of compact bundles of fibrillated cells, while Antoni B tissue is less cellular, featuring a loosely textured stroma. On T2WI, schwannomas commonly present a heterogenous pattern with avid hyperintensity, indicating the cystic degeneration and myxoid alterations in histologic Antoni B regions. Both Antoni A and Antoni B tissue types commonly exist together in the same tumor, with neither one necessarily being dominant. The varied morphology, which can include intra-tumoral cysts, different cellular patterns, calcifications, and hemorrhage regions, contributes to the heterogeneity in MRI signal intensity. Additionally, the pathological features often differ across various areas of the same tumors.3,4,12,13,19 Orbital schwannomas often display a central area of low signal with a peripheral hyperintense rim, known as the "target sign".4,20 The "target sign," seen in 46.2% of schwannomas, was identified as a characteristic MRI finding in orbital schwannoma, as reported by Wang et al.3 The "tail sign," also observed in 46.2% of our schwannomas, indicates a direct and central continuity with a small nerve branch. The presence of a "tail sign" can help depict the relationship between a soft tissue mass and the adjacent nerve, a suggestive feature for neurogenic tumors.21 Among the CVM lesions, none exhibited a target sign or tail sign. The heterogeneity on T2WI with avid hyperintensity and the presence of a target sign or tail sign collectively suggest a greater likelihood of schwannoma rather than CVM.
On non-contrast MRI, our study revealed that the presence of a linear T2 hypointensity was observed in 57.7% of CVM cases, whereas it was found in 7.7% of orbital schwannomas (p = 0.008). The linear T2 hypointensity may correlate with the histology of CVM, characterized by multiple large vascular channels separated by fibrous septa.5,6,8,18 For contrast-enhanced MRI, nodular enhancement at early phase and final diffuse filling in the late phase is a well-known characteristic of CVM.5,6,8,22 In our study, 92% of CVM demonstrates nodular enhancement, whereas all schwannomas show diffuse enhancement at early phase. The nodular early enhancement pattern has the highest sensitivity and specificity, both at 92.0% and 100% respectively, with a diagnostic accuracy of 94.3% among various imaging features for the diagnosis of CVM. Therefore, it is considered the most useful feature for discriminating between the CVMs and orbital schwannomas. The nodule enhancing starting point in CVM is correlated with low-flow arterial supply and the slow progressive accumulation of contrast medium within dilated venous spaces filling during the late venous phase. During delayed phase, 76% of CVM shows heterogeneous enhancement, likely due to slower flow of contrast, resulting in incomplete filling even at the delayed phase. Excluding early-phase enhancement, there was no significant difference in the enhancement pattern and delayed enhancement findings between the two groups. Thus, dynamic enhancement image, especially at early phase, can aid in diagnosis of orbital CVM. Notably, two cases of CVM (8%) exhibited diffuse early-phase enhancement, which has also been reported in previous studies.5 Although the exact reasons remain unclear, it is possible that this may be associated with a faster hemodynamic pattern compared to typical orbital CVM.4,15
Recent studies have indicated that DWI may aid in distinguishing orbital schwannomas from other orbital masses.10,11 Myxoid masses, including schwannomas, tend to exhibit higher ADC values compared to non-myxoid masses.10,11,23 Razek et al. also reported the ADC value of schwannoma to be 1.9 ± 0.20 x 10− 3 mm2/s (mean ± SD), which was statistically significant (p = 0.001), when compared to that of venous malformation (mean ± SD: 1.38 ± 0.70 x 10− 3 mm2/s). However, it is worth noting that in their study, only three schwannomas and one CVM were analyzed.24 Our study shows that mean ADC values for schwannomas tended to be higher than those of CVM lesions, but this difference did not reach statistical significance (1.64 ± 0.56 x 10− 3 mm2/s and 1.26 ± 0.21 x 10− 3 mm2/s, respectively; p = 0.078).
Histologic characteristics of CVM may help explain its observed lower ADC value. CVM is characterized by multiple vascular channels covered by endothelial cells and abundant stroma. The vascular lumen is filled with blood and variable regions of thrombosis, reflecting vascular stasis or extremely slow flow. The stromal structure shows perivascular hypercellularity or hyperplastic elements.6 Presence of intraluminar blood, thrombosis, and a prominent stromal structure may contribute to the lower ADC value compared to orbital schwannoma. Correspondingly, the median HU on precontrast CT was higher in CVM compared to orbital schwannoma (p = 0.001).
Treatment strategies and surgical goals may differ between orbital schwannomas and CVMs. Regarding surgical approach, schwannomas often require a broader surgical field due to their thinner, more adherent capsules, which are prone to rupture during surgery. In contrast, CVMs have a thicker fibrous capsule less adherent to surrounding structures, allowing easier and more complete removal, often achievable through minimally invasive transconjunctival approach aided by a cryoprobe.14,15 In our cases, although a transconjunctival approach was generally used, schwannoma patients more frequently required lateral bony marginotomy compared to CVMs (23.1% vs. 7.7%). Regarding surgical goals for schwannomas, minimizing complications takes precedence over complete excision, particularly in challenging locations like the orbital apex or intramuscular lesions of extraocular muscles. Piecemeal subtotal resection can be preferred to avoid risks of vision loss or strabismus.2,13 In our series, three schwannoma patients underwent subtotal piecemeal resection without complications such as visual deficits or strabismus requiring corrective surgery. No recurrence or progression of residual lesions was observed over an average follow-up of 4.3 years, though patients should be cautioned about potential recurrence with subtotal resection. Regarding alternative treatment options, stereotactic radiosurgery has been reported to show excellent outcomes for orbital apex lesions, especially for CVMs, though more evidence is needed for schwannomas.2,25 Given the differences in management considerations, radiological differentiation between orbital schwannomas and CVMs is beneficial for informed decisions about appropriate management strategies.
Multiple studies have examined orbital schwannomas and CVMs, highlighting their clinical characteristics, radiologic findings, and surgical outcomes.2,3,5,8,12,13,15,22 However, differentiating these conditions from other orbital tumors remains challenging despite contrast-enhanced MRI.5,8,15,22 To our knowledge, no prior study has delineated the clinical attributes and treatment outcomes of orbital schwannoma and CVM, while concurrently examining imaging features, with a focus on quantitative parameters such as ADC in DWI and CT value. Additionally, our study suggests that subtotal resection of anatomically-challenging schwannomas does not always lead to recurrence, indicating that prioritizing complete removal at the risk of vision loss may be unnecessary.
This study has some limitations. The retrospective nature of this study carries inherent drawbacks such as selection bias. Although statistical differences emerged in our MRI analysis between orbital schwannomas and CVMs, the sample size remains limited, further decreasing when examining ADC and CT values. Future prospective studies with larger populations may be necessary to validate and extend these findings, although this could be challenging given the rarity of orbital schwannoma. Moreover, variations in quantitative MRI and CT parameters from examinations conducted at referring hospitals introduce a potential source of variability. For future studies, it is important to carefully consider the reproducibility and repeatability of ADC measurements and CT values in such situations.
In conclusion, orbital schwannomas and CVMs exhibit similar clinical presentations, affecting individuals in their fifth decade with gradual proptosis as a primary symptom. Despite these shared features, their radiologic characteristics markedly differ, offering valuable insights for pre-operative diagnosis. Notably, schwannomas tend to display a heterogeneous appearance on T2-weighted imaging, diffuse early-phase enhancement, distinct tail and target signs, and lower CT value. Conversely, the presence of a linear T2 hypointensity and nodular early-phase enhancement suggest a potential diagnosis of CVM. Physicians should recognize distinctive features for accurate diagnosis and appropriate management. For schwannomas, it is crucial to avoid prioritizing total excision over preventing significant complications like vision loss and strabismus, especially in anatomically challenging cases, considering the low recurrence risk after subtotal resection. Careful consideration of these factors is essential for optimal patient outcomes.