DOI: https://doi.org/10.21203/rs.3.rs-1392890/v1
Purpose: This retrospective study aimed to analyze the relationship between the volume of the fractured and the normal orbit in patients with unilateral orbital fractures with and without traumatic optic neuropathy (TON).
Subjects: Data of 25 patients with unilateral orbital fractures who underwent computer tomography between January 2016 and December 2020 were investigated. Emergency imaging was performed within 2 h of arrival at the emergency room. The subjects were categorized into two groups: unilateral orbital fractures with and without TON.
Methods and Measures: The assessment of TON was performed during a comprehensive ophthalmologic examination by an ophthalmologist. The stereographic orbit was reconstructed and the volume was calculated. Other variables examined included age, sex, and cause of orbital trauma. The variables were compared using paired t-tests. Statistical significance was set at p < 0.05.
Results: The orbital volume of the non-fractured orbit was 27.50 ± 2.26 and 27.48 ± 2.64 cm3 in the groups with and without TON, respectively. The average volume of the fractured orbit in the TON group was 27.78 ± 2.56 cm3, and there was no significant volumetric difference between the fractured and non-fractured sides in this group. However, the average volume of the fractured orbit without TON was 28.76 ± 3.18 cm3, larger than that of the non-fractured orbit (p=0.016).
Conclusions: Non-expansion of the fractured orbit was a risk factor for indirect TON in patients with unilateral orbital fractures. Volumetric analysis from primary imaging would expedite the diagnosis and treatment of TON, resulting in optimal outcomes.
Traumatic optic neuropathy (TON) was described by Hippocrates more than 2,000 years ago.1 It is a rare condition causing catastrophic sequelae in visual function. The prevalence of TON is approximately 0.5 to 2%2 among cases of head injury, and is higher in cases of craniofacial injury.3 Previous studies have shown its prevalence to be positively associated to male sex and young age, the main causes being traffic accidents, falls, and body assaults.4,5 TON can be caused by direct injury of the eyeball, such as stab injury; however, the most common cause is indirect trauma or a non-penetrating wound causing acute injury of the optic nerve, resulting in impaired visual function. The exact pathology of indirect TON is not well understood. The probable etiologies of this traumatic visual condition include shock wave,6 direct impact force within milliseconds,7 coup contrecoup forces, rapid globe rotation,8 and direct or indirect pressure from edema disrupting the blood supply.9 Other mechanisms remain to be fully elucidated.
Generally, the diagnosis of TON is primarily based on clinical ophthalmic signs. In cranio-maxillofacial trauma, evidence of optic nerve dysfunction, including relative afferent pupillary defect, partial or total visual loss, color vision alteration, or visual field defects of variable degree suggest the diagnosis of TON.10 Because patients with brain or facial trauma are often confused or unconscious, the importance of the ophthalmic examination cannot be overemphasized. In these patients, imaging is frequently performed to clarify the extension of head or maxillofacial injury.11 Computed tomography (CT) is the most accessible method for detecting neurological and maxillofacial problems, including orbital conditions.12 Conventionally, this imaging technique can be used for the detection of a hematoma or bony fragments impinging on the optic nerve, which is useful in surgical management.6,10,11
In the era of patient-specific information, the investigation of orbital volume is becoming a fundamental requirement. For example, orbital volume is clearly one of the most helpful tools for estimating enophthalmos. Many studies have investigated the bony wall of orbit fractures followed by an increase in orbital volume.13,14,15,16 On the other hand, orbital wall or rim fractures are also clearly related to TON.4 Unfortunately, there is little evidence on the relationship between orbital volume and TON.
To address this relationship, this study aimed to analyze the orbital volume of the fractured and non-fractured sides in patients with unilateral orbital fractures. The analysis was performed in subjects with and without TON using a three-dimensional CT analysis program. These orbital fracture schemes were used to determine whether TON could be suspected by means of orbital volume measurements.
Data of patients with unilateral orbital trauma who underwent comprehensive ocular examination and CT in the Division of Ophthalmology, Sisaket General Hospital, from January 2016 to December 2020 were investigated retrospectively. CT imaging was performed within 2 h of the arrival of the patient at the emergency room. The exclusion criteria were: (1) direct or open globe injury, previous ophthalmic surgery, or facial fracture; (2) age < 18 years; (3) signs of congenital or pathologic condition in the eye, orbit, or facial bone (i.e., pre-existing blindness, glaucoma, cataract, tumor, hemifacial microsomia); and (4) insufficient imaging or ophthalmic examination data. Images were obtained using a single scanner (Siemens definition AS, Siemens, Germany) with the following imaging conditions: thickness 2 mm, scan time 7 s, tube potential 120 kVp, and tube current 90 mAs. Digital Imaging and Communications in Medicine (DICOM) files were analyzed using 3-D analysis software (Mimics Research 22.0, Materialize, Belgium). The specific orbital bone threshold value17 (-200–100 Hounsfield units) was calibrated. This study was approved by the Institutional Review Board of Sisaket General Hospital (approval no. 059/2564). The requirement for consent was waived, and the study complied with the tenets for human research protection established by the Declaration of Helsinki.
The subjects were categorized into two groups: unilateral orbital fracture with TON (OFwT) and without TON (OFwoT). TON was defined as an acute injury condition resulting in decreased visual acuity with a relative afferent pupillary defect. Open globe trauma, orbital compartment syndrome, and marked retrobulbar hemorrhage are absent.6 The diagnosis was recorded in the patient’s electronic medical records by an ophthalmologist. The two orbits of each subject were defined as the affected and normal sides.
Orbital volumes were measured using a semi-automatic method.12 Briefly, the orbital area of the selected group of slices was automatically generated using a flood fill tool, and then audited and refined manually using a digitizer (Intuos CTL6100, Wacom Co, Saitama Japan). The area of interest is shown in Fig. 1. The stereography of the orbit was reconstructed, and the volume (cm3) was calculated automatically (Fig. 2). Age, sex, and cause of orbital trauma were analyzed. A researcher (A.Y.), blinded to the subjects’ group assignment, analyzed the orbital volume twice with a 3-week interval. All statistical results except for the intraclass correlation coefficient (ICC) were analyzed using the average of two measurements.
The data were analyzed using the SPSS statistical analysis software (version 25.0; SPSS Inc., Chicago, IL, USA). The ICC was assessed to evaluate the agreement between the first and second measurements. The orbital volumes between sides were compared using a paired-t test, after verifying the normality of the data with a Kolmogorov-Smirnov test. For all analyses, p-values less than 0.05 were regarded as statistically significant.
The initial cohort consisted of 30 subjects. After applying the exclusion criteria, a total of 25 subjects (23 men and 2 women; mean age: 36.76 ± 15.09 years [range: 18–71]) were selected. Of these, 12 (11 male and 1 female) and 13 (12 male and 1 female) were classified in the OFwT and OFwoT groups, respectively. The most common cause of orbital fracture was motorcycle accidents, except one case of body assault in the OFwT group and one case in the OFwoT group caused by a fall. In the OFwT group, 6 patients were treated with observation and the others were administered steroids or erythropoietin. After a follow-up of at least 6 months post-trauma, however, only four patients with TON had improved visual function. Demographic data are shown in Table 1.
|
Unilateral Orbital Fracture |
All Data (n = 25) |
|||
|---|---|---|---|---|
|
With TON (n = 12) |
Without TON (n = 13) |
|||
|
Age (Mean±S.D, years) |
30.92 ± 11.58 |
42.15 ± 16.34 |
36.76 ± 15.09 |
|
|
Sex |
||||
|
Male (n) |
11 |
12 |
23 |
|
|
Female (n) |
1 |
1 |
2 |
|
|
Affected (Fracture) side |
||||
|
Right side (n) |
5 |
7 |
12 |
|
|
Left side (n) |
7 |
6 |
13 |
|
TON: Traumatic optic neuropathy
The orbital volume was not statistically different between the two measurements, with an ICC of 0.97. The average orbital volumes of the normal side were 27.50 ± 2.26 cm3 and 27.48 ± 2.64 cm3 in the OFwT and OFwoT groups, respectively. A Kolmogorov-Smirnov test used to analyze the normality of the orbital volume gave a p-value of 0.200. The average orbital volume of the affected side in the OFwT group was 27.78 ± 2.56 cm3. When the affected and normal orbits were compared, no significant difference was observed in the OFwT group (p = 0.165). However, the affected side was significantly larger than the normal side in the OFwoT group (p = 0.016). These comparisons are presented in Table 2.
|
Orbital Volume (cm3) |
|||
|---|---|---|---|
|
÷ |
Affected side (Fracture side) |
Normal side |
p - value† |
|
Unilateral orbital fracture without TON (n = 13) |
28.76 ± 3.18 |
27.48 ± 2.64 |
0.016 |
|
Unilateral orbital fracture with TON (n = 12) |
27.78 ± 2.56 |
27.50 ± 2.26 |
0.165 |
| TON: Traumatic optic neuropathy | |||
| † Paired t-test between the affected orbit and normal orbit | |||
We hypothesized that the relationship between affected and normal orbital volume in the OFwT group would be different from that in the OFwoT group. Our results supported this hypothesis, as the difference in volume between the affected and normal sides was not statistically different in the OFwT group, while, in subjects with OFwoT, the orbital volume of the affected side was significantly larger than that of the normal side. These data provided direct evidence from volumetric analysis of orbital fractures without expansion of orbital volume, which is considered a characteristic of indirect TON.
Although various putative factors have been identified, the etiology of indirect TON remains unclear. Diffuse axonal damage is thought to be the main mechanism involved.18 Based on cadaver studies, other factors include direct shearing injury to axons, disruption of the blood supply, and pressure from micro-hematomas and edema.9 As a demarcated, conical shaped matrix, which, apart from its anterior aspect, is bound on all sides by hard tissue, the orbit contains orbital fat, extraocular muscles, lacrimal apparatus, and neurovascular structures including the globe19. Thus, the orbit has limited compliance. Without significant orbital volume enlargement, which is regarded as a common sequela of orbital fracture,14–16 the fracture would result in an increase in intra-orbital pressure of the entire orbit, leading to orbital compartment syndromes such as retrobulbar hemorrhage. These conditions can be diagnosed by clinical signs of stony hard proptosis and elevated intra-orbital pressure, including imaging with mass effect, and especially massive hemorrhage.20 To the best of our knowledge, this is the first study to reveal the relationship between orbital volume and TON.
To date, our understanding of orbital volume has focused on enophthalmos correlated to orbital fracture with volume increase and destruction of at least one orbital wall or lateralization of the lateral wall.13–16 A previous prospective observational study21 showed that the mean orbital volume of a fractured orbit was significantly larger than that of a non-fractured orbit, while the authors did not find concomitant blindness in their subjects. In the present study, the subjects in the OFwoT group also showed consistent results. We suspected that the expansion of orbital volume after orbital fractures would be regarded as a vision protective mechanism. This assumption remains to be fully explained by well-designed studies in the near future.
To diagnose TON, prompt ophthalmic examination is necessary.6 Nevertheless, concomitant neurological deficits may delay such investigation.10 The orbital volume from primary CT imaging could be considered as an alternative tool for diagnosing TON and improve the sensitivity of such diagnosis. In the TONTT trial of traumatic optic neuropathy treatment,20 significantly worse visual outcomes were observed in patients with a longer time interval between trauma and treatment with steroids or erythropoietin. Accordingly, early detection of this catastrophic condition would be allowed by the specific feature of lack of volume expansion after orbital fracture, leading to prompt management and satisfactory treatment results.
However, there were some limitations to this retrospective study, specifically the small sample size and the uniformity of the orbital fractures examined in terms of mechanism, shockwave measurement, and rotational force of the globe.
In conclusion, the results of this study showed that the volume of the fractured orbit was a predictor of indirect TON in unilateral orbital fractures. Volumetric analysis from primary imaging would expedite TON diagnosis and treatment, resulting in optimal outcomes.
Financial Declaration: None
Conflict of interest: No conflicting relationship exists for any authors
Author contribution: W. Senarak: Conceptualization, Methodology, Software, Writing - Original Draft A. Yongvikul: Conceptualization, Methodology, Writing - Review & Editing, Visualization J.Y. Kim: Visualization, Investigation, Writing - Review & Editing J.K. Ku: Formal analysis, Visualization J.K. Huh: Conceptualization, Methodology, Resources Supervision, Project administration. All authors: final approval and agree to be accountable for all aspects of the work.