The superior ophthalmic vein (SOV) is the largest vein in the orbit. It begins at the anteromedial part of the orbit from the confluence of the supraorbital vein, supratrochlear vein, and angular vein, and runs posteriorly to drain into the cavernous sinus. The average diameter of a normal SOV is about 2 mm, and dilation is defined as 3 mm or more.
SOV dilation is an imaging finding that may suggest a condition threatening vision or life. Although SOV dilation is detected on CT or MRI, it does not by itself confirm a specific diagnosis. It is essential to search for the underlying cause.
The most common causes of SOV dilation are carotid-cavernous fistula (CCF), arteriovenous malformation (AVM), and SOV thrombosis. Other possible causes include orbital cellulitis, thyroid eye disease, orbital tumors, and increased intracranial pressure.
The estimated annual incidence of SOV thrombosis (SOVT) is extremely rare, about 3–4 cases per million people4), but it is a serious condition that can progress to cavernous sinus thrombosis and carries a risk of blindness.
The orbital venous system has no valves, and its venous plexus connecting the face, nasopharynx, and cavernous sinus allows inflammation or thrombosis to spread bidirectionally.
SOV dilation itself is an imaging finding associated with various diseases, and the frequency of it as an independent disease is not clear. The most common cause, SOV thrombosis, has an incidence of approximately 3–4 cases per million per year, making it extremely rare 4). However, the frequency of causative diseases such as carotid-cavernous fistula should also be considered.
Various symptoms appear depending on the underlying disease causing SOV dilation.
SOV thrombosis is characterized by acute orbital pain, pain with eye movement, and a “blurry” swelling of the eyelid1). Some cases of SOVT occur without conjunctival hyperemia, and limitation of eye movement is an important indicator of progression1).
Findings vary depending on the underlying disease, but the following may be commonly observed.
Carotid-cavernous fistula is known for the triad of pulsatile proptosis, bruit, and conjunctival chemosis (“Medusa head”). Diplopia due to oculomotor (oculomotor, abducens, trochlear) nerve palsy, retinal hemorrhage, and tortuosity and dilation of retinal veins are also observed. However, in dural carotid-cavernous fistula, there may be cases with poor reflux into the superior ophthalmic vein, and characteristic ocular symptoms such as chemosis may be mild, leading to delayed diagnosis, so caution is needed.
In orbital varices, intermittent proptosis with head position changes (bending forward, Valsalva maneuver, coughing, etc.) is a characteristic finding. It may also be accompanied by pupillary dilation, retinal vascular dilation, and elevated intraocular pressure.
The etiology of SOV dilation is diverse. The main categories are shown below.
| Etiological classification | Representative diseases |
|---|---|
| Vascular malformations | Carotid-cavernous fistula, orbital/facial AVM |
| Thrombosis | SOV thrombosis, cavernous sinus thrombosis |
| Infectious | Orbital cellulitis, sinusitis |
| Inflammatory | Thyroid eye disease, idiopathic orbital inflammation |
| Neoplastic | Orbital lymphoma, parasellar meningioma |
Carotid-cavernous fistula is the most common cause of SOV dilation. Direct CCF involves direct flow from the internal carotid artery into the cavernous sinus, often secondary to trauma. Indirect CCF (dural CCF) is a shunt via dural branches, triggered by congenital AVM, hypertension, or diabetes. Direct CCF has high shunt volume and severe symptoms, while indirect CCF has low shunt volume and often a chronic course.
SOVT is divided into infectious and non-infectious types. Infectious causes include orbital cellulitis, sinusitis, and dental infections 1)9). Non-infectious causes include hypercoagulable states, autoimmune diseases, malignancies 2), and thyroid eye disease 5). Recently, SOVT due to systemic hypercoagulability associated with COVID-19 infection has been reported 7). A case of simultaneous immune thrombocytopenia and SOVT after ChAdOx1 nCoV-19 vaccination has also been reported 8).
Orbital varices account for 90% of causes of intermittent proptosis. They are classified into congenital (due to venous wall fragility) and acquired (associated with intracranial AVM) types. They may be accompanied by varices in the lower extremities.
Carotid-cavernous fistula is a condition in which an abnormal communication (fistula) forms from the internal or external carotid artery to the cavernous sinus. High-pressure arterial blood flows into the low-pressure cavernous sinus, causing reflux and dilation of the SOV, leading to symptoms such as pulsatile proptosis, conjunctival injection and edema, and vascular bruit. For details, see the “Causes and Risk Factors” section.
Imaging tests play a central role in diagnosing SOV dilation and identifying its cause.
The characteristics of the main examination methods are shown below.
| Examination Method | Main Role |
|---|---|
| MRI/MRA/MRV | High detection rate for SOV; first choice |
| Contrast-enhanced CT | Excellent bone detail; rapid imaging |
| Catheter angiography | Definitive diagnosis of vascular etiology |
Coronal T2-weighted imaging is optimal for understanding the three-dimensional structure of the SOV and shows a high detection rate. In carotid-cavernous fistulas, blood flow velocity in the cavernous sinus increases, so it appears as a signal void on MRI. MRA can depict abnormal vascular images of the cavernous sinus in addition to SOV dilation. MRI is also useful for follow-up of indirect carotid-cavernous fistulas and evaluation of thrombosis after endovascular surgery.
Contrast-enhanced CT can detect SOV dilation and filling defects, and is also suitable for evaluating infectious causes such as sinusitis. It provides a large amount of bone information and has a short scanning time, making it useful in emergency cases and children. In orbital varices, CT imaging immediately after the Valsalva maneuver may detect masses that are difficult to visualize at rest. If phleboliths are seen as spherical calcifications, this supports the diagnosis of varices.
This is the gold standard for definitive diagnosis of vascular causes. It can not only confirm the diagnosis of carotid-cavernous fistulas but also serve as treatment such as endovascular embolization. In dural carotid-cavernous fistulas, both the internal and external carotid arteries can be responsible, so it is necessary to perform angiography of all four vessels: bilateral internal and external carotid arteries.
B-mode ultrasound can detect the mass shadow of the SOV. It is used adjunctively for evaluation after changes in body position or the Valsalva maneuver.
Diseases presenting with SOV dilation are diverse and require differentiation from preseptal cellulitis, orbital cellulitis, cavernous sinus thrombosis, thyroid eye disease, idiopathic orbital inflammation, and orbital tumors 1). SOVT without conjunctival congestion may be misdiagnosed as preseptal cellulitis, and the appearance of restricted eye movement is a key to differentiation 1).
Treatment of SOV dilation varies depending on the underlying disease.
Carotid-Cavernous Fistula / AVM
Endovascular embolization: Transvenous or transarterial approach to occlude the fistula is the mainstay.
Observation: For dural carotid-cavernous fistulas with low shunt volume and minimal ocular symptoms, spontaneous closure may be awaited. The spontaneous closure rate is reported to be slightly less than 50%.
Symptomatic treatment: Glaucoma eye drops, manual carotid compression.
SOV Thrombosis
Anticoagulation therapy: Initiate heparin or LMWH, then switch to oral medication. This is the central treatment for both infectious and non-infectious cases.
Antibiotics: For infectious cases, broad-spectrum antibiotics are administered intravenously.
Surgical drainage: Indicated in cases with abscess formation.
Orbital Varix
Observation: If vision is good and there is no eye movement disorder, observe. Instruct the patient to avoid positions that enlarge the varix.
Surgery/Embolization: Perform excision or embolization when vision impairment or eye movement disorder worsens.
Treatment of carotid-cavernous fistula/AVM is determined in collaboration with neurosurgery, based on comprehensive ophthalmological and neurosurgical findings. Ophthalmic treatment mainly involves administering pressure-lowering drugs for high intraocular pressure. Direct carotid-cavernous fistulas rarely close spontaneously after 3 weeks, and if left untreated, rupture of the cavernous sinus, cerebral hemorrhage, subarachnoid hemorrhage, etc., can lead to poor prognosis.
Systemic steroid therapy is effective for SOV expansion associated with idiopathic orbital inflammation. Anticoagulation therapy is added as needed.
Cavernous sinus thrombosis is often infectious; high-dose intravenous antibiotics are administered, and collaboration with otolaryngologists and neurosurgeons is required.
Anticoagulation therapy is considered standard treatment for non-infectious SOVT, but its exact role is debated. In infectious SOVT, antibiotics are prioritized first, and anticoagulation is used concurrently to prevent thrombus progression. If abscess drainage is possible, anticoagulation may need to be discontinued 1). For details, see the “Standard Treatment” section.
SOV dilation occurs through the following three mechanisms:
When a fistula forms between the high-pressure internal carotid artery and the low-pressure cavernous sinus, the pressure in the SOV flowing into the sinus increases. This results in pulsatile proptosis and conjunctival chemosis. Direct carotid-cavernous fistulas have a large shunt volume and severe symptoms. Dural carotid-cavernous fistulas have a relatively small shunt volume and often follow a chronic course.
Since the oculomotor, trochlear, abducens, and trigeminal nerves run along the wall of the cavernous sinus, increased sinus pressure can cause ocular motor nerve palsy and trigeminal nerve dysfunction. Elevated episcleral venous pressure increases resistance to aqueous outflow, leading to open-angle secondary glaucoma.
In SOVT, local inflammation or a hypercoagulable state causes venous stasis, leading to thrombus formation. Due to the structural characteristics of the orbital venous system, which lacks venous valves, the thrombus can extend toward the cavernous sinus.
In COVID-19, systemic inflammatory response, endothelial injury, and hypercoagulability promote thrombus formation 7). Platelet activation, enhanced coagulation cascade, and suppression of fibrinolysis are involved as immunothrombosis.
It has also been reported that a dilated and tortuous SOV can physically compress the optic nerve, causing visual field defects without an increase in venous pressure 3). In such cases, intraocular pressure may be normal and conjunctival injection may be absent, making diagnosis difficult.
Sigurdsson et al. (2024) reported treatment of 3 cases of non-infectious SOVT associated with carotid-cavernous fistula with methylprednisolone 500 mg/day for 3 days intravenously followed by oral steroids 4). All cases showed poor improvement with anticoagulation alone but marked improvement after adding glucocorticoids. Two cases recovered to 20/20 vision and one to 20/25. Among 15 reported cases of glucocorticoid use for SOVT in the literature, 10 showed good visual recovery, but individual judgment was deemed necessary considering the risk of glucocorticoid-induced thrombotic promotion.
Khurram et al. (2021) reported the first case of incidental SOVT in a 61-year-old man with COVID-19 pneumonia and saddle pulmonary embolism 7). SOVT and pulmonary embolism completely resolved 3 weeks after starting LMWH. It was suggested that COVID-19-related systemic hypercoagulability and immunothrombosis contribute to SOVT development.
Bayas et al. (2021) reported a 55-year-old woman who developed bilateral SOVT, immune thrombocytopenia, and ischemic stroke 10 days after the first dose of the ChAdOx1 nCoV-19 vaccine 8). Platelet count recovered with dexamethasone 40 mg/day for 4 days, but transient right hemiparesis and aphasia occurred on hospital day 8. Attention was drawn to the possibility of thromboembolism after vaccination.
Hirano et al. (2023) reported a 70-year-old man with visual field defects only, caused by optic nerve compression from a dilated and tortuous SOV due to an orbital arteriovenous fistula 3). This was an atypical case with normal intraocular pressure and no conjunctival injection. Transvenous coil embolization via the facial vein reduced the SOV, and visual acuity improved from 0.3 to 0.9 on postoperative day 7.
Yamada et al. (2025) reported a 72-year-old woman with spontaneous thrombosis of the SOV while awaiting treatment for a cavernous sinus dural arteriovenous fistula, leading to venous congestion of the cerebellum and brainstem due to rerouting of shunt flow to the posterior cranial fossa 6). Since endovascular access was lost, microscopic surgical occlusion of the petrosal vein via retrosigmoid craniotomy was performed, resulting in complete neurological recovery and fistula obliteration.
COVID-19 infection can cause a systemic hypercoagulable state and may lead to SOVT 7). Additionally, SOVT with immune thrombocytopenia has been reported after some COVID-19 vaccinations 8). Although both are rare complications, SOVT should be considered in the differential diagnosis when ocular symptoms appear in COVID-19 patients.
