High index of suspicion: diagnosing a carotid-cavernous fistula

Abstract

A woman in her 70s presented with acute bilateral retro-orbital headache, diplopia, chemosis and eye swelling. Ophthalmology and neurology were consulted after detailed physical examination and diagnostic workup including laboratory analysis, imaging and lumbar puncture. The patient was diagnosed with non-specific orbital inflammation and was started on methylprednisolone and dorzolamide–timolol for intraocular hypertension. The patient’s condition improved slightly, but a week later, she developed subconjunctival haemorrhage in the right eye, which prompted investigation for a low-flow carotid-cavernous fistula. Digital subtraction angiography showed bilateral indirect carotid-cavernous fistula (Barrow type D). The patient underwent bilateral carotid-cavernous fistula embolisation. Her swelling improved considerably on day 1 after the procedure and her diplopia improved over the following weeks.

Background

Acute painful red eye is a medical emergency in most situations. The differential is wide, and outcomes range from mild discomfort to definitive loss of vision. Aetiologies include but are not limited to acute angle-closure glaucoma, inflammation, vascular disease, infection and trauma. Prompt evaluation by an ophthalmologist for detailed ocular examination is mandatory especially if warning signs of vision loss are present.^{1 2} In some cases, corrective measures must be taken immediately with the goal of preserving vision when possible. From the internist’s perspective, it is important to recognise emergent situations, to perform the initial workup and to identify patients who need urgent evaluation at a centre dedicated to ophthalmological care.

Carotid-cavernous fistulas are an unusual cause of acute red eye, defined as an abnormal connection between the carotid artery and/or its branches and the cavernous sinus. The presentation is usually non-specific, but patients commonly have ophthalmological manifestations. Signs and symptoms depend on the type of fistula but both direct and indirect fistulas manifest with eyelid swelling, conjunctival hyperaemia, ptosis and proptosis.^{3 4}

Case presentation

In this case, we describe a woman in her 70s with a history of coronary artery disease requiring previous percutaneous coronary intervention, hypertension and hyperlipidaemia who presented with 2 weeks of intractable bilateral retro-orbital headache, diplopia, non-pulsatile proptosis and eye swelling. The patient reported sustaining a fall with head trauma several months prior to her presentation at which time she experienced a similar self-limited headache. Her diplopia was binocular and worse with distant vision. She had bilateral abducens nerve palsy with chemosis and increased intraocular pressure (IOP) (figure 1). She was initially seen at an outside hospital where she was diagnosed with bilateral orbital cellulitis for which she received antibiotics. Neurology and ophthalmology were consulted when she arrived at our facility and antibiotics were held.

Figure 1.

Eyes on presentation.

Investigations

The IOP was measured in the ophthalmology clinic by applanation tonometry. It was noted to be elevated at 32–35 on the right eye and 34–39 on the left eye. IOP pulsation was not observed. CT of the facial bones and orbits with contrast (figure 2) and MRI of the orbits showed bilateral proptosis, bilateral perineuritis, retro-orbital inflammation with prepontine dural enhancement, mild enlargement of the extraocular muscles and enlargement of bilateral superior ophthalmic veins without thrombosis. The physical examination and the imaging findings were inconsistent with orbital cellulitis and the patient was started on methylprednisolone for suspected non-specific orbital inflammation, dorzolamide–timolol for intraocular hypertension and artificial tears for chemosis. Her headaches were initially persistent and uncontrolled with non-opioid analgesics, but their intensity lessened with intermittent doses of morphine and improved after starting corticosteroids. IOP quickly improved after 2 days of dorzolamide–timolol, and no further evaluation such as gonioscopy was performed.

Figure 2.

CT of the head.

Due to findings of non-specific orbital inflammation and broad differential, an extensive workup was done including immunological markers, lumbar puncture for autoimmune and malignant indicators, and CT scan (with contrast) of the chest, abdomen and pelvis to evaluate for a possible primary malignancy. The cerebrospinal fluid was unremarkable except for high protein despite being on steroids. Meningitis and encephalitis panels were negative, and oligoclonal bands were absent.

After the patient was started on corticosteroids, her condition improved slightly, but days later, she developed a subconjunctival haemorrhage. Due to this new finding, as well as corkscrew vessels on slit-lamp examination, intractable headaches, previous MRI findings described above and continued conjunctival injection and hyperaemia with extraocular muscle deficits, the ophthalmologist suspected the presence of an indirect low-flow carotid-cavernous fistula. The patient had digital subtraction angiography that confirmed the diagnosis and showed bilateral indirect carotid-cavernous fistula (Barrow type D) fed predominantly by the bilateral meningeal arteries but also by meningeal branches from the bilateral internal carotid arteries.

Differential diagnosis

The differential in this case is quite broad, from inflammatory causes including Tolosa-Hunt syndrome, autoimmune syndromes, neurosarcoidosis and other granulomatous diseases, malignant causes including ocular lymphoma, infectious causes including syphilis and tuberculosis, thyroid orbitopathy, and vascular causes including cavernous sinus thrombosis and carotid-cavernous fistula.

Although bilateral superior ophthalmic vein engorgement is commonly seen in carotid-cavernous fistulas, it has also been reported in other pathologies like cavernous sinus thrombosis, orbital pseudotumour and Graves’ disease presenting as thyroid orbitopathy,^{5 6} and the diagnosis was not readily made to avoid anchoring on a specific cause. The few studies that have evaluated the statistical integrity of this finding in the setting of carotid-cavernous fistula show good sensitivity but poor specificity even when limited to trauma-induced causes (type A), where the diagnosis is more readily made based on clinical history.⁷ The overall specificity is likely worse than even represented in those studies. Due to the uncertainty of the diagnosis, steroids were immediately started to avoid permanent vision loss complicating autoimmune diseases, and lumbar puncture was done to rule out other potential aetiologies needing immediate treatment.

The diagnosis of low-flow carotid-cavernous fistula was eventually confirmed after extensive workup and ophthalmology follow-up.

Treatment

After the diagnosis of indirect type D carotid-cavernous fistula was made, the patient was treated with bilateral carotid-cavernous fistula embolisation (figure 3).

Figure 3.

Postoperative day 1 after embolisation.

Outcome and follow-up

On the first day after embolisation, the patient’s swelling had improved dramatically, and 8 weeks later, when she followed up in the neurology clinic, her diplopia had resolved except with right-ward gaze.

Discussion

Carotid-cavernous fistulas are classified into four main types (Barrow types A–D), which can be divided into direct (type A) and indirect (types B, C and D). Direct fistulas most often occur due to previous instrumentation, trauma or rupture of an internal carotid artery aneurysm. Indirect fistulas are more commonly due to chronic conditions including hypertension, fibromuscular dysplasia or conditions that can cause carotid artery dissection. Type D fistulas are the most common fistula presenting spontaneously.⁴

Carotid-cavernous fistulas present with different signs and symptoms depending on their location, flow, size and type. Both direct and indirect fistulas can manifest with eyelid swelling, conjunctival hyperaemia, ptosis and proptosis. Ophthalmic manifestations are seen mostly with anteriorly draining fistulas and are explained by the stagnation of blood in the orbit.⁴ Carotid-cavernous fistulas abnormally connect the carotid artery to the cavernous sinus. The orbit venous system drains in the cavernous sinus predominantly through the superior ophthalmic vein. As such, this connection impairs the venous drainage from the orbit due to overflow of blood in the cavernous sinus and there is a resultant arterialisation and dilation of conjunctival and episcleral veins. This is reflected by chemosis and proptosis with the corkscrew appearance of the vessels on physical examination. Moreover, blood backflows into the venules that drain the extraocular muscles and causes engorgement limiting extraocular movement. This engorgement can manifest as non-specific enlargement of the extraocular muscle on the MRI with clinical signs of diplopia and bilateral abducens nerve palsy. The presence of proptosis and diplopia can mimic other diseases and can be a falsely localising sign making the diagnosis of carotid-cavernous fistula harder to make, and can cause confusion when other causes of extraocular muscle enlargement need to be explored.^{8 9}

Treatment of carotid-cavernous fistulas varies depending on the type. Some providers advocate for a conservative approach using non-surgical management. If symptoms do not improve or are intolerable, catheter-directed therapy is considered either via transvenous or transarterial approach with coil embolisation, stenting or liquid embolic agents. In most cases, treatment is successful and improvement in symptoms is reported even in some patients who do not demonstrate angiographic cure.^{10 11}

Learning points.

• This case is a reminder that carotid-cavernous fistulas can mimic any eye disease and can easily be missed even in an expert centre despite exhaustive imaging studies.

• Carotid-cavernous fistula should always be in the differential when patients present with ophthalmological symptoms that are not otherwise explained and there should be a high level of suspicion in those cases.

• It is the role of the internist to notice warning signs in any red eye presentation and consult ophthalmology in a timely manner. Diagnosing and treating symptoms early improve outcomes and chances of recovery.

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