The first step towards a successful diagnosis is a thorough medical history, including recent traumatization to the head, recent surgical intervention, pregnancy, and labor, as well as comorbidities that may increase the risk for the development of a fistula, such as hypertension, collagen vascular disease, atherosclerosis, etc.

Patients usually initially consult an ophthalmologist, since ocular symptoms are the only predominant symptoms caused by a carotid cavernous sinus fistula. Individuals affected by the condition commonly present with a red eye, attributed to the edema of the conjunctiva due to the blockage of the blood flow and the high blood pressure [9]. They report ocular bruit, namely distinct vibrations caused by the turbulence in intracranial blood vessels; bruit is experienced by the patient as a buzz or a swishing sound. Exophthalmos is common, as well as vision impairment; diplopia or a recent deterioration in the ability to see may be reported by the patient. Lastly, such fistulae may lead to palsies of the cranial nerves and affected individuals frequently complain of pain on the face, which follows the path of the 1st division of the trigeminal nerve.

An examination will help to reveal further signs which are compatible with the existence of a carotid cavernous sinus fistula. Intraocular pressure is often augmented and the development of angle-closure glaucoma may be attributed to the elevated pressure in the veins of the episclera or to retinal ischemia, which leads to a secondary neovascularization [10]. Vitreous or intraretinal hemorrhagic phenomena may be observed, alongside edema of the optic disc and a dilatation of the retinal veins [11]. Ophthalmoplegia is also often exhibited, which may imitate the manifestations of Graves' ophthalmopathy [12]. Eye pulsations may be felt or seen with a bare eye and proptosis, accompanied by exposure keratopathy, are also common manifestations.

Both direct and indirect carotid cavernous sinus fistulae can be diagnosed via a computerized tomography scan (CT), a magnetic resonance imaging scan (MRI) and an orbital echogram. These procedures can outline the existence of a congested cavernous sinus and superior ophthalmic veins [13].

A cerebral arteriography can help to establish a definitive diagnosis since it can illustrate the course of the vessels and reveal the existence of a pathological channel of communication with great precision.

Patients diagnosed with carotid cavernous sinus fistulae receive symptomatic treatment, to relieve them of the ocular complications caused by the cavernous congestion and surgical treatment to repair the fistula. As far as supportive treatment is concerned, eye lubricants or a tarsorrhaphy may be necessary in order to treat exposure keratopathy. Patients presenting with glaucoma may require hyperosmotic medication or aqueous suppressants.

As far as surgical options are concerned, the aim is to achieve the closure of the fistula, either via embolization or via electrothrombosis or balloon occlusion [14] [15]. Some indirect fistula may resolve on their own; the rest require surgical intervention to prevent long-lasting and severe complications. Under some circumstances, fistulae may find an alternative route and reappear after surgery.

Prognosis depends on the type of fistula, the time that has passed from its formation until treatment and the symptoms a patient presents with.

Individuals that are diagnosed with dural, indirect fistulae generally present with milder symptomatology. Most of the ocular symptoms, such as eye pulsations, bruit, and thrill are resolved instantaneously following a surgery, whereas other symptoms like a conjunctival hematoma, disc swelling or glaucoma pay persist for up to half a year until they are resolved. Patients that are diagnosed with a high-pressure direct fistula may recover completely after surgical intervention, but some may be left with residual defects such as partial vision impairment or ophthalmoplegia.

Most cases of carotid cavernous sinus fistulae occur as a result of traumas sustained to the head, following various types of accidents or falls. Fistulae exhibit an increased frequency amongst patients who have sustained a fracture of the basal or facial skull. On the other hand, postmenopausal women usually present with fistulae that arise of their own accord, without any prior traumatization. Risk factors for the development of a spontaneous carotid cavernous sinus fistula are:

• Hypertension
• Pregnancy
• Atherosclerotic lesions
• Connective tissue disorders

Under some still unclear circumstances, the low-pressure fistulae that develop between branches of the carotid arteries and the cavernous sinus can resolve without intervention, possibly due to increased clotting [5] [6].

A carotid cavernous sinus fistula is an uncommon occurrence. The existence of high-pressure, direct fistulae can damage vision severely if it is left undiagnosed or untreated because it can lead to vision impairment or loss, as well as glaucoma. The fistula may also lead to intracerebral hemorrhage, hemorrhage in the subarachnoid space and increased mortality/ morbidity in some cases.

Low pressure, spontaneous fistulae are usually diagnosed amongst female patients over 50 years of age, whereas direct canals that are a result of trauma are observed amongst younger, more active individuals. In general, the condition seems to exhibit a predilection for women in their middle age.

Carotid cavernous sinus fistulae are abnormal connections between the internal or external carotid artery or its branches and the cavernous sinus, which belongs to the venous system [7]. As a result, high-pressure arterial blood is directed into the cavernous sinus, leading to a congested sinus and ocular symptoms, due to the direct connection of the cavernous sinus to the superior ophthalmic vein.

Generally, direct fistulae comprise up to 9 out of 10 fistulae that are diagnosed. The pathological connection is between the carotid artery itself and the cavernous sinus. A great amount of arterial blood is exchanged via this fistula. They are commonly induced by a severe traumatization that damaged the walls of the carotid artery. On the other hand, dural fistulae are those that are formed between the cavernous sinus and meningeal branches of the carotid artery. They do not develop a traumatic background but arise spontaneously.

In a nutshell, carotid cavernous sinus fistulae can be categorized broadly into two categories: direct fistulae, between the part of the internal carotid artery that lies within the cavernous sinus, and the indirect, that develop between branches of the internal or external carotid artery and the cavernous sinus. More specifically:

• Type A fistula represents a direct connection between the cavernous sinus and the segment of the internal carotid artery that is found within the sinus.
• Type B is an indirect, dural channel between branches of the internal carotid artery that lie within the sinus and the sinus itself.
• Type C is also an indirect shunt between meningeal branches of the external carotid artery and the cavernous sinus
• Type D describes type B + C.

Fistulae belonging to the C group are the most common [8].

There are not specific suggestions that could prevent the development of such a fistula. After a traumatic injury to the head, all patients, regardless of age, should undergo extensive examinations and evaluation, especially if a fracture of the basal or facial skull is suspected. The only way to prevent permanent damage and severe complications is to diagnose the condition in time and treat it appropriately.

The cavernous sinus is a venous sinus located in the skull, specifically on the sides of the pituitary fossa. Under various circumstances, channels between the cavernous sinus and the external or internal carotid artery can form, which lead to the redirection of vascular blood flow. The augmented carotid arterial pressure leads to blood entering the cavernous sinus through these abnormal channels (fistulae).

The cavernous sinus is a venous structure and differs from its arterial counterparts, as far as the amount of blood it can receive is concerned and also with regard to the pressure it can maintain. The entrance of high-pressure blood from the arteries leads to a blockage of the flow or a reversed flow within the walls of the cavernous sinus. Given the fact that the structure is connected to the superior ophthalmic veins, and either drains to or from them, this change of flow balance leads to an excessive amount of blood being redirected into the ophthalmic veins; as a result, ocular symptoms emerge, such as diplopia, orbital pain, blindness, ophthalmoplegia and other symptoms.

A carotid cavernous sinus fistula (CCF) can either arise after a specific incident, such as a head traumatization which leads to damage to the internal carotid artery or in a spontaneous way [1]. Spontaneous fistulae do not usually form between the carotid artery itself and the cavernous sinus, but rather between the sinus and smaller arterial branches. These fistulae contain blood that flows with lower pressures, in comparison to fistulae between the principal artery and the sinus, which are high-pressure canals. Tortuous conjunctival vessels along with retinal hemorrhages can also present with CCF [2].

It seems that the majority of the CCF cases that lead to the appearance of ocular symptoms are reported from industrialized countries of the West [3] [4].

A fistula is an abnormal channel that allows for the communication between two structures that normally do not communicate. A carotid cavernous sinus fistula is such a channel, that is formed between the carotid artery, one of the major arteries of the body, and the cavernous sinus, a venous sinus that lies within the head. It may be a result of trauma to the head, or it may develop spontaneously, particularly in middle-aged women.

The fistula allows high-pressure blood from the artery to enter the cavernous sinus and thus, the latter is congested. This congestion leads to eye-related symptoms because the cavernous sinus is directly connected to the superior ophthalmic veins. Patients with such fistulae often present with a red, bulging and pulsating eye, report buzzing sounds and have trouble to move their eyes in all normal directions. They may also complain that their vision has deteriorated and the ophthalmologist may diagnose increased pressure within the eye.

Treatment comprises measures to alleviate the symptoms and surgical treatment in order to close the fistula and stop the abnormal communications between the carotid artery and the cavernous sinus.

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