Posterior fossa emissary veins

A 35-year-old male patient was referred for bilateral chronic otitis and mixed type hearing loss. The patients had two tympanoplasty surgeries for cholesteatoma. The patient was evaluated with thin-section spiral computed tomography (CT) of the temporal bone and temporal magnetic resonance imaging (MRI), and diffusion-weighted imaging (DWI).

Temporal bone CT showed bilateral bony grooves, laterally on the superior surface of the petrous bones, extending into the posterosuperior portion of temporomandibular joints (Figure 1). The diameter of left petrosquamosal emissary vein was 4.5 mm, whileas the right has diameter of 2.5 mm. The patient also had bilateral mastoid emissary veins which cross the mastoid part of temporal bone, extending from occipital vein to sigmoid sinus (Figure 2).

CT also demonstrated near-total opacification of bilateral middle ear cavities; ill-defined left incus and eroded scutum, compatible with earlier left-sided cholesteatoma. These findings explained conductive hearing loss.

The patient had also pial and ependymal low signal intensity and cerebellar atrophy, consistent with superficial siderosis, causing the loss of sensorineural hearing.

Figure 3 demonstrates incidentally detected left condylar emissary vein, in another patient.

Emissary veins are embryonic remnants usually regressing during fetal life. They originate from the sigmoid sinus and communicate with extracranial veins.  They reported being more frequent on the left side (18% over 5). They are generally very thin (<1 mm in diameter), although emissary veins with up to 4 mm diameter could be present. The sigmoid sinuses and transverse sinuses could be smaller in the presence of large emissary veins.

The emissary veins have a relatively small role in healthy people and could act as a safety valve, as an exit for cerebral blood flow in patients with skull base and neck lesions and patients with bilateral internal jugular vein obstruction to prevent dangerous increases in intracranial pressure. 

There are three emissary venous pathways that have been described in humans encompassing the upper, middle, and lower thirds of the sigmoid sinus. Three emissary venous pathways are:

• The lower or posterior condylar vein exits the sigmoid sinus above the jugular bulb and usually is of moderate size.
• The middle or mastoid emissary vein crosses the mastoid foramen and unites with the sigmoid sinus via the posterior auricular or occipital vein
• The upper petrosquamosal emissary vein is generally very small and originates at the junction of the transverse and sigmoid sinuses.

It has been postulated that petrosquamosal emissary vein has two drainage pathways into the basilar and cavernous sinuses: one inferiorly into the retromandibular vein, then in external jugular vein, as in our case and one medially into the pterygoid venous plexus.

The petrosquamosal sinus may be the main outflow pathway of the transverse sinus, and particular care is required in these people during surgical procedures due to the risk of venous ischemic and hemorrhagic consequences with the sacrification of this outflow pathway. It is illustrated in picture 4.

Superficial siderosis is an uncommon condition causing neurological dysfunction, which results from the slow haemorrhages into the leptomeninges leading to deposition of hemosiderin. Its incidence could increase with the more widespread use of brain MRI, and as in our case, incidental cases could be detected. Typical symptoms include sensorineural hearing loss and cerebellar dysfunction (ataxia).