1

Introduction

Despite of the better understanding of anatomy, certain structures of the temporal bone remain ill defined. The petrosquamosal sinus (PSS) is a rare and almost unknown emissary vein of the temporal bone . It traverses the cranium and connects the intracranial venous sinuses and the extracranial veins. These veins are valveless, so, blood can flow bidirectionally, thereby allowing cooler blood from the evaporating surfaces of the head to cool the brain. Although anatomic and radiologic descriptions of the PSS have been previously presented , few reports in the recent literature provide detailed descriptions of the PSS. The vein may be opened to bleed, it can cause difficulty in proper eradication of the disease especially in hands of inexperienced surgeons, and it can also be the source of infection or thrombosis. Preoperative imaging can help to show the site and course of the vein. Because large, persistent PSS presents a patient risk for mastoidectomy, otologists should be aware of it in advance.

A 59-year-old woman with a two-year history of intermittent right otorrhea and ipsilateral hearing loss was referred to our department. The patient had no other complaints, such as tinnitus, otalgia, vertigo, or facial weakness. The otoscopic examination showed the right attic destruction. High resolution computed tomography of the temporal bone (TB CT) showed soft tissue density in the right middle ear cavity and sclerotic change in the right mastoid region. There also was lateralization of sigmoid sinus with bony dehiscence of lateral cortex. The tegmen tympani and head of malleus were also erosed. TB CT also showed an abnormal bony canal on the right mastoid ( Figs. 1 & 2 ). The course of the canal was anteroinferior, extending from the superior part of the sigmoid sinus to the posterior part of the temporomandibular joint. These findings were consistent with bilateral PSS. There were no additional anomalies of the middle ear, facial nerve, or skull base. According to those findings, she was diagnosed with chronic otitis media with cholesteatoma, and surgery was scheduled. The surgical procedures were performed gently, taking care not to injure the vessel by retrograde mastoidectomy (atticoantrostomy). Bypassing Preserving the PSS, mastoid air cells were eliminated and tympanoplasty with scutumplasty was also completed.

Axial high-resolution computed tomography with bone algorithm of the right temporal bone (TB CT). Axial view shows soft tissue density in the right middle ear cavity and sclerotic change in the right mastoid region. There is lateralization of sigmoid sinus (arrowhead) with bony dehiscence of lateral cortex. Axial view also shows an abnormal bony canal (arrow) in the right mastoid. The course of the canal was anteroinferior, extending from the superior part of the sigmoid sinus to the posterior part of the temporomandibular joint. The canal exits the temporal bone through the posterior glenoid foramen (B, arrow). These findings are consistent with the petrosquamosal sinus. There were no additional anomalies of the middle ear, facial nerve, or skull base.

Coronal high-resolution computed tomography with bone algorithm of the left temporal bone. Axial view shows soft tissue density in the right middle ear cavity and sclerotic change in the right mastoid region. There is lateralization of sigmoid sinus (arrowhead) with bony dehiscence of lateral cortex. Coronal view also shows the petrosquamosal sinus (arrow) is arising from the superior part of the sigmoid sinus. The PSS starts from the rostral portion of the transverse sinus. It passes tortuously through superior portion of mastoid bone.

2

Discussion

In humans, most of the cerebral venous drainage reaches the posterior fossa before being directed primarily towards the internal jugular veins (IJV) or the vertebral venous system (VVS). The external jugular veins (EJV) are primarily involved in venous drainage of the viscerocranium and neurocranium, with a variable, but generally limited, participation in the cerebral venous drainage itself. There are two possible pathways connecting the cerebral drainage to the EJV system. The most common pathway involves drainage of the superficial and deep middle cerebral veins into the pterygoid plexus by way of the cavernous sinus and/or the emissary veins of the middle cranial fossa. The second pathway involves a connection between the rostral portion of the transverse sinus and the veins of the temporal fossa through a PSS . This pathway normally regresses during fetal and early postnatal life and may be absent in the human adult. The PSS initiates at the unification point between the transverse and sigmoid sinuses, coursing horizontally above the petrosquamosal suture of the temporal bone, which is located at the union of the petrous and squamous bones and plays a major role in the variations of mastoidectomy. One report has described a remaining vascular tract in the midportion of the petrosquamosal suture, arising from an ascending branch of the occipital artery and showing possible cessation of the embryologic development causing the persistent PSS . The PSS ends via the foramen retroarticulare, and through this the blood flowing in the PSS is directed into the retromandibular vein and then into the external jugular vein. The vessels coursed anteroinferiorly over the superior portion of the temporal bone and terminated near the posterior part of the temporomandibular joint. Although this drainage pathway is prominent in most mammals where it constitutes the major route of cerebral venous drainage, it is rarely of functional significance in humans, in which the IJV and VVS represent the major outflow pathways. PSS most likely plays a minor role in encephalic drainage because it is not connected to the vertebral venous plexus.

In contrast to the anatomical studies, radiological identification of PSS is quite uncommon. The first radiological description of PSS was given by Marsot-Dupuch et al. in 2001 . The incidence of a persistent PSS as described in the literature is between 1% in routine CT examinations, as noted by Koesling et al. , and up to 23% in corrosion casts, as noted by San Millán Ruiz et al. . The reason for this discrepancy lies in the small calibre of PSS. It is amenable for identification by imaging techniques only when it persists as a large channel or is contained in a bony canal within the petrosquamous suture . The number of PSSs in association with other inner ear malformations in the literature is higher than that in the study of routine CT scans by Koesling et al. .

The importance of PSS is increasingly being appreciated in recent times. Besides being a rare cause of tinnitus, PSS may sometimes become a cause of potential uncontrolled bleeding during middle ear surgery or during translabyrinthine approach to posterior fossa surgery. PSS may be opened at the exit in making incision or when striping up the periosteum in various operative procedures or in its course in removing bone for extensive infections. As a rule bleeding is trivial but in case of large ones it may be troublesome. The sacrifice of the PSS would lead to fatal venous ischemic and hemorrhagic consequences, especially when the PSS represents the major or only drainage route of the transverse sinus. Possibility of retrograde spread of infection or tumour of external auditory canal through PSS has also been mentioned . Recognition of these anomalous venous channels is therefore of paramount importance to the otologist.