Emissary veins are small valveless veins passing through skull foramina to connect the intracranial dural sinuses with the extracranial scalp veins. The major emissary veins are: mastoid emissary (through the mastoid foramen, connecting sigmoid sinus to posterior auricular or occipital veins), parietal emissary (through parietal foramina, connecting superior sagittal sinus to scalp veins), occipital emissary (connecting confluence to occipital vein), and condylar emissary (through posterior condylar canal).
The valveless nature of emissary veins allows bidirectional blood flow, creating pathways for spread of scalp and neck infection into the intracranial dural sinuses (otogenic meningitis via mastoid emissary) and for haematogenous tumour metastasis from the skull to intracranial structures. They are also the drainage pathway for intracranial venous hypertension when the major sinuses are thrombosed. The mastoid emissary vein is identified and occluded with bone wax during mastoid surgery.
Mastoiditis can spread intracranially through the mastoid emissary vein connecting the sigmoid sinus to the posterior auricular vein without direct bony erosion, producing sigmoid sinus thrombophlebitis and retrograde spread to produce otogenic meningitis; mastoidectomy with sigmoid sinus decompression is required alongside systemic antibiotics.
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