Syndromes of the orbital fissure, cavernous sinus, cerebello- pontine angle, and skull base.

Correspondence to: Professor Ian Bone, Institute of Neurological Sciences, Southern General Hospital, 1345 Govan Road, Glasgow G51 4TF, UK; i.bone@ clinmed.gla.ac.uk _________________________ T his article outlines the clinical presentation, neuroradiological approach, and the application of helpful ancillary investigations in the diagnosis of a range of cranial nerve syndromes. These syndromes are characterised by combinations of cranial nerve lesions that occur because of the involvement of contiguous nerves that direct investigation to a specific site. The syndromes highlight some important basic anatomy—in particular, knowledge of the contents of the cranial nerve exit/entry foramina (table 1) as well as the sites at which cranial nerves are clustered (table 2). The rate of presentation, with or without more widespread neurological or systemic involvement, hints at the particular pathological process at play. Advances in neuroimaging mean that the clinician and neuroradiologist have a range of available modalities to choose from. Discussion and appropriate selection of these is essential to accurate diagnosis and correct management. Magnetic resonance (MR) imaging is the modality of choice to depict many of the lesions involved in producing these syndromes based on their water content and molecular environment. Data can be acquired in multiple planes with differing contrast and is especially useful for showing soft tissue anatomy and pathology. There is now also widespread availability of spiral (helical) volume computed tomography (CT) where high definition planes or surfaces can be reconstructed to produce detailed depiction of bone, contrast filled vessels, and enhancing soft tissues based on their differences in electron density. Bone hardening and streak artefacts are reduced by this technique. Intravenous extracellular contrast agents (gadolinium based for MR imaging and iodine based for CT) are essential components of the examination. Catheter digital subtraction angiography (DSA) is still a key examination for showing vessels and as a route for therapeutic embolisation but has been superseded in many situations by less invasive CT angiography (CTA) or MR angiography (MRA). Plain radiographs are rarely used and are only justified in the assessment of remodelling of the skull, diffuse density changes, or gross fractures.