Saturday, August 16, 2008

Essentials of Diagnosis

• History: antecedent meningeal infection; infections of paranasal sinuses or skull base.

• Signs: cranial nerve deficits, especially nerves two, four, five, six, and seven.

• Laboratory: evidence of meningeal infection in CSF studies; MRI imaging of skull base and paranasal sinuses showing signs of an inflammatory process.

Involvement of the cranial nerves by infectious processes is usually the result of infection in the subarachnoid space at the base of the brain or in structures at the base near or through which cranial nerves course (eg, the cavernous sinus, as discussed previously). In the differential diagnosis, one must be aware that cranial nerves are also commonly involved by neoplastic processes, by inflammatory processes of other types (eg, sarcoidosis), or by vasculitic diseases, especially those affecting medium and small size vessels (eg, giant cell arteritis, polyarteritis nodosa, and diabetes mellitus).

Involvement of cranial nerves three, four, and six and the first division of five can be seen together because they traverse the cavernous sinus. Thus infections or infective thrombosis in the cavernous sinus can involve these nerves together.

Any meningeal infection, particularly if prolonged, can involve the cranial nerves at the base of the brain, so that palsies, particularly of the third, sixth, seventh, and eighth cranial nerves, can be seen in the course of pyogenic meningitis of any type or in more slowly evolving meningitides caused by tuberculosis, fungal infections, syphilis, or Lyme disease. Lyme disease seems to have a particular affinity for the seventh nerve; unilateral or bilateral facial paralysis is commonly seen. Involvement of the fifth nerve can be seen from reactivation of varicella-zoster virus in the trigeminal ganglion producing the well-known shingles eruption in one or more divisions of the nerve. Involvement of the intracranial portion of the carotid artery may occur in the course of ophthalmic zoster with thrombosis of the vessel and a middle cerebral distribution stroke. In contrast, reactivation of HSV in the same ganglion usually results only in the production of fever blisters on the lip.

The common Bell's palsy involving the facial nerve is tenuously associated with herpes simplex infections, prompting some authorities to recommend treatment of such cases with acyclovir. Involvement of the facial nerve by varicella results in the Ramsey-Hunt syndrome, with the appearance of vesicles in the ear. The facial nerve is susceptible to a variety of infections; in addition to those mentioned above, it is vulnerable to bacterial infections in the petrous apex. It, along with the eighth nerve, can be involved in Pseudomonas infections of the ear in diabetics (malignant external otitis).


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