In focal neuropathy, occurring in patients with diabetes, a neuropathy of another origin must always be excluded. In patients with ophthalmoplegia, preservation of pupillary function in a nearly complete third nerve palsy strongly suggests a diabetic origin, however, even in such cases, it is wiser to perform a noninvasive investigation of the area. Magnetic resonance angiography will permit exclusion of a compressive lesion of the third nerve by a large aneurysm of the carotid artery within the cavernous sinus, of the posterior communicating artery, or a fusiform aneurysm of the top of the basilar artery. Imaging will also permit to exclude tumors occurring at the base of the brain or in the basal skull. In patients with progressive involvement of several cranial nerves without imaging abnormalities, examination of the CSF might detect malignant cells characteristic of a carcinomatous meningitis. In patients with diabetes who develop a focal or multifocal neuropathy of the limbs, causes other than diabetes should be considered. The first step in this context is to determine if the lesions are located in the spinal roots or in the peripheral nerves, a distinction which might be difficult clinically and electro-physiologically. In addition, the lesions might be mixed. A nerve and a muscle biopsy might be considered, especially when another cause of focal or multifocal neuropathy is considered. When a patient with diabetes develops proximal weakness without much pain, a superimposed cause of motor neuropathy or of motor neuron disease must be considered, and appropriate investigations undertaken.
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