4Ancillary Tests in Neurology

Fundamentals

Neurological conditions can often be correctly diagnosed from the history and physical examination alone, but ancillary tests of various kinds are nonetheless vitally important, in many patients, to confirm the diagnosis and identify the etiology precisely. In this section, we will discuss imaging studies including conventional radiographs, CT and MRI, electrophysiological tests (including EEG, EMG, electroneurography, and evoked potentials), and ultrasonography, as well as the laboratory testing of bodily fluids (blood, CSF) and the histopathological or cytological study of biopsy specimens.

Whenever an ancillary diagnostic test is proposed, the specific indication for the test should be considered carefully and critically:

Only after thorough and meticulous clinical history taking and neurological examination.

Only after the formulation of a clinical differential diagnosis, in which all of the competing diagnoses are ranked by probability.

The study that should be performed is the one whose result is most likely to be important for further diagnostic and therapeutic management,

but only if this will be of clear benefit to the patient,

and only if the risks of performing the study do not outweigh any potential benefit that its findings might bring.

Multiple studies providing the same diagnostic information should not be performed merely for repeated confirmation of the findings.

A study should not be performed if, regardless of its result, another study will have to be performed that is likely to yield at least as much information.

Only very rarely should studies be performed to confirm a diagnosis that is already practically certain.

If a genetic study is contemplated, the potential consequences should be discussed thoroughly with the patient and his or her family before the study is performed.

The costs must not be forgotten.

Because of the high expense of some diagnostic tests—in particular, certain types of imaging study and some of the newer molecular biological studies—the physician is obliged to be cost conscious and order them only when necessary. It is understood, however, that an essential test should not be left out just because of the expense.

Conventional Skeletal Radiographs

Even though newer techniques are available, conventional radiograph images can still be of diagnostic use, with or without tomographic (sectional) views. Plain radiograph views of the skull and spine are occasionally indicated for neurological diagnosis.

Skull radiographs are performed for very few purposes nowadays and are hardly ever indicated. (They cannot be used as a substitute for CT in head trauma; if a CT is indicated, but unavailable for some reason, then the patient should probably be transported to a center where a CT can be performed.) Plain films of the skull enable visualization of:

fractures (though much less well than on CT, see Fig. 4.1),

congenital malformations of the bony skull, and

various developmental disorders.

Skull radiographs are useless in the diagnostic evaluation of headache or intracranial processes.

Plain radiographs of the spine are sometimes useful for the demonstration of:

fractures,

bony tumors (which, however, are more easily seen by CT or MRI—cf. Fig. 4.2),

degenerative diseases and slippage (olisthesis) of the spine,

infections involving bone,

axial skeletal deformities,

dynamic abnormalities (abnormal mobility or instability of individual spinal segments; their demonstration requires special radiological techniques, socalled “functional studies”).

Most findings of these types, with the notable exception of dynamic spinal abnormalities, can be more readily seen in a CT or MRI scan.

Fig. 4.6 e−h

image is a gray-scale map of the different intensities of MR signal coming from the tissue (Fig. 4.6) and can be computed in any desired plane of section. Gadolinium− DTPA can be given intravenously as a contrast medium for MRI.

The MR signal intensity of tissue is a function of its local physical and chemical properties, which determine, for example, the length of time that the hydrogen nuclei need to return to their initial orientation (T1 and

T2 relaxation times). The signal intensity is further influenced by the technical parameters of the scanner (e. g., the strength of the applied magnetic field and the frequency of the emitted impulses). The MRI signal characteristics of various normal and pathological tissues in the brain are listed in Table 4.2.

MR angiography. When the spin-echo technique is used in MRI scanning, flowing blood gives rise to a signal