Fat and bone marrow have a high-signal intensity in MR images, whereas fascia, ligaments, and cortical bone lack signal intensity. In T1-weighted images, normal muscle has a low signal and dystrophic muscle, a slightly increased signal; in T2-weighted images, dystrophic muscle has a slightly increased (brighter) signal. Given its sensitivity to these dystrophic changes in muscle, MRI is particularly effective in determining the topographic distribution of muscle involvement in a dystrophy (see Chap. 45). Spectroscopic MRI in metabolically determined myopathies has the capacity to quantitate levels of selected biochemical constituents of muscle, including intracellular pH and levels of metabolic intermediates such as phosphocreatine. This technique is particularly effective in demonstrating subnormal generation of intracellular acidosis after a limb is exercised in disorders of glycogenolysis and of glycolysis. Some individuals with mitochondrial disease of muscle will demonstrate rapid depletion of energy supplies and profound delays in recovery that can be quantified and used as an end point for treatments.
As addressed in an earlier section, magnetic resonance techniques have been developed that allow the imaging of nerves. This may be an aid in assessing traumatic nerve injury and in demonstrating neuromas and other tumors, hypertrophy or atrophy of a nerve trunk or plexus.
NEUROPSYCHOLOGICAL TESTS,
PERIMETRY, AUDIOMETRY, AND
These methods are used in defining and quantitating the nature of the psychologic or sensory deficits produced by disease of the nervous system. They are performed most often to obtain confirmation of a disorder of function in particular parts of the nervous system or to quantitate, by subsequent examinations, the progression of the underlying illness such as a dementia. A description of these methods and their clinical uses is found in the chapters dealing with cerebral function (Chap. 21), developmental disorders of the cerebrum (Chap. 27), dementia (Chap. 20), and disorders of vision (Chap. 12) and of hearing and equilibrium (Chap. 14).
Numerous genetic markers of heredofamilial disease have become available to the clinician and have greatly advanced both diagnosis and categorization of neurologic disease. The main examples are analyses of DNA extracted from blood or other cells for the identification of mutations (e.g., muscular dystrophy, spinocerebellar atrophies, and genetically determined polyneuropathies, and the quantification of abnormally long repetitions of certain trinucleotide sequences, most often used for the diagnosis of Huntington chorea). The use of these tests is elaborated in Chap. 38. The study of mitochondrial genetics has allowed the detection of an entire category of diseases that affect this subcellular structure, as detailed in Chap. 37.
BIOPSY OF BRAIN, NERVE, MUSCLE, AND OTHER TISSUE
The application of light, phase, and electron microscopy to the study of these tissues may be highly informative. The findings are discussed in Chaps. 36 (skin and conjunctivum in the diagnosis of metabolic storage diseases), 45 (muscle), and 46 (nerve). Temporal artery biopsy is indicated when giant cell arteritis is suspected (Chap. 33). Brain biopsy, aside from its main use in the direct sampling of a suspected neoplasm, may be diagnostic in cases of granulomatous angiitis, some forms of encephalitis, and infectious abscesses. Biopsy of the pachymeninges or leptomeninges may disclose vasculitis, sarcoidosis, other granulomatous infiltrations, or an obscure infection, but its sensitivity is low. This is usually performed in concert with a biopsy of the underlying brain. Biopsy is now generally avoided in cases of suspected prion disease because of the risk of transmitting the causative agent. Abdominal fat pad biopsy is used in the diagnosis of amyloidosis.