The analysis and interpretation of data elicited by a careful history and examination may prove to be adequate for diagnosis. Special laboratory examinations then do no more than corroborate the clinical impression. However, it happens more often that the nature of the disease is not discerned by "case study" alone; the diagnostic possibilities may be reduced to two or three, but the correct one is uncertain. Under these circumstances, one resorts to ancillary examinations. The aim of the neurologist is to arrive at a final diagnosis by artful analysis of the clinical data aided by the least number of laboratory procedures.
Only a few decades ago, the only laboratory tests available to the neurologist were examination of a sample of cerebrospinal fluid, radiography of the skull and spinal column, contrast myelography, pneumoencephalography, and electroencephalography. The physician's armamentarium has been expanded to include a multitude of neuroimaging modalities, biochemical, and genetic methods. Some of these new methods give the impression of such accuracy that there is a temptation to substitute them for a careful, detailed history and physical examination. Reflecting the limitations of laboratory diagnosis, in a carefully examined series of 86 consecutively hospitalized neurologic patients reported by Chimowitz and colleagues, laboratory findings (including MRI) clarified the clinical diagnosis in 40 patients but failed to do so in the remaining 46. Moreover, it is common in practice for ancillary testing to reveal abnormalities that are of no significance to the problem at hand. Consequently, the physician should always judge the relevance and significance of laboratory data only in the context of clinical findings. Hence the neurologist must be familiar with all laboratory procedures relevant to neurologic disease, their reliability, and their hazards.
What follows is a description of laboratory procedures that have application to a diversity of neurologic diseases. Procedures that are pertinent to a particular symptom complex or category of disease—e.g., audiography to study deafness; electronystagmography (ENG) in cases of vertigo; electromyography (EMG) and nerve conduction studies, as well as nerve and muscle biopsy, where there is neuromuscular disease—are presented in the chapters devoted to these disorders.
The information yielded by examination of the cerebrospinal fluid (CSF) is crucial in the diagnosis of certain neurologic diseases, particularly infectious and inflammatory conditions, subarachnoid hemorrhage, and processes that alter intracranial pressure. Combinations of findings, or formulas, in the CSF generally denote particular classes of disease; these are summarized in Table 2–1.
Table 2–1 Characteristic CSF Formulas |Favorite Table|Download (.pdf)
Table 2–1 Characteristic CSF Formulas
WBC >50/mm3, often greatly increased
20–50 mg%; usually lower than half of blood glucose level
Gram stain shows organisms; pressure increased
Viral, fungal, spirochetal infection
Normal or slightly reduced
Special culture techniques required; pressure normal or slightly increased