Neurologic diagnosis is frequently determined solely on the basis of careful history and examination. In that case, ancillary testing is unnecessary or simply corroborates the clinical impression. It also happens that the diagnoses can be reduced to a few possibilities but that testing is necessary to arrive at the correct one. The aim of the neurologist is to arrive at a diagnosis by artful integration of clinical data with laboratory procedures. Commonly the clinician already has at his disposal some laboratory information when the patient presents for a consultation. This may orient or distract from the correct course of action.
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 electrophysiologic tests. The physician’s armamentarium has been expanded to include a multitude of neuroimaging modalities, biochemical and immunologic assays, and genetic analyses. Some of these new methods give the impression of such accuracy that there is a temptation to substitute them for a detailed history and physical examination. Moreover, it is common in practice for laboratory 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 tests that have application to a diversity of neurologic diseases. Certain procedures that are pertinent to a particular category of disease—e.g., audiography to study deafness; electronystagmography (ENG) in cases of vertigo; as well as nerve and muscle biopsy, where there is neuromuscular disease—are presented in the chapters devoted to these disorders.
EXAMINATION OF CEREBROSPINAL FLUID
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. Patterns of findings, or “formulas,” in the CSF generally denote particular classes of disease; these are summarized in Table 2-1. The fluid is most often obtained by lumbar puncture, the technique and indications for which are described below.
Table 2-1CHARACTERISTIC CSF FORMULAS ||Download (.pdf) Table 2-1CHARACTERISTIC CSF FORMULAS
|CONDITION ||CELLS ||PROTEIN ||GLUCOSE ||OTHER FEATURES |
|Bacterial infection ||WBC >50/mm3, often greatly increased ||100–250 mg% ||20–50 mg%; usually lower than half of blood glucose level ||Gram stain shows organisms; pressure increased |
|Viral, fungal, spirochetal infection ||WBC 10–100/mm3 ||50–200 mg% ||Normal or slightly reduced ||Special culture techniques required; pressure normal or slightly increased |
|Tuberculous infection ||WBC >25/mm3 ||100–1,000 mg% ||<50, often markedly reduced ||Special culture techniques and PCR may be needed to detect organisms |
|Subarachnoid hemorrhage ||RBC >500/mm...|