The classic neurological symptoms of early disseminated Lyme disease are meningitis, cranial neuritis, and radiculoneurotis. These occur alone or in combination in approximately 15% of untreated patients. A unilateral or bilateral facial nerve palsy is also a relatively common symptom of early disseminated disease. Symptoms usually last for weeks to months but can become chronic. Chronic neurologic manifestations of the disorder, which include encephalopathy, polyneuropathy, and leukoencephalopathy, usually occur late in the illness. Although there have been reports of cases with severe cognitive impairment, including psychosis and dementia, and vasculitic lesions, such cases are rare. A mild chronic Lyme encephalopathy (LE) is the most common neurologic symptom in patients with late-stage disease. Months to years after disease onset, sometimes following long periods of latent infection, a small percentage of patients develop a mild to moderate encephalopathy. The symptoms tend to be diffuse and nonspecific and can include memory loss, naming problems, sleep disturbance, fatigue, and depression.
The diagnosis of LE is difficult. It is generally believed that those with immunity to B. burgdorferi and abnormal cerebral spinal fluid (CSF) are more likely to have a neurological basis to their illness. Even if a patient is seropositive, it is difficult to know whether active infection is causing encephalopathy because serologic testing only indicates exposure to the
B. burgdorferi, not active infection. The standard neurological examination is usually normal. The CSF examination may show a positive polymerase chain reaction to B. burgdorferi DNA, intrathecal production of antibody to B. burgdorferi, or increased CSF protein. Traditional brain neurophysiological and neuroimaging techniques have also not been shown to be highly sensitive to the pathophysiology of LE. The routine electroencephalograph is typically normal. Magnetic resonance imaging (MRI) abnormalities have been described in Lyme patients, but these are nonspecific and relatively infrequent. When MRI abnormalities are present they are usually white matter lesions, suggesting the possibility of an inflammatory process. Single photon emission computed tomography (SPECT) imaging has shown some promise in identifying brain abnormalities in Lyme patients. Because the spatial resolution of SPECT is relatively poor, it can be used together with MRI to provide information about metabolic activity in specific brain regions (Fig. 1). SPECT has proved sensitive in identifying pathophysiologic abnormalities in other neurobehavioral disorders such as Alzheimer's disease and other dementias. In 1997, Logigian and colleagues studied a series of LE patients using a quantitative SPECT technique. In their analysis, 10 transaxial slices were imaged from each brain and divided into 4320 macrovoxels (Fig. 2). Quantitative SPECT has an advantage over visual image analysis because regional radiotracer uptake is analyzed statistically for each macrovoxel using an analysis of covariance. Patient scans are compared macrovoxel by macrovoxel to normal subjects to determine which brain regions are hyperperfused.
The Lyme patients showed patterns of multifocal hypoperfusion, most notably in the subcortical areas including the basal ganglia and white matter of the cerebral hemispheres, which was not apparent in normal controls. Patients with more objective evidence of LE, including CSF abnormalities, demonstrated significantly lower cerebral perfusion than Lyme patients with encephalopathic symptoms without objective evidence of central nervous system (CNS) involvement. Studies using visual ratings to assess the reduction in regional uptake of radiotracer have similarly reported decreased perfusion in Lyme patients. The reduction of metabolic activity in frontal and temporal lobe structures may provide a clue to the neuroanatomic basis of LE. That these same white matter regions form the large-scale neurocognitive networks involved in mediating memory and attention also suggests a possible explanation for the cognitive
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