Correct clinical diagnosis of patients with parkinsonism is not always possible in spite of the continuous effort in defining diagnostic criteria (1). Parkinsonism, defined as the combination of bradykinesia and rigidity (2), may be a predominant clinical feature of several diseases, including idiopathic Parkinson's disease (IPD) and several other entities commonly known as "Parkinson-plus" syndromes or atypical parkinsonian disorders (APDs). In these entities, parkinsonism is accompanied by other clinical signs, or red flags (3), that should warn the physician of the existence of a degenerative disorder. Even though rather specific clinical patterns have been described in patients with APDs, such as predominantly autonomic failure, cerebellar, or pyramidal dysfunction, in multiple system atrophy (MSA), axial rigidity, ocular motility disorders, and falls early in the course of the disease, in progressive supranuclear palsy (PSP), myoclonus and asymmetrical higher cortical limb dysfunction, in corticobasal degeneration (CBD), and fluctuating cognitive deficits, visual hallucinations, and REM sleep behavior disorder, in diffuse Lewy-body disease (LBD), these signs are not always evident or they may pass unrecognized by nonspecialized neurologists. In some conditions, such as for instance CBD, similar clinical expressions may be common to different pathologies (4), and the same disease may encompass diverse clinical presentations (5). In others, such as MSA with parkinsonian features (MSA-P), patients may behave like IPD until death (6), making it almost impossible to establish a clinical separation between the two diseases. Nowadays, the definite clinical diagnosis still resides in the pathological postmortem examination (1,7,8).
When in doubt, the clinician may seek help in laboratory exams. Unfortunately, however, there are not yet laboratory methods that can supply a diagnosis of certainty for APDs. Although single photon emission computer tomography (SPECT) and positron emission tomography (PET) have shown postsynaptic dopaminergic deficits in APDs (9), these tests are not yet capable of differentiating between the various degenerative syndromes featuring parkinsonism. Though electrophysiological studies do not usually provide the diagnosis, they can be of great help in the recognition of patho-physiological mechanisms underlying the presentation of some symptoms and signs. In parkinsonism, pallidal hyperactivity might be responsible for reduced activation of thalamocortical projections, and for an abnormal control of brainstem circuits (10-12). The situation might be slightly different in APDs, in which the basal ganglia pathology is accompanied by neuronal loss and atrophy in many nuclei of the brainstem and cerebellum, and dysfunctions might be present in various circuits. It is
From: Current Clinical Neurology: Atypical Parkinsonian Disorders Edited by: I. Litvan © Humana Press Inc., Totowa, NJ
therefore logical that, although some clinical neurophysiological manifestations are common to both types of disorders, there are a few specific and distinctive features of each syndrome that may be clinically useful.
Neurophysiological studies are suited to demonstrate, document, and quantify clinical observations. They are not expensive, and mainly noninvasive. They may occasionally bring information on neurological functions that is not obtainable with other means. One such example is, for instance, the measure of excitability in neuronal structures or circuits. In this chapter, we describe the neurophysiological observations made in patients with the principal disorders grouped under the term APDs, and discuss how the results compare between different patient groups.
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