Patients with APDs usually present with the main clinical signs characteristic of parkinsonism, i.e., bradykinesia and rigidity. Although clinicians identify bradykinesia and rigidity with no need for neurophysiological recordings, these are convenient for quantitation of the dysfunction. Bradykinesia and, specially, rigidity are present with varying degree and localization in patients with parkinsonism. They may not be observed at all in patients at early stages of the cerebellar variant of multiple system atrophy (MSA-C), though they will eventually appear during the course of the disease.
Hypokinesia and bradykinesia are abnormalities of movement, and the best way to quantify them is with the use of reaction time task paradigms. Akinesia is defined as a delay in movement initiation, whereas bradykinesia is defined as the slowness of movement execution (13). There are many studies on reaction time in patients with IPD, but considerably less in APDs. Reaction time can be studied using different methods, ranging from the execution of a task (14,15), the release of a switch or a lever (16,17), the onset of limb displacement (18,19), or the onset of electromyogram (EMG) activity (20,21). In paradigms of simple reaction time (SRT), the subject knows all details about the requested motor performance before the imperative signal is delivered. In paradigms of choice reaction time (CRT), subjects have to process some of the information contained in the imperative signal itself. When measuring SRT as the onset of EMG activity in wrist extensors in patients with IPD, PSP, and MSA, Valldeoriola et al. (22) found a significantly larger delay of reaction time in patients with PSP in comparison to the other groups of patients (Fig. 1). This is in fitting with previously reported results of an investigation of reaction time to progressively complex tasks in PSP patients (23). These authors found an increased central processing time in patients with PSP compared with both IPD and control subjects. PSP patients might have degraded cognition, with a delay in stimulus identification and categorization processes (24).
The mechanisms underlying hypokinesia can be studied using EMG analysis of reaction time tasks. Hallett and Khoshbin (10) found that IPD patients were unable to appropriately scale the size of the first agonist burst to the requirements of a ballistic movement. They proposed that such defect represented a physiological mechanism of bradykinesia. It is likely that the increased pallidal inhibition of thalamocortical excitatory connections accounts for such an abnormal "energization" of the motor cortex. A proper study of the triphasic pattern in patients with APDs has not been done so far. However, in a small group of PSP patients, Molinuevo et al. (25) observed abnormalities of the triphasic pattern that were similar to those observed in IPD patients.
Rigidity in patients with parkinsonism manifests as a difficulty of complete muscle relaxation, with often permanent tonic background EMG activity (11). Several neurophysiological tests have been used to assess rigidity, although direct clinico-neurophysiological correlations have proven more difficult than with bradykinesia. Rigidity has been considered to be the cause of some neurophysi-
ological findings in IPD, such as an increased size of the F wave (26), increased size of long loop reflex responses to stretch (27) or to electrical stimuli (28-30), abnormalities in the silent period induced by transcranial magnetic stimulation (31,32), reduced reciprocal inhibition (33,34), and reduced autogenic (Ib) inhibition of the soleus H reflex (35). However, none of those tests is specific for rigidity, which continues to be assessed by clinical evaluation of the resistance to passive movements.
Like with bradykinesia, most tests directed to the evaluation of rigidity have been proven in patients with IPD, and more scarcely in patients with APDs. However, it is not unreasonable to admit that when and where rigidity is present, patients with APDs would present similar abnormalities as those described for IPD. Direct surface electrophysiological recording of a muscle in rigid patients in resting conditions may be enough to notice that there is increased muscle activity with respect to normal subjects. Electrophysiological evidence for that can be found when testing the relaxation time after a sustained contraction (36). The stretch reflex, which is the most paradigmatic electrophysiological test for limb rigidity, has not been properly tested in patients with APDs. In these patients, rigidity often predominates in axial muscles and is very mild in limb muscles, making it more difficult for neurophysiological evaluation. The shortening reaction (37-40) is a relatively poorly studied long latency reflex that occurs in the muscle shortened during a passive movement. Its frequent presence in patients with IPD could reflect the difficulties of these patients in modulating sensory signals generated by either joint afferents (38), tendon organ afferents (39), or both. Unfortunately, however, there have not been recent studies on such an interesting phenomenon.
The pathophysiology of rigidity may be related to abnormalities in propriospinal reflexes. In their study of IPD patients, Delwaide et al. (35) postulate that reduced autogenic inhibition mediated by Ib interneurons would be a neurophysiological correlate of rigidity. Interestingly, however, in the sole study published so far on spinal physiological mechanisms in APD patients, Fine et al. (41) reported increased Ib inhibition in patients with PSP. The observation of such an opposite behavior between IPD and PSP patients might be useful for differential diagnosis but points to the fact that neurophysi-
ological observations might only be one manifestation of dysfunctional mechanisms. Further studies are required to find out the mechanisms by which the Ib interneurons are modulated in a different direction in PSP and IPD, and what is the exact role of this dysfunction in the generation of rigidity.
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