John McAuley

Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing

Abstract The output from the central nervous system to muscles may be rhythmic in nature. Previous recordings investigating peripheral manifestations of such rhythmic activity are conflicting. This study attempts to resolve these conflicts by employing a novel arrangement to measure and correlate rhythms in tremor, electromyographic (EMG) activity and muscle vibration sounds during steady index finger abduction. An elastic attachment of the index finger to a strain gauge allowed a strong but relatively unfixed abducting contraction of the first dorsal interosseous (1DI). An accelerometer attached to the end of the finger recorded tremor, surface electrodes over 1DI recorded EMG signals and a heart-sounds monitor placed over 1DI recorded vibration. This arrangement enabled maintenance of a constant overall muscle contraction strength while still allowing measurement of the occurrence of tremulous movements of the finger. Ten normal subjects were studied with the index finger first extended at rest and then contracting 1DI to abduct the index finger against three different steady forces up to 50% of maximal voluntary contraction (MVC). Power spectral analysis of tremor, EMG activity and muscle vibration signals each revealed three frequency peaks occurring together at around 10 Hz, 20 Hz and 40 Hz. Coherence analysis showed that the same three peaks were present in the three signals. Phase analysis indicated a fixed time lag of tremor behind EMG of around 6.5 ms. This is compared with previous measurements of electromechanical delay. Other experiments indicated that the three peaks were of central nervous origin. Introducing mechanical perturbations or extra loading to the finger and making recordings under partial anaesthesia of the hand and forearm demonstrated preservation of all the peaks, suggesting that they did not originate from mechanical resonances or peripheral feedback loop resonances. It is concluded that, at least for a small hand muscle, there exist not one but a number of separate peak frequencies of oscillation during active contraction, and that these oscillations reflect synchronization of motor units at frequencies determined within the central nervous system. It is proposed that the multiple oscillations may be a means of frequency coding of motor commands.

Identification of psychogenic, dystonic, and other organic tremors by a coherence entrainment test

The differentiation of psychogenic from organic tremors, particularly those of a dystonic nature, can be difficult on clinical grounds. Entrainment of tremulous movements of different body parts into a single rhythm has been used clinically as a means of distinguishing these tremor forms, based on the inability of a patient with hysterical tremor to generate voluntary tapping oscillations independent of their ongoing tremor oscillation. The coherence entrainment test is a quantified electrophysiological entrainment test performed on accelerometer or surface EMG tremor signals. This test was carried out on 25 patients referred with suspected psychogenic tremor or dystonic tremor and on 10 normal subjects attempting to tap two independent voluntary oscillations. Using established and new clinical diagnostic criteria, patients were assigned the following final clinical diagnoses: 6 cases of clinically definite dystonic tremor, 5 cases of probable dystonic tremor, 2 cases of classic essential tremor, 5 cases of clinically definite psychogenic tremor, 3 cases of probable psychogenic tremor and 4 uncertain cases. On comparing these clinical diagnoses with those reached by a coherence entrainment test subsequently carried out on each patient, there was 100% concordance in both clinically definite and clinically probable patients. In uncertain cases, when later clinical information came to light, this also corroborated with the coherence entrainment diagnosis. No normal subjects were able to “mimic” organic tremor. The coherence entrainment test appears to be a sensitive and specific means of distinguishing psychogenic tremor from dystonic and other organic tremors.

Electrophysiological aids in distinguishing organic from psychogenic tremor

The clinical differentiation of tremors of organic and psychogenic origin can be difricult. We describe a patient with unilateral upper limb tremor that was initially considered to have a psychogenic cause, but subsequent frequency analysis of EMG signals and accelerometer recordings indicated that the tremor was organic in nature. An ischemic lesion in the contralateral lentiform nucleus found on MRI supported this conclusion. Quantitative electrophysiologic studies may thus be useful in distinguish organic from psychogenic tremor.

Common 3 and 10 Hz oscillations modulate human eye and finger movements while they simultaneously track a visual target

1 A 10 Hz range centrally originating oscillation has been found to modulate slow finger movements and anticipatory smooth eye movements. To determine if an interaction or linkage occurs between these two central oscillations during combined visuo‐manual tracking, frequency and coherence analysis were performed on finger and eye movements while they simultaneously tracked a visual target moving in intermittently visible sinusoidal patterns. 2 Two different frequencies of common or linked oscillation were found. The first, at 2–3 Hz, was dependent on visual feedback of target and finger tracking positions. The second, at around 10 Hz, still occurred when both target and finger positions were largely obscured, indicating that this common oscillation was generated internally by the motor system independent of visual feedback. Both 3 and 10 Hz oscillation frequencies were also shared by the right and left fingers if subjects used these together to track a visual target. 3 The linking of the 10 Hz range oscillations between the eyes and finger was task specific; it never occurred when eye and finger movements were made simultaneously and independently, but only when they moved simultaneously and followed the target together. However, although specific for tracking by the eyes and fingers together, the linking behaviour did not appear to be a prerequisite for such tracking, since significant coherence in the 10 Hz range was only present in a proportion of trials where these combined movements were made. 4 The experiments show that common oscillations may modulate anatomically very distinct structures, indicating that single central oscillations may have a widespread distribution in the central nervous system. The task‐specific manifestation of the common oscillation in the eye and finger suggests that such mechanisms may have a functional role in hand‐eye co‐ordination.

Analysis of force profile during a maximum voluntary isometric contraction task

This study analyses maximum voluntary isometric contraction (MVIC) and its measurement by recording the force profile during maximal‐effort, 7‐s hand‐grip contractions. Six healthy subjects each performed three trials repeated at short intervals to study variation from fatigue. These three trials were performed during three separate sessions at daily intervals to look at random variation. A pattern of force development during a trial was identified. An initiation phase, with or without an initiation peak, was followed by a maintenance phase, sometimes with secondary pulses and an underlying decline in force. Of these three MVIC parameters, maximum force during the maintenance phase showed less random variability compared to intertrial fatigue variability than did maximum force during the initiation phase or absolute maximum force. Analysis of MVIC as a task, rather than a single, maximal value reveals deeper levels of motor control in its generation. Thus, force parameters other than the absolute maximum force may be better suited to quantification of muscle performance in health and disease. Muscle Nerve 29: 401–408, 2004

Dopaminergic therapy in acute encephalitis lethargica.

We describe a patient with an acute illness, with clinical features characteristic of encephalitis lethargica, who responded repeatedly to trials of an apomorphine infusion and subsequently to oral levodopa therapy. This observation implies a direct acute neurotropic effect of the encephalitis on nigral dopaminergic cells. Dopaminergic medication may therefore be helpful in the acute stage of encephalitis lethargica as well as in the delayed post‐encephalitic parkinsonian phase of the syndrome.

Alcohol‐induced paroxysmal nonkinesogenic dyskinesia after pallidal hypoxic insult

We describe the first case of paroxysmal nonkinesogenic dyskinesia secondary to pallidal ischaemia, which is uniquely and specifically triggered by alcohol.

PO091 Breathing difficulty in adult onset alexander’s disease

Sleep apnoea and respiratory difficulties are reported in adult onset Alexander’s disease (AOAD), an autosomal dominant leukodystrophy that presents mainly with progressive ataxia. We demonstrate for the first time that the respiratory symptoms can result from association of palatal tremor with a similar tremor of laryngeal and respiratory muscles that interrupts normal inspiration and expiration. A 60-year-old woman presented with progressive ataxia, palatal tremor and breathlessness. Magnetic resonance imaging revealed bilateral T2 hyperintensities in the dentate nuclei, hypertrophic olivary degeneration and medullary atrophy. AOAD was confirmed genetically with a positive glial fibrillary acidic protein (GFAP) mutation. Electrophysiological study revealed 1.5 Hz rhythmic laryngeal and respiratory muscle activity. Her respiratory symptoms were significantly improved at night with variable positive pressure ventilation. This case illustrates that palatal tremor in AOAD, and potentially in other conditions, may be associated with treatable breathlessness due to a similar tremor of respiratory muscles.