Identifying fasciculation potentials in motor neuron disease: A matter of probability.

INTRODUCTION: Fasciculations, the spontaneous activity of single motor units (MUs) are characteristic, but nonspecific for motor neuron disease (MND). We aimed to identify MU discharge properties to optimally differentiate MND patients from healthy controls. METHODS: High-density surface electromyography recordings were performed in the thenar muscles during 10 min of rest. MU discharges were classified as "isolated" when the interspike intervals (ISIs) before and after were > 250 ms, "continual" when both ISIs were ≤ 250 ms, or as "other". RESULTS: In patients (n = 30) compared with controls (n = 14), more MUs were active (9 vs. 3, P < 0.001) and generated relatively more isolated discharges (35% vs. 10%, P = 0.01). Two or more MUs with isolated discharges occurred more frequently in patients compared with controls (24% vs. <1% of 10-s windows, P < 0.001). CONCLUSIONS: More frequent occurrence of multiple MUs showing isolated discharges may improve identification of patients with MND.

Increased supernormality in patients with multiplet discharges: Evidence for a common pathophysiological mechanism behind multiplets and fasciculations.

OBJECTIVE: To determine whether there is a relation between electrically evoked multiplet discharges (MDs) and motor axonal excitability properties. We hypothesized that electrically evoked MDs share their underlying pathophysiological mechanism with fasciculations. METHODS: High-density surface EMG and motor nerve excitability recordings of the thenar muscles were performed in 22 patients with motor neuron disease (MND) in their differential diagnosis and who were referred for EMG examination. RESULTS: Supernormality (hyperexcitable phase following the refractory period) was significantly increased in patients with MDs (n=10) compared to patients without MDs (n=12) (25.5% vs 17.0%; p=0.02). Depolarizing threshold electrotonus differed significantly between both groups as well (TEdpeak, 76.6% vs 66.6%, p<0.01; TEd90-100ms, 51.7% vs 44.3%, p<0.01) CONCLUSIONS: Our findings imply that the same pathophysiological excitability changes are involved in generating MDs and fasciculations. Yet, MDs may be quantified more easily, and may be more specific for abnormal distal excitability than fasciculations, because fasciculations may originate along the motor axon as well as in the neuron cell body. SIGNIFICANCE: MDs are potentially useful as objective measure of increased distal axonal excitability at individual motor unit level and might complement clinical studies in MND.

Diagnostic accuracy of electrically elicited multiplet discharges in patients with motor neuron disease.

OBJECTIVE: To determine and compare the diagnostic accuracy of electrically elicited multiplet discharges (MDs) and fasciculation potentials (FPs) in motor neuron disease (MND). METHODS: Patients were eligible when they had MND in their differential diagnosis and were referred for electromyogram (EMG). Stimulated high-density surface EMG of the thenar muscles was performed on the same day as standard EMG examination. High-density recordings were analysed for presence of MDs and needle EMG of any muscle investigated in the cervical region for presence of FPs. RESULTS: Of the 61 patients enrolled in this diagnostic study, 24 patients were clinically diagnosed with amyotrophic lateral sclerosis (ALS) and 11 patients with progressive muscular atrophy (PMA). Another diagnosis was made in 26 patients. Sixteen patients in whom MDs were detected were diagnosed with either ALS (n = 11) or PMA (n = 5; sensitivity = 47.1%, PPV = 94.1%). MDs were detected in only one patient initially diagnosed with PMA, but in whom later on, multifocal motor neuropathy could not be excluded (specificity = 96.2%). Electrically elicited MDs had a higher specificity than FPs (96.2% vs 53.9%, p < 0.001, n = 26) and lower sensitivity (47.1% vs 85.3%, p = 0.002, n = 34). When considering presence of MDs in MND as neurogenic EMG abnormality, lower motor neuron involvement of ≥ 1 EMG region increased from 50% to 73.5% (p = 0.008, n = 34). CONCLUSIONS: Electrically evoked MDs are highly specific for ALS and PMA and are an early sign of lower motor neuron dysfunction.

Adaptation of the cervico- and vestibulo-ocular reflex in whiplash injury patients.

The aim of this study was to investigate the underlying mechanisms of the increased gains of the cervico-ocular reflex (COR) and the lack of synergy between the COR and the vestibulo-ocular reflex (VOR) that have been previously observed in patients with whiplash-associated disorders (WAD). Eye movements during COR or VOR stimulation were recorded in four different experiments. The effect of restricted neck motion and the relationship between muscle activity and COR gain was examined in healthy controls. The adaptive ability of the COR and the VOR was tested in WAD patients and healthy controls. Reduced neck mobility yielded an increase in COR gain. No correlation between COR gain and muscle activity was observed. Adaptation of both the COR and VOR was observed in healthy controls, but not in WAD patients. The increased COR gain of WAD patients may stem from a reduced neck mobility. The lack of adaptation of the two stabilization reflexes may result in a lack of synergy between them. These abnormalities may underlie several of the symptoms frequently observed in WAD, such as vertigo and dizziness.

Interaction between ocular stabilization reflexes in patients with whiplash injury.

PURPOSE: In the past few decades, the automobile has become an increasingly more popular means of transport, which has led to an increasing number of rear-end collisions and consequently has resulted in more patients with whiplash-associated disorders (WADs). Recently, it was found that the gain of one of the ocular stabilization reflexes-the cervico-ocular reflex (COR)-is elevated in patients with whiplash injury. The COR responds to proprioceptive signals from the neck and acts in conjunction with the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) to preserve stable vision on the retina during head motion. Therefore, an investigation was conducted to determine whether the reported elevation of the COR in WADs is accompanied by changes in VOR or OKR. METHODS: Eye movements of 13 patients and 18 age-matched healthy controls were recorded with an infrared eye-tracking device. RESULTS: Analysis confirmed a significant increase in COR gain in whiplash patients. Meanwhile the VOR and OKR gains remained the same. No correlation was found between the gains of the reflexes in individual patients. This is in contrast to earlier observations in elderly subjects and subjects with labyrinthine defects, who showed increases in COR gain and decreases in VOR gain. CONCLUSIONS: Impaired neck motion, altered proprioception of the neck, or disorganization in the process of VOR plasticity could explain the lack of change in VOR gain.