Fatigue and activity dependent changes in axonal excitability in amyotrophic lateral sclerosis

Background

While patients with amyotrophic lateral sclerosis (ALS) may complain of fatigue, the underlying mechanisms appear complex, with dysfunction of central and peripheral nervous systems independently reported as contributing factors. The aim of the present study was to further delineate the mechanisms underlying increased fatigability in ALS by measuring activity dependent changes in axonal excitability following a maximum voluntary contraction (MVC).

Methods

Nerve excitability changes were recorded before and after an MVC of the abductor pollicis brevis in 16 patients with ALS and 25 controls.

Results

In patients with ALS, there was a greater increase in threshold (36.5 (5.9)%; controls 19.6 (3.5)%; p<0.05) as a result of MVC, with reduction in the amplitude of the compound muscle action potential generated by a submaximal stimulus (ALS 49 (7.6)%; controls 41.0 (5.4)%). These changes were associated with an increase in superexcitability (ALS 65.1 (25.4)%; controls 42.3 (5.7)%) and reduction in strength–duration time constant (ALS 20 (4.9)%; controls 10 (2.5)%; p<0.01), indicative of axonal hyperpolarisation. The increase in threshold was more pronounced in patients with ALS with predominantly lower motor neuronal involvement.

Conclusions

Higher firing rates of surviving motor axons attempting to compensate for neurogenic weakness are likely to explain the greater activity dependent changes in ALS. As such, the present study suggests a further peripheral factor underlying the development of fatigue in ALS.Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder that affects motor neurones in the spinal cord, brainstem and motor cortex.^1,2 The consequences of this neurodegeneration are motor deficits in the limbs, bulbar and respiratory muscles.³ Although the mechanisms of neuronal dysfunction, and ultimately the development of symptoms in ALS, remain unknown, glutamate excitotoxicity,^4,5,6 increased levels of inducible nitric oxide synthase levels⁴ and, in cases of inherited ALS, oxidative stress secondary to mutations in the superoxide dismutase‐1 gene, have been proposed.^7,8,9,10Increased fatigability, defined as an inability to sustain a predictable maximal force during voluntary contraction, is a common symptom of ALS.^11,12,13 The mechanisms underlying fatigue in ALS are complex, and contributions from both the central and peripheral nervous systems have been reported.^11,12 Central fatigue refers to a reduced excitatory drive to motor neurones, secondary to central nervous system dysfunction, resulting in incomplete motor unit recruitment and submaximal motor unit discharge rates. In contrast, peripheral fatigue typically refers to impaired muscle activation, caused by dysfunction at or below the anterior horn cell.^13,14 Perhaps somewhat counterintuitively, fatigue in ALS appears to be independent of muscle strength and disease severity.^15,16 Regardless of the underlying mechanism, fatigue in ALS severely impacts on the patient''s quality of life.^15,16The ability to sustain a motor output may be assessed by measuring changes in axonal membrane threshold following a voluntary contraction. Specifically, in peripheral nerves, voluntary contraction activates the axonal membrane Na⁺/K⁺ pump,¹⁷ which attempts to return the resting membrane potential to baseline after contraction has ceased,^18,19,20,21 resulting in activity dependent hyperpolarisation. The magnitude of activity dependent hyperpolarisation is determined by the impulse load²² and, in neurological diseases where the safety margin for impulse conduction has been reduced as occurs for instance in demyelinating neuropathy, may be sufficient to induce conduction failure.^23,24,25 In an attempt to further delineate the mechanisms underlying fatigability and weakness in ALS, the present study measured activity dependent changes in axonal excitability induced by voluntary contraction.