**2. Background**

Amyotrophic lateral sclerosis (ALS) is a group of fatal, neurodegenerative disorders, which is characterised pathologically by progressive degeneration and loss of motor neurones in the anterior horn cells of the spinal cord, motor nuclei of the brainstem and the descending pathways within the corticospinal tracts. The term amyotrophic lateral sclerosis (ALS) is used synonymously with motor neurone disease (MND) in the USA, but in the UK and Australia is used only to refer to patients who have a combination of upper and lower motor neurone dysfunction. (Talbot, 2002).

mutation carriers and supports the observation that sudden, catastrophic loss of motor neurones occurs immediately prior to the onset of symptoms and the development of the disease, rather than a gradual attrition of motor neurones over time. These results suggest that there may be a biological trigger initiating rapid cell loss, just prior to the onset of symptoms.

Current treatment for sporadic ALS or Cu, Zn superoxide dimutase 1 (SOD 1 mutation) familial ALS, produces only a modest increase in survival. The excitatory amino acid neurotransmitter, glutamate, may be involved in the pathogenesis of ALS. Riluzole, an anti-glutamate agent, remains the only disease modifying therapy available for ALS and has been used since 1995. (Cheah et al, 2010). Treatment of human ALS patients or transgenic Cu, Zn superoxide dimutase 1 (SOD 1) mice, most commonly produce a modest but significant increase in survival. (Bensimon et al, 1994). It has also been shown to have a small beneficial effect on

Using the statistical motor unit number estimation (MUNE) technique, (Daube, 1995), a longitudinal study was performed to determine whether early institution of Riluzole can reduce that rate of motor unit loss in familial amyotrophic lateral sclerosis (fALS). Motor unit numbers were estimated from the right abductor pollicis brevis (APB) and right extensor digitorum brevis (EDB) muscles. Our subjects had a presumptive diagnosis of fALS, as electromyography (EMG) was "normal" with an absence of fasciculation and fibrillation potentials, normal motor unit potentials and normal recruitment. MUNE is more sensitive that EMG and once changes occur on conventional EMG studies, the window of opportunity to influence the progression of this condition has been missed. They were all commenced on Riluzole therapy in the pre-symptomatic phase, as soon as loss of motor units was detected using motor unit number estimation (MUNE). After commencing Riluzole, "symptomatic" improvement occurred, especially a decrease in muscle fasciculations and an improvement in MUNE. Riluzole is not a disease altering agent but possibly if given early in the pre-sympto‐ matic phase of the disease, before significant motor neurone loss has occurred, it may have

This effect may have implications for the management of asymptomatic carriers of the SOD 1

Regular follow-up of SOD1 carriers with MUNE may lead to early diagnosis, creating an opportunity for future approaches and therapies aimed at preserving motor neurones rather than replacing lost motor neurones. Detecting the onset of motor neurone loss in asymptomatic individuals will identify those who may benefit from early institution of an active management program to improve their quality of life, until more effective treatment modalities are available

Amyotrophic lateral sclerosis (ALS) is a group of fatal, neurodegenerative disorders, which is characterised pathologically by progressive degeneration and loss of motor neurones in the

bulbar function, but not muscle strength.

184 Current Advances in Amyotrophic Lateral Sclerosis

some therapeutic benefit.

for this devastating condition.

**2. Background**

gene, as these subjects are at risk of developing ALS.

It is primarily a condition of middle to late life, with onset of symptoms between the ages of 50 and 70 and a mean age of onset of 57.4 years. (Ringel et al., 1993). Occasionally, it arises as early as the 2nd decade or as late as the 9th decade. In a natural history study, the overall median survival is 4.0 years from the onset of symptoms, but only 2.1 years from the time of diagnosis. (Ringel et al., 1993). In a study performed at the Mayo clinic, approximately 50% of patients died within 3 years of referral, but 20% were still alive at 5 years and 10% were still alive at 10 years. (Mulder & Howard, 1976).

Aging, motor neurone diseases and many peripheral neuropathies are all associated with loss of motor neurones or axons. When the disorders are recent or rapidly progressive, the extent of the loss may be indicated by weakness and wasting. In slowly progressive denervating conditions, like MND, loss of more than 50-80% of motor units may occur with little or no clinically apparent weakness.

It has been showed that patients with substantial chronic denervation could maintain normal muscle twitch tension until loss of about 70-80% of motor units occurred. (McComas, 1971). The surviving motor neurones enlarge their territories, through collateral sprouting (reinner‐ vation) to keep pace with cell loss, to maintain the muscle maximum compound muscle action potential (CMAP), until late in the disease. At this point, collateral reinnervation is no longer able to provide full functional compensation. (Campbell et al., 1973).

In MND, needle electromyography often reveals evidence of chronic reinnervation (increased motor unit action potential amplitudes and duration with reduced recruitment), but provides little direct evidence to the extent of motor neurone and axonal loss. The supramaximal CMAP amplitude also provides little direct evidence of the extent of motor neurone loss. Normal CMAP amplitudes might mistakenly suggest that motor neurone loss has not occurred yet. (Shefner, 2001).

Motor unit number estimation (MUNE) is a more reliable method for following changes in neurogenic disorders than the CMAP amplitude. It estimates the number of functioning lower motor neurones innervating a muscle or a group of muscles i.e. the number of motor units, which can be excited by electrical stimulation. It is therefore an indirect measure of motor neurone loss, rather than a measure of primary pathology. It can identify that the number of motor units may be well below normal, in the presence of normal CMAP amplitudes. (Brown, 1972).

Pre-symptomatic loss of motor neurones has been identified in an animal model of the disease (transgenic mice expressing mutant human SOD1-G93A). The initial loss in the pre-sympto‐ matic phase related to severe motor axonal degeneration due to vacuolar changes in motor neurones and a slow decrease in CMAP amplitudes. After a period of stabilisation, there was a gradual loss of motor neurones and a rapid decrease in CMAP amplitude, at the onset of weakness due to myelin alteration. At this point, there was a striking loss of motor units. There was also decrease in evoked motor potentials (an indirect measure of the number of motor units), prior to the onset of symptoms. The onset of disease in transgenic G93A mice involves a sharp decline of muscle strength and a transient explosive increase in vacuoles derived from degenerating mitochondria, but little motor neurone death. These did not die until the terminal stage. (Kong & Xu, 1998). The decline exhibited kinetics consistent with both a constant and exponentially decreasing risk of neuronal death. An escalating risk forced by cumulative damage was not responsible for cell death. (Azzouz et al., 1997).

on intron 4 and intron 1 and 14 'apparently' sporadic cases described with 6 different SOD1

The Role of the Statistical Method of Motor Unit Number Estimation (MUNE) to…

http://dx.doi.org/10.5772/56559

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Most are autosomal dominant in inheritance, but there is one confirmed autosomal recessive mutation, the D90A mutation in exon 4. This is unique in that it exists in dominant families in a heterozygous state, but in a number of pedigrees, specifically those of Scandinavian ancestry,

Mutations in the heavy polypeptide 200kDa subunit of neurofilaments (NEFH) have been identified in sporadic MND cases, (Figlewicz et al., 1994) and in one FALS case. (Al-Chalabi et al., 1999). Accumulation of neurofilaments in cell bodies and axons of motor neurons is a pathological hallmark of early stages of many neurodegenerative diseases. These mutations lie in the region of the protein involved in cross-linking and thus may disrupt normal aggre‐ gation of filaments. Thus far, 1 insertion and 5 deletion mutations have been identified on exon 4. Analysis of the NEFH locus on chromosome 22 however has failed to detect linkage in MND families. (Vechio et al., 1996). Genome search on a large pedigree with autosomal dominant juvenile onset MND found strong evidence for linkage to chromosome 9q34 (ALS4). The average age of onset is 17 years, with slow progression of disease. (Chance et al., 1998). There is also an autosomal recessive, juvenile onset MND, with linkage to a locus on chromosome

The other 90% of all MND patients have the sporadic form. There is no recognisable phenotypic difference between FALS and sporadic MND. The male: female ratio is 1:1 in FALS and 1.7:1 in sporadic MND. (De Belleroche et al., 1995). This decreases with increasing age of onset and approaches 1:1 after the age of 70. (Haverkamp et al., 1995). The site of onset is variable. Survival does not seem to be affected by age or gender, but rather the site of symptom onset. Generally, bulbar onset disease has a worse prognosis, and upper limb onset is more favour‐

homozygous mutations are required for disease. (Andersen et al., 1997).

mutations. (Shaw et al., 1998).

**Figure 1.** Number of SOD1 mutations identified for each exon

15 (ALS5). (Hentati et al., 1998).

able. (Mulder et al., 1986).

It is possible that the high metabolic activity in motor neurones, combined with the toxic oxidative properties of the mutant SOD1, causes massive mitochondrial vacuolation in motor neurones, resulting in degeneration, earlier than other neurones, triggering the onset of weakness. The involvement of mitochondrial degeneration in the early stages is consistent with a direct effect of toxicity, mediated by properties gained by the mutant enzyme in catalysing redox reactions. (Beckman et al., 1993).

Until recently, it has not been possible to address this in humans, as pre-symptomatic diagnosis was not possible. Now, with the ability to identify Cu, Zn superoxide dismutase 1, (SOD1) mutation carriers, a group of human pre-symptomatic subjects can be studied to determine whether there was gradual lifelong pre-symptomatic loss of motor neurones or whether sudden catastrophic loss of motor neurones occurs just prior to the onset of clinical symptoms.
