Motor Neuron Disease

The answer first

Motor neuron disease (MND) is a progressive, fatal neurodegenerative disorder of motor neurons in which both upper motor neurons (UMN) and lower motor neurons (LMN) degenerate, producing a combination of UMN and LMN signs. Amyotrophic lateral sclerosis (ALS) is the commonest form and the two terms are used interchangeably in practice. The diagnostic signature is UMN and LMN signs coexisting in the same body region (for example, a spastic, hyperreflexic arm that is also wasted and fasciculating). No other common neurological disease does this ^[9][10].

The diagnosis is clinical, supported by electromyography that confirms widespread LMN denervation and by exclusion of the mimics. The three questions that drive every decision are: is this truly MND (combined UMN and LMN signs in multiple regions, not a mimic)?; which body systems are involved now (bulbar, respiratory, nutritional, cognitive)?; and what is the respiratory status — because respiratory failure is the usual cause of death? ^[1]

The clinical decision rules a registrar must own at viva: ^[1]

1. Combined UMN and LMN signs in the same region is the hallmark. A wasted, fasciculating tongue with a brisk jaw jerk is bulbar ALS. A spastic leg with an extensor plantar response that is also wasted and areflexic is paradoxical — that paradox is MND. The mistake is to attribute all signs to one process (a cervical myelopathy) when two levels of the neuraxis are involved ^[2].
2. MND is a clinical diagnosis, made by excluding mimics. There is no confirmatory blood test or scan. The job is to demonstrate combined UMN/LMN involvement in multiple regions, confirm active denervation with EMG, and actively exclude cervical myelopathy, multifocal motor neuropathy with conduction block (anti-GM1, treatable), Kennedy disease (X-linked, androgen receptor), B12 deficiency and lead toxicity ^[8].
3. Respiratory failure is the cause of death, and non-invasive ventilation is the single most powerful survival intervention. Median survival without respiratory support is 3 to 5 years; NIV extends survival by a median of 7 to 13 months and improves quality of life ^[7]. A sniff nasal inspiratory pressure below 40 cmH2O, or an FVC below 50 per cent predicted (or below 1.5 L), is the trigger to discuss and offer NIV.
4. Multidisciplinary care and riluzole both extend survival. Attendance at a specialist MND clinic independently prolongs survival ^[14], and riluzole 50 mg twice daily adds a median of about 3 months ^[5]. These are the two disease-modifying interventions with robust evidence; everything else is symptom control.

The organising principle is multidisciplinary, symptom-led, prognosis-aware care: make the diagnosis accurately and kindly, exclude the treatable mimics, start riluzole early, surveil respiratory and nutritional function, intervene with NIV and PEG at the right time, and begin advance care planning early. ^[1]

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References15ShowHide

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