Occasional essay: Upper motor neuron syndrome in amyotrophic lateral sclerosis.

The diagnosis of amyotrophic lateral sclerosis (ALS) requires recognition of both lower motor neuron (LMN) and upper motor neuron (UMN) dysfunction.1 However, classical UMN signs are frequently difficult to identify in ALS.2 LMN involvement is sensitively detected by electromyography (EMG),3 but, as yet, there are no generally accepted markers for monitoring UMN abnormalities,4 the neurobiology of ALS itself and disease spread through the brain and the spinal cord.5 Full clinical assessment is therefore necessary to exclude other diagnoses and to monitor disease progression. In part, this difficulty regarding detection of UMN involvement in ALS derives from the definition of ‘the UMN syndrome’. Abnormalities of motor control in ALS require reformulation within an expanded concept of the UMN, together with the neuropathological, neuroimaging and neurophysiological abnormalities in ALS. We review these issues here. Sir Charles Sherrington (1857–1952) defined the LMN6 7 as the anterior horn cell and its motor axon, constituting the final common pathway for reflex action.8 In 1906, Sherrington,7 following Hughlings Jackson’s insights, concluded that motor acts were initiated in the brain by sensory input, thus building on activation of this simple reflex pathway, a view further developed by Sir Francis Walshe (1885–1973).9 Merton10 likened the effect of reflex action to a follow-up length servo, an influential hypothesis that was subsequently modified as servo assistance to emphasise that stretch reflexes support movement, generated centrally, rather than drive it.11 12 Despite these ideas, the UMN syndrome is not well defined.7 13–15 The clinical criteria (table 1) used by generations of neurologists to define the ‘corticospinal’ or ‘pyramidal’ syndrome, a term frequently but erroneously regarded as synonymous with ‘UMN syndrome’, rest on surprisingly uncertain pathophysiological underpinnings. The term UMN was introduced by Sir William Gowers (1845–1915) in his manual …