Spasticity in ALS
Spasticity is often undetectable or absent in weak and wasted muscles in ALS. In primary lateral sclerosis, a disorder in which LMN signs are absent and the spinal motoneurons are intact, spasticity, often with rigidity, is a prominent clinical abnormality. Spasticity is a more complex response to UMN dysfunction than increased tendon reflex activity. Classically, spasticity consists of a velocity-dependent increase in muscle tone, associated with increased stretch responsiveness, and the clasp-knife phenomenon, representing autologous inhibition occurring in response to the imposed stretch. Spasticity is dependent on increased excitatory drive, and reduced inhibitory projections, on spinal segments. The dominant physiological abnormalities in spasticity have been summarised by DeSeilligny and Burke as decreased post-activation depression, facilitated Group I and Group II excitation and decreased recurrent Renshaw cell inhibition (see table 12.1 page 578 of Pierrot-Deseilligny and Burke). A number of ill-defined additional mechanisms are also operative, including increased α motor neuronal tone ('α rigidity'), increased stiffness of muscles in the spastic limb, and increased non-corticospinal extrapyramidal discharge, leading to the phenomenon of 'spastic rigidity'. However, classical spasticity differs from spastic rigidity and also from decerebrate rigidity, with the clasp-knife phenomenon, representing switching off the stretch-induced motor neuronal discharge by autogenetic inhibition. In ALS, the spinal motor neuron and interneuron wiring has been disrupted by the degenerative process, so that increased tone in this disease is variable and often atypical of the classical, velocity-dependent, stretch-sensitive, increased tone of spasticity as emphasised by the Sherrington school of physiology. There is no information on the relative pace of degenerative change in the α and γ motor neurons, or in internuncial neurons or Renshaw cells in the cord in ALS, but these factors will influence the output of the cord, including the development of spasticity.