Laboratory features of motor neuron disease Investigation provides useful confirmatory evidence even in the typical clinical syndrome. The EMG, as expected, displays widespread fibrillations (evidence of active denervation) and fasciculations and enlarged motor units (denoting reinnervation), and motor nerve conduction studies reveal only slight slowing, without focal motor conduction block. If the atrophic paresis is restricted to an arm or hand, raising the question of cervical spondylosis, evidence of denervation in many widely separated somatic segments favors the diagnosis of ALS. In questionable cases, it is good practice to insist that denervation be demonstrated in at least 3 limbs before concluding that the process is ALS. (The currently favored “El-Escorial” criteria that are used for the purposes of clinical research mandate that this finding be present.) Widespread denervation of the paraspinal muscles and of the genioglossus or facial muscles is also strongly suggestive of the disease but electromyographic testing of these muscles demands considerable experience and is uncomfortable for patients. A muscle biopsy is sometimes helpful in corroborating neurogenic denervation. Sensory nerve action potentials should be normal but may show slightly low amplitude; tests of motor nerve conduction have a normal velocity, but the amplitudes become progressively lower as the disease progresses—in the earliest stages, they too may be normal. When in a typical case the amplitudes of sensory nerve action potentials are reduced, there is usually an underlying entrapment neuropathy, diabetes, or other late-life neuropathy. Sensory evoked potentials are mildly abnormal in a proportion of patients, but the explanation for this finding is unclear. (Sensory complaints and minimal sensory loss have been commented on above.)
The CSF protein is usually normal or marginally elevated. Serum creatine kinase is moderately elevated in patients with rapidly progressive atrophy and weakness, but it is just as often normal. Motor evoked potentials elicited from the cortex are also prolonged in patients with prominent corticospinal signs. In this group, the MRI may show slight atrophy of the motor cortices and wallerian degeneration of the motor tracts (Fig. 38-10). These changes may be diagnostically useful and appear as increased FLAIR and T2 signal intensity in the posterior limb of the internal capsule, descending motor tracts of the brainstem, and spinal cord, all of which are subtle and may be missed. All these laboratory findings, particularly the degeneration of the lateral columns of the cord and changes in the internal capsules, pertain also to primary lateral sclerosis with the notable exception of EMG findings of denervation and of elevations of creatine kinase (CK).