Amyotrophic lateral sclerosis
This is the usual form of motor neuron disease, manifesting with both upper and lower motor neuron features in at least three limbs, and in some cases with involvement of bulbar musculature also being apparent.
A few patients present with an initial phase in which the symptoms and signs are largely, if not entirely, attributable to upper motor neuron involvement.
In this situation, the differential diagnosis includes other causes:
- progressive spastic paraparesis,
- especially multiple sclerosis,
- cervical spondylosis,
- primary lateral sclerosis.
The presence of sensory symptoms and signs clearly point towards an alternative diagnosis, but these are not always present in either multiple sclerosis or cervical spondylosis. In general, the tempo of amyotrophic lateral sclerosis is somewhat more rapid than that for patients with progressive spastic paraparesis due to multiple sclerosis or cervical spondylosis.
Although there may be no clinical signs of lower motor neuron involvement in the early stages of amyotrophic lateral sclerosis, EMG may reveal evidence of denervation, and serial EMG studies over several months may show that anterior horn cell involvement is becoming more widespread.
EMG may show also denervation in cervical spondylosis, but this is restricted to upper limb muscles and shows little spread in myotome involvement with follow-up.
In multiple sclerosis, MRI reveals high-signal lesions in the cerebral white matter and spinal cord, the CSF contains oligoclonal bands, and in some patients the visual evoked potentials are delayed.
In amyotrophic lateral sclerosis, brain and axial spinal cord images reveal symmetrical, high signal in the corticospinal tracts on T2-weighted images in two-thirds of patients (Thorpe et al. 1996;), a characteristic finding when present and entirely different from the multifocal and asymmetrical lesions found in multiple sclerosis; the CSF may show an elevated protein but there are no oligoclonal bands, and the visual evoked potentials are normal.
MRI is required to demonstrate cervical spondylosis, but it should be remembered that a degree of spondylosis will frequently coexist in middle-aged patients who develop either multiple sclerosis or amyotrophic lateral sclerosis: careful judgement of all available clinical and investigative data is needed to determine the relative importance of the spondylosis and the underlying neurological discase in contributing to the patient's clinical state.