These two patients allow definition of the onset of ALS. Both developed El Escorial definite ALS.5
Although patients often describe the sudden onset of fasciculation or of the recognition of weakness in a limb, clinicians have tended to regard this not so much as the onset of the disease, but of awareness of the deficit. These two patients developed ALS while under observation for spinal pain of rheumatological origin. They both had normal EMG studies <2 years prior to the onset of their ALS. In addition, patient 1 showed relatively localised EMG changes at the time of the diagnostic EMG, 4 months after the onset of ALS. At this time there were mild abnormalities in the upper limb muscles, but marked partial denervation in both legs; however, the upper limb findings indicate the change from possible to probable ALS (laboratory supported) by the revised El Escorial criteria.5
In patient 2, prospective neurophysiological investigation showed progressively more severe changes in the muscles that were most affected initially compared with those not affected at presentation; eventually, loss of motor units were also disclosed in the latter (table 1).
EMG studies of the pattern of spread of denervation and reinnervation in ALS from a site of clinical presentation to other limbs have indicated that even when there is apparently focal onset of loss of motor units there are subtle abnormalities in spinal segments far removed from the site of clinical onset.6
This supports the usefulness of the electrophysiological studies, as laid out in the revised El Escorial criteria.5
Aggarwal and Nicholson3
prospectively followed a group of 17 people with a family history of ALS and known SOD‐1
mutations during a 3 year period, using motor unit number estimation and clinical assessment. Two of these subjects developed ALS during this period. Their patient 1 showed normal conventional EMG and normal motor unit number estimate (MUNE) in several muscles when he entered into the study, but 6 months later, there was a reduction in number of motor units in the extensor digitorum brevis muscle. However, symptomatic weakness developed only a year after study entry, when the EMG was diagnostic of ALS. The observations in this patient with familial SOD‐1 related ALS resemble ours in sporadic ALS. There was a rather rapid onset of the loss of motor units that seems to be characteristic of the disease, which has been observed in serial studies using MUNE in patients presenting shortly after symptom onset.3
We have previously reported a patient in whom benign fasciculation and cramp preceded the onset of ALS.2
Conventional EMG was normal shortly before weakness developed, but was thereafter abnormal. These observations show that ALS develops rapidly from a finite onset, and should not be regarded as a disorder gradually developing in the context of age related motor unit loss. There is usually an initial phase of rapid motor unit loss but, later in the disease, there is often relatively less rapid progression.1,7
It is interesting that in our patient 2 an elective surgical procedure seemed to be associated with a phase of rapid progression, which may also be an important feature.