Hi Betty,
Yes, this is true. There is a standard that doctors are required to meet, for a definite diagnosis of ALS. I have some of it quoted below. This is for a "Definite" diagnosis. There are other levels of diagnosis, from suspected, (which is no longer part of the official terminology any longer) to possible, then, probable, (and of course definite)
The clinical diagnosis of ALS, without pathological confirmation, may be categorized into various levels of certainty by clinical assessment alone depending on the presence of UMN and LMN signs together in the same topographical anatomic region in either the brainstem [bulbar cranial motor neurons], cervical, thoracic, or lumbosacral spinal cord [anterior horn motor neurons].
The terms Clinical Definite ALS and Clinically Probable ALS are used to describe these categories of clinical diagnostic certainty on clinical criteria alone:
Clinically Definite ALS
is defined on clinical evidence alone by the presence of UMN, as well as LMN signs, in three regions.
The EMG signs of LMN dysfunction required to support a diagnosis of ALS, should be found in at least two of the four CNS regions: brainstem [bulbar/cranial motor neurons], cervical, thoracic, or lumbosacral spinal cord (anterior horn motor neurons).
For the brainstem region it is sufficient to demonstrate EMG changes in one muscle (e.g. tongue, facial muscles, jaw muscles).
For the thoracic spinal cord region it is sufficient to demonstrate EMG changes either in the paraspinal muscles at or below the T6 level or in the abdominal muscles.
For the cervical and lumbosacral spinal cord regions at least two muscles innervated by different roots and peripheral nerves must show EMG changes.
Electrophysiological features compatible with UMN involvement include:
1. Up to 30% increase in central motor conduction time determined by cortical magnetic stimulation
2. Low firing rates of motor unit potentials on maximal effort.
Clinically Probable ALS
is defined on clinical evidence alone by UMN and LMN signs in at least two regions with some UMN signs necessarily rostral to (above) the LMN signs.
The terms Clinically Probable ALS - Laboratory-supported and Clinically Possible ALS are used to describe these categories of clinical certainty on clinical and criteria or only clinical criteria:
Clinically Probable - Laboratory-upported ALS
is defined when clinical signs of UMN and LMN dysfunction are in only one region, or when UMN signs alone are present in one region, and LMN signs defined by EMG criteria are present in at least two limbs, with proper application of neuroimaging and clinical laboratory protocols to exclude other causes.
Clinically Possible ALS
is defined when clinical signs of UMN and LMN dysfunction are found together in only one region or UMN signs are found alone in two or more regions; or LMN signs are found rostral to UMN signs and the diagnosis of Clinically Probable - Laboratory-supported ALS cannot be proven by evidence on clinical grounds in conjunction with electrodiagnostic, neurophysiologic, neuroimaging or clinical laboratory studies. Other diagnoses must have been excluded to accept a diagnosis of Clinically possible ALS.
Clinically Suspected ALS
it is a pure LMN syndrome, wherein the diagnosis of ALS could not be regarded as sufficiently certain to include the patient in a research study. Hence, this category is deleted from the revised El Escorial Criteria for the Diagnosis of ALS.
Importantly:
Electrophysiological features suggesting other disease processes include:
1. Evidence of motor conduction block.
2. Motor conduction velocities lower than 70%, and distal motor latencies over 30%, of the lower and upper limit of normal values respectively.
3. Sensory nerve conduction studies that are abnormal. Entrapment syndromes, peripheral neuropathies and advanced age may render sensory nerve action potentials difficult to elicit in the lower extremities.
4. F-wave or H-wave latencies more than 30% above established normal values.
5. Decrements greater than 20% on repetitive stimulation.
6. Somatosensory evoked response latency greater than 20% above established normal values.
7. Full interference pattern in a clinically weak muscle.
8. Significant abnormalities in autonomic function or electronystagmography.