In order of relevance
In many countries, ALS is regarded as just one form of several related Motor Neurone Diseases. In these countries the broader (and perhaps more accurate) term "Motor Neurone Disease" is used instead of ALS.
There are some symptoms that occur in "textbook" cases of ALS that can differentiate it from other Motor Neurone disorders. The diagnosis of typical or so called "sporadic" ALS requires the presence of upper motor neuron signs (spasticity, rigidity, abnormal reflexes), lower motor neuron signs (weakness, atrophy) and progression.
The pattern of upper and lower motor neuron signs that a neurologist detects are usually found in certain patterns commonly spread throughout several regions of the body. EMG and nerve conduction studies are helpful in identifying lower motor neuron problems in areas where there may not be much weakness.
There are a large number of other illnesses that share some or many features of ALS and they must be evaluated or ruled out before a definite diagnosis can be made. An experienced practitioner who is familiar with the many presentations of ALS can make an accurate diagnosis with appropriate diagnostic testing and time to ensure that "look-alike" diseases have been excludedOTHER MOTOR NEURON DISEASES
Primary Lateral Sclerosis
This motor neuron disease is similar to ALS but only the upper motor neurons are degenerating. Although rare, some lower motor neurons may appear to be involved with mild atrophy.
PLS patients have most problems with spasticity and stiffness. The disease can progress slowly over many years. Patients often respond well to medical therapy to reduce their spasticity and stiffness.
Progressive Muscular Atrophy
Patients display progressive weakness that can be rapidly progressive or very slow. Eventually, atrophic muscles are always noticed but patients would not be expected to have brisk reflexes, stiffness, or spasticity.
When considering a diagnosis of progressive muscular atrophy, neuropathy (possibility of nerve disease) must be excluded.
Spinal Muscular Atrophy
The primary difference between the types of SMA involves the age of onset of symptoms. The age of onset of SMA can vary from infancy to adulthood. The types of problems and clinical presentation are similar to that of patients with progressive muscle atrophy.
Progressive Bulbar Palsy
Often, consideration for a stroke or myasthenia gravis must be eliminated before a diagnosis of progressive bulbar atrophy is made.
There are variations in presentation, including weakness in the legs. Patients display weakness and atrophy in those areas, but stiffness and brisk reflexes would not be expected. The gene mutation that causes Kennedy's disease is located on the X chromosome. Consequently, only males can be affected with this disorder.
One theory underlying this type of motor neuron degeneration involves an abnormality in the receptor responsible for detecting testosterone. Some of the associated symptoms of the disease are a result of an abnormal testosterone receptor. These include the development of breast tissue in males, testicular atrophy, and occasionally male sterility.
Clinical trials using testosterone therapies are underway and to date have not been completely successful in eliminating symptoms in this disorder.
This disease is characterized by its clinical presentation and no specific laboratory findings are helpful in making the diagnosis. The prognosis, however, is significantly more benign, as patients experience much less disability compared to generalized motor neuron disease.
Inclusion Body Myositis (Mimicking Motor Neuron Disease)
The result of severe cervical myelopathy is a combination of both upper and lower motor neuron signs that can be confused with the presentation of generalized motor neuron disease.
The lower motor neuron signs, however, should be limited to the area of the cervical spine most affected by the myelopathy. MRI imaging of the cervical spine is usually sufficient to discover the presence of cervical myelopathy.
A less common variation of neuropathy affects only motor nerves, resulting in weakness that can be confused with motor neuron degeneration.
Motor neuropathy may be an autoimmune disorder that involves selected areas of the myelin sheath blocking electrical conduction in the nerve. Weakness can begin in one area and remain at that location for several years. Alternatively, weakness can be found at several locations at one time (multi focal).
This disorder is diagnosed with a nerve conduction study showing focal areas of disrupted electrical conduction (conduction block) in the arms or legs. Fortunately, multi focal motor neuropathy responds to immuno-suppressant drugs.
The course of the disease frequently waxes and wanes and requires repeated treatments, occasionally for many years.
Other neuropathies that may resemble the presentation of ALS are even less common but should be evaluated when there is appropriate clinical suspicion. Porphyria is a disease involving abnormal metabolism in the body and can be associated with a predominantly motor neuropathy.
Weakness in prophyric neuropathy develops over several days to weeks. Often it is associated or preceded by significant abdominal pain, nausea, vomiting, possibly constipation, rapid heart rate, and fluctuating blood pressure.
Patients may also recognize a history of similar abdominal complaints without weakness in the past. Blood test and nerve conduction studies are often sufficient to implicate this diagnosis.
Other neuropathies result in predominantly motor symptoms. As with all neuropathies, the presentation is predominantly that of a lower motor neuron process and blood testing is usually sufficient to implicate the diagnosis.
Mercury intoxication can result in many neurologic complications, including motor neuropathy. This is a very rare source of confusion for the diagnosis of ALS.
One specific muscle disease, inclusion body myositis, is more frequently confused with ALS due to both the nerve and muscle involvement. Inclusion body myositis often affects patients at about the same age as those with ALS and may involve the same areas of weakness (distal). Often, a muscle biopsy is required to make the diagnosis of inclusion body myositis.
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