Anterior horn cells of the spinal cord receive excitatory and inhibitory motor control from brainstem descending tracts, and corticospinal tracts, and limbic system. It also receives afferent information direct and indirect from the peripheral nerves. This system is called the intraneuronal system (paths), which innervates the skeletal muscles. It is sometimes called the final common path.
Grey matter ( anterior horn cells): divided into three cords: Dorsal cord: which innervates the distal muscles of the extremities. Ventral cord innervates the proximal muscles of extremities. Medial cord innervates the axial and truncal muscles.
There are three types of motorneurons:
Alpha motorneurons: it is the large and has its own axon which branches on the muscles and large dendrites that provide an extensive receptive field.
Beta and Gamma motor neurons are smaller ones.
Motor unit: comprises single alpha motorneuron, its axons, and its muscle fibers.
Collateral sprouting: following the death of motorneuron, adjacent axons from healthy motorneurons may reinnervate denervated muscle fibers.
Clinical features of LMNs:
1) Muscle weakness. Because of the collateral phenomenon, for example, in a patient with poliomyelitis, muscle weakness becomes detectable only after the loss of more than 50% of LMNs.
2) Muscle atrophy and hyporeflexia.
3) Hypotonicity or flaccidity. It means loss of the normal muscle resistance to passive movement. It is the opposite of spasticity.
4) Fasciculations. It is caused because of the hyperexcitability of the motor neurons. It emerges on the muscle surface as a fine, rapid, flickering and sometime vermicular movements irrespective to time or location. In tongue, fasciculation is fine vermicular movement. Fasciculation can occur in healthy individuals. It can be seen in chronic spinal muscular atrophy (SMA), Kennedy disease, and post-poliomyelitis syndrome.
5)Muscle cramps. It is caused by hyperexcitability of the motor neurons. It is abrupt, involuntary shortening of the muscles along with palpable or visible knotting. It is relieved by stretching or massage.
1) EMG & NCS: loss of motor units reflects a decrease in the Maximum compound action potential (CMAP) of the motor conduction.
With the axonal loss, there is a modest slowness in conduction velocity, which is commensurate with a dropout of the large myelinated axons.
Whereas in the demyelination process, the is a noticeable slowness in the conduction velocity and severe cases, a block can be seen. The newly generated axons that innervate a denervated muscle has a slow conduction velocity.
In chronic reinnervation process, the surviving motor units may innervate a large number of muscle fibers, and this causes the potential to become broader in duration and higher in amplitude. Hence, the shape of the motor units potentials is broader, polyphasic, and has higher amplitude.
The needle portion of this test in LMN diseases is crucial, the denervated muscle fibers cause spontaneous discharge, which shows as fibrillation and positive sharp waves. Fasciculation may also detectable.
Decrease the number of surviving motor units leads to a decline in the firing rate. The denervation of the muscle fibers triggers the reinnervation process; this causes the motor units to remodel continuously. Further, the newly formed neuromuscular junctions (NMJ) are electrically unstable. All these changes can be seen in the needle part as irregular and polyphasic.
2)Muscle biopsy: it may show early signs of muscle fiber denervation and can be useful in ruling out other causes of weakness.
The denervated muscle fibers appear as small, angular, and stain darkly by oxidative enzymes and other esterase stains.
As the process of denervation progress, small groups of atrophied muscle fibers ( group atrophy) may appear. However, in ALS, there is no sufficient time to form fiber type grouping.
The random pattern of healthy muscle fibers is lost in repeated denervation, and reinnervation disease like Spinal Muscular Atrophy (SMA), and even large muscle biopsy may show one type of muscle fiber, and this is called fiber type grouping.
Poliomyelitis: it is caused by poliovirus ( which is RNA virus), transmitted via Fecal-Oral route. The virus enters the pharyngeal and intestinal lymphoid tissues before it goes through the bloodstream to the CNS.
C/F: after 3-6 days of the incubation period, the patient may suffer flu-like symptoms ( Fever, cough, diarrhea, myalgia, headache, malaise), it resolves after 2-3 days. 2-3% of the patients develop aseptic meningitis with severe headache, which tends to resolve after 7-14 days.
Less than 1% of patients who develop the acute paralytic syndrome, which is manifested as localized fasciculation, severe myalgia, fever, hyperesthesias, and asymmetrical paralysis. The most common site for the paralysis is the leg, but it can occur anywhere including bulbar paralysis.
On physical exam, there will be a significant weakness, hypotonia, loss of deep tendon reflexes, and muscle atrophy after a few weeks. Nonetheless, about 80% of patients recover their strength after 6 months up to 2 years. The sensation is often normal. 2/3 of patients recover with some degree of impairment.
Diagnosis: CSF analysis: an increase in protein, normal glucose, a pleocytosis ( a polymorphonuclear cells in the acute stage, and lymphocytes later). IgM antibody to poliovirus can be detected in the CSF.
Stool and pharyngeal culture is positive for poliovirus.
EMG & NCS: Low CMAPs, with slowness or block in velocities. However, the sensory nerve action potentials (SNAPs) are normal.
The needle portion will show profuse sharp waves and fibrillation +/- fasciculation, which means diffuse axonal loss.
The best way to diagnose polio now is the Polymerase Chain Reaction (PCR).
Differential Diagnosis (DDX): West nile flavivirus, Japanesse encephalitis, enterovirus 67/71, or coxaskie virus A7.
Others, Guillain Bare Syndrome (GBS), amyotrophic neuralgia, Myasthenia Gravis (MG), acute intermittent porphyria, HIV neuropathy, and periodic paralysis.
Treatment: All patient in their acute phase require ICU admission to monitor ventilation and cardiovascular function. After the acute stage, aggressive rehabilitation and multidisciplinary team are mainstays of treatment. Immunization against polio is a must for all healthcare team.
Post-poliomyelitis syndrome: Occurs at least 10 years after the acute attack. It is characterized by weakness and atrophy of the previously affected limb, myalgia, and arthralgia. Dx: via EMG. Exclude other causes — treatment: symptomatic and supportive.
West Nile Virus (WNV): it is transmitted from birds to humans. The bridge victor is Culex Mosquito. Human-to-human can occur via body fluids, including breastfeeding and intrauterine exposure.
It causes meningitis, encephalitis, and in some cases, Polio-like flaccid paralysis. The disease pattern follows the birds' migration. WNV can cause GBS.
Diagnosis (Dx): CSF for IgM or PCR to detect the virus.
Treatment: supportive, no vaccine for humans, only for horses and birds.
Multifocal motor neuropathy with conduction block (MMNCB) : it is an autoimmune condition, and it is similar to chronic inflammatory deymilnating polyneuropathy (CIDP) except that MMNCB presented with pure motor symptoms. Blood test: anti-GQib
DDX: Benign focal amyotrophy and progressive muscular atrophy variant of ALS.
Treatment: it is treatable with high dose immunoglobulin.
Benign focal amyotrophy: LMN symptoms that are restricted to one limb. On autopsy, the affected area of spinal cord flattened. Anterior horn cells markedly atrophied and lost both large and small motor fibers. It is often begins in late teens, but it can also presented in the fourth decades of life. More in men than women. It is panless, slowly progressive, weakness and atrophy. The most common pattern is the C7-T1 innervated muscle in the hand and/ or forearm with sparing brachioradialis muscle. It is called “ the oblique pattern”. There is no UMN signs, and if there is any, then ALS should be considered. The condition progress steadly in the first two years, and stabilizes by five years.
EMG & NCS: may show modest reduction in the CMAPs. The needle part: some fibrillation, fsciculations, and motor units changes.
Note: you should always exclude ALS and MMNCB. In AlS, the deep tendon reflexes (DTRs) are almost always Hyperactive in the evolution of ALS. In any radiculopathy, there should be a pain and sensory impairment.
Parsonage turner syndrome/ Neuralgic amyotrophy: usually starts with pain before weakness and wasting in the distribution of motor nerves that are derived from brachial plexus.it may involve sensory part.
Treatment: Physical and occupational therapy. The use of assistive devices ( splinting and braces). Tendon transfer can be considered in selected patients with focal weakness.
Spinal Muscular Atrophy ( SMA): it is congenital anterior horn cell destruction. The most common genetic inheritance is autosoma recessive. SMA has four types.