Disorders of neuromuscular origin can be caused by muscular diseases, neuromuscular junction or their neurological supply. Neurological disorders can be in the peripheral nerves, motor cells in the spinal cord, motor tracts in the spinal cord, brain stem nuclei, tracts and cerebral cortex.
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Histopathology of Myotonic Dystrophy Type 2

Image: “Histopathology of Myotonic Dystrophy Type 2 (DM2, PROMM). Muscle biopsy showing mild myopathic changes and grouping of atrophic fast Fibres (Type 2, highlighted). Immunohistochemical staining for Type-1 (“slow”) Myosin.” by Marvin 101 – Own work. License: CC BY 3.0


Overview

Neuromuscular disorders usually lead to impaired movement, muscle weakness, fatigue, twitches and rigidity. This group of disorders can be genetic or acquired. Upper motor neuronal dysfunction lead to spasticity and hyperreflexia while lower motor neuronal lesions lead to paralysis with muscle fasciculations.

Amyotrophic lateral sclerosis is an example of both upper and lower motor neuron lesions. Some upper motor neuron diseases affect the brain and include stroke, Parkinson’s disease, Huntington’s chorea, Creutzfeldt-Jakob disease and multiple sclerosis (MS).

Muscular disorders include a group of genetic muscular dystrophies e.g. Duchenne’s and Becker’s muscular dystrophies and mitochondrial dystrophies.

Neuromuscular junction disorders involve a group of disorders e.g. myasthenia gravis, Lambert-Eaton myathenic syndrome, tetanus and botulism. Peripheral nerves can be involved with neuropathy, trauma and demyelinating diseases e.g. Charcot-Marie-Tooth (CMT) disease. The following are a few examples of neuromuscular disorders that affect the muscles, nerves or upper motor neurons.

Amyotrophic Lateral Sclerosis (ALS)

ALS is a fatal degenerative disorder that is characterized by muscle weakness and disability due to both upper and lower motor neuron lesions. The disease is usually sporadic but is less commonly genetically inherited.

The upper motor lesions are due to involvement of neurons in the corticospinal tract of the spinal cord, medulla, pons, internal capsule and Brodman area 4 in the cortex while the lower motor lesions are due to involvement of the motor nuclei in the brain stem and spinal cord.

Clinical presentation of ALS

Initial presentation of ALS is asymmetric extremity weakness that affects the upper or lower limbs. Foot drop or lateral hand weakness and atrophy are the initial common presentations.

Weakness progresses to involve axial trunk muscles with head drop or bulbar muscles with dysarthria and dysphagia, respiratory muscles are affected in rare cases. Upper motor neuron manifestations include spasticity, hyperreflexia and ankle clonus. Bulbar upper motor neuron symptoms include dysphagia and dysarthria. Sometimes, pesudopulbar affect, which is inappropriate laughing or crying spontaneously triggered by stimuli, occurs. Lower motor neuron manifestations include weakness, muscular atrophy and fasciculation.

ALS

Image: “Amyotrophic lateral sclerosis. MRI (axial FLAIR) demonstrates increased T2 signal within the posterior part of the internal capsule, consistent with the clinical diagnosis of ALS” by Frank Gaillard. License: CC BY-SA 3.0

Autonomic manifestations present with urinary urgency, constipation and sweating. Cognitive impairment with dementia can also be found in up to 50% of patients.

Diagnosis is clinical with careful history and clinical examination. Electromyography can be applied with muscle biopsy and neuroimaging to exclude other causes of upper or motor neuron disease. Electromyography results will show combined acute and chronic denervation.

Management with respiratory support, physical therapy, pain medications and muscle relaxants can help as palliative therapy for patients with ALS. Riluzole is proven to delay the need for ventilation support and improve survival for few months.

Duchenne Muscular Dystrophy (DMD)

This is an X-linked recessive muscular dystrophy due to dystrophin gene mutation. The disease manifests early in the first 2 years of life with motor developmental delay and pelvic girdle weakness. Early manifestations include delayed walking, difficulty rising from a seated position or climbing up the stairs. This is because the disease affects the lower limbs earlier than the upper limbs and proximal muscles earlier than distal muscles. Repeated falls and bone fractures are common among almost all patients with DMD. Cognitive impairment with behavioral dysfunction is also common.

Cardiac involvement: the disease progresses to dilated cardiomyopathy, mitral regurgitation and different types of arrhythmias. Respiratory failure due to muscular weakness and scoliosis are primary causes of death in DMD patients.

Signs of DMD

Pseudohypertrophy of the calf muscles is noted in most patients while other voluntary muscles show weakness and wasting. Lumbar lordosis, waddling gait and Gower’s sign where the patients use their hands for support while standing up are also common.

Serum creatine kinase is markedly elevated than normal range and muscle biopsy with genetic testing are confirmatory for the diagnosis. Muscle biopsy shows degeneration and regeneration with fibrosis, fat and connective tissue deposition in the muscles.

Becker Muscular Dystrophy (BMD)

This is an X-linked recessive muscular dystrophy that affects the dystrophin gene. Compared to DMD, Becker dystrophy presents later in life with milder but variable severity.

Patients have preserved motor power and activity until later in life with no or less cognitive impairment compared to DMD. Cardiomyopathy is more evident in patients with BMD, which can be explained by preserved muscular power and activity leading to more work load on the cardiac muscles with abnormal dystrophin. Heart failure associated cardiomyopathy, arrhythmias and heart block is more evident.

Guillain-Barré Syndrome (GBS)

Microscopic Image of campylobacter jejuni

Image: “Campylobacter jejuni, which triggers about 30% of cases of Guillain-Barré syndrome”. License: Public Domain

GBS is an autoimmune disorder with antibodies against peripheral nerves secondary to a preceding infection. An infection with campylobacter jejuni, cytomegalovirus or Epstein-Barr virus is responsible for the formation of antibodies which lead to demyelination of the peripheral nerves.

Clinical picture of GBS

Over a period of 2-4 weeks, the disorder will manifest as progressive symmetric muscle weakness or paralysis that involves limbs, respiratory muscles, face and bulbar muscles accompanied by diminished or absent reflexes. Respiratory or oropharyngeal involvement requires prolonged ventilatory support and airway protection. Facial nerve paresis, pain and autonomic dysfunction may be present.

CSF analysis will show albuminocytologic dissociation, which is elevated protein with normal white cell count. Plasmapharesis and immunoglobulin therapy (IVIG) are the main modes of treatment as well as breathing support and physical therapy.

Myasthenia Gravis (MG)

MG is an autoimmune disorder with T-cell dependent antibodies against post-synaptic acetylcholine receptors at the motor endplate. The disorder can affect ocular, bulbar, extremity and even respiratory muscles causing weakness and fatigue that fluctuates throughout the day.

Weakness is more evident later during the day or after exercise. Ocular disease presents with diplopia or ptosis while pharyngeal muscle weakness presents with dysarthria and dysphagia. Muscles of the jaw, face, neck, trunk and respiratory muscles are affected in the generalized form of the disease.

Respiratory muscle weakness is known as myasthenic crisis due to respiratory failure which can be complicated by dysphagia and aspiration. It can be precipitated by infection, surgery or immunosuppressant withdrawal.

Management of MG

Diagnosis is based on clinical presentation, laboratory tests and imaging. Anti-acetylcholine receptor antibody, anti-striated muscle antibody, anti-muscle specific kinase antibody are all highly sensitive and specific tests for ocular and generalized MG.

Up to 15% of patients with MG will have enlarged thymoma on chest radiographs. Repetitive nerve stimulation and single fiber electromyography will show impaired neuromuscular transmission at the motor endplate. Improvement of symptoms after acetylcholine esterase inhibitors (edrophonium) injection was used as an initial screening test but lacks specificity.

Treatment of MG

Anti-acetylcholine esterase medications are used to control symptoms while immunosuppressive therapy e.g. steroid, azathioprine is used for disease control.

Plasmapharesis and intravenous immunoglobulins are used in severe cases. Some medications can worsen MG weakness including fluoroquinolone, quinine, magnesium, aminoglycoside antibiotics, procainamide and beta blockers.

Lambert-Eaton Myasthenic Syndrome (LEMS)

This is an autoimmune disease affecting the neuromuscular junction that presents with weakness in the setting of small cell lung cancer. The weakness is due to decreased release of acetylcholine from nerve terminals with normal vesicles in the presynaptic terminals and normal receptors in the postsynaptic terminals. It is thought to be due to antibodies against presynaptic voltage gated calcium channels that allow calcium influx and release of acetylcholine.

Patients present with progressive proximal muscle weakness and fatigue. Respiratory failure is common. Ptosis, extraocular muscle paresis and cranial nerve involvement is also common.

Treatment with guanidine increases presynaptic acetylcholine release resulting in improvement of muscular weakness. Other therapies include pyridostigmine which is an acetylcholine esterase inhibitor, IVIG, azathioprine, which is an immunosuppressant and aminopyridine, which prolongs membrane depolarization.

Spinal Muscular Atrophy (SMA)

SMA is defined as a degeneration of the anterior horn cells of the spinal cord that can manifest prenatally or early in life. There are four types of the disorder; the most severe form is called Warding-Hoffman disease.

Infants with this disorder usually die from respiratory failure within the first year of life. Patients present with lower motor neuron symmetrical weakness or even paralysis of the proximal muscles with absence of deep tendon reflexes. Bulbar muscle involvement leads to a weak cry, poor suckling, aspiration and tongue twitches.

Respiratory muscle weakness affects the intercostal muscles but not the diaphragm, leading to respiratory failure and paradoxical breathing with the chest wall moving inwards during inspiration.

The diagnosis of SMA is made by electromyography, nerve conduction studies and muscle biopsy. Electromyography shows fibrillation in abnormal spontaneous activity in the muscles while a muscle biopsy shows atrophic and hypertrophied muscle fibers.

Management is mainly supportive for respiratory failure. Nusinersen is a drug approved for treatment of SMA that increases survival of motor neuron protein in affected patients.

Myotonic Dystrophy (DM)

This is an autosomal dominant disorder caused by expansion of the CTG (DM1) or CCTG (DM2) trinucleotide repeat in the dystrophica myotonica protein kinase gene that results in arrhythmia, balding, infertility, cataract and diabetes mellitus.

Cognitive impairment and mental retardation can develop in patients with DM1. Cancer susceptibility is higher in patients with DM specially brain, ovarian, colon and endometrial tumors.

Diagnosis is made using electromyography, muscle biopsy and genetic testing. Treatment is mainly supportive and involves use of pacemakers, respiratory support, physical therapy, CNS stimulants and analgesics depending on the clinical presentation.

Charcot-Marie-Tooth (CMT) Disease

It is also called hereditary motor sensory neuropathy and is caused by genetic mutation of one of the myelin genes. The disease can be classified into type 1 and 2 depending on the mutation. It is a combination of motor and sensory manifestations and deformities due to nerve demyelination.

Patients develop muscle weakness with diminished or absent reflexes that lead to atrophy of the calf muscles and intrinsic muscles of the hands and feet. Deformities develop in the form of Pes cavus foot deformity, stork leg deformity and even kyphoscoliosis. Sensation is also affected causing absent deep proprioception and vibration sensation.

Diagnosis is made through nerve conduction studies, which show slow conduction of 60% normal in both sensory and motor nerves in type 1 CMT while a nerve biopsy shows defective myelination with onion bulbs of demyelination and remyelination with abundant Schwann cells. Genetic testing detects the type of mutation and confirms the diagnosis.

Type 2 CMT has similar presentation of distal motor weakness, sensory loss and diminished reflexes and deformity but is less severe than type 1 and presents later in life.

Management of CMT disease is mainly supportive. Physical therapy and even surgery are indicated for deformities. Ascorbic acid, neutrophin-3 and progesterone antagonists help promote myelination and slow the disease progression.

Spinal and Bulbar Muscular Atrophy

This group of genetic disorders presents with muscle weakness and atrophy. Upper and lower motor neuron lesions can both be present due to degeneration of neurons in the brain stem and spinal cord. Endocrine manifestations are present due to X-linked recessive mutation in the androgen receptor (AR) gene, which leads to androgen insensitivity.

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