Diagnosis of neuromuscular pathology
Neuromuscular pathologies present with varying clinical signs.
The most common signs are alterations in muscle size (atrophy or hypertrophy), muscle weakness and altered gait.
Clinical signs can be localised, multifocal or generalized.
Unilateral atrophy is easier to observe by comparing muscles on either side of the body.
In cases of generalized atrophy, we should remember what normal musculature is for that particular breed.
For example, the musculature in a dairy cow is less prominent than that of an animal destined for meat, and mild generalized atrophy in a draft horse will be much more difficult to detect than in a racing horse.
Weakness may be evident when an animal cannot get up or when it refuses to exercise. Careful attention should be given to gait analysis. In an animal with generalized weakness, due to muscular or peripheral nerve dysfunction, the step will be shortened or seem problematic and all four legs will be carefully positioned under the body to ensure greater stability when stationary.
Signs and medical history
- Breed – specific hereditary disorders
- Vaccinations and travel
- Information on diet
- Other associated pathologies
- Other affected animals
- Evolution of the illness
- Diagnosis of neuromuscular pathology
What are the criteria for talking about a Neuromuscular disease?
- Have orthopaedic, articular and cardiovascular pathologies been ruled out?
- Is there any weakness?
- Is there exercise intolerance?
- Abnormal gait?
- Changes to the muscle mass (atrophy or hypertrophy)?
- Disphagia, rigurgitation, voice changes?
Exclude upper motor-neurone pathologies
Clinical signs of neuromuscular pathology
- Generalized weakness and intolerance to exercise
- Stiff gait
- Generalized or localized muscle atrophy
- Pain on palpation of muscle
- Dysphonia, dysphagia, regurgitation
- Muscle hypertrophy
- Normoreflexic, hyporeflexic, areflexic
Localize disorder: muscle, nerve or neuromuscular junction
- Careful prognosis.
- Identification of specific treatment.
- Inappropriate treatment can lead to late or wrong diagnosis.
- Delays in starting treatment can cause death.
- Availability of DNA tests for genetic disease.
Delayed diagnosis . . . inapproriate treatment …
Tests to run when Neuromuscular pathology is suspected …
- Complete medical and neurological examination
- Haemachrome tests, serum biochemistry profile including electrolytes and CK, urine analysis
- Thyroid examination
- Acetylcholine receptor antibody titre – MG is an excellent mimic!
- Plasma lactate concentration
Clinical – pathological fndings
If the plasma membrane of the muscle fibres is damaged or in necrosis, some of the enzymes contained in it will be found in the blood.
The serum levels of some of these components are indicators of myofibral damage.
One of the most commonly used parameters is creatine kinase (CK).
Aspartate aminotransferase (AST) and lactate dehydrogenase levels can also be checked but they are not specific indicators of muscle damage as they are present in other tissues.
Clinical-pathological findings (cont.)
- Idiopathic HyperCKmia: there can be “leakages” from the sarcolemma, with CK and other enzyme release without muscle fibre being in necrosis.
- Although laboratory tests for CK and AST are relatively standardised, normal values can vary significantly.
- An increase in CK or AST:
- 2-3 times the normal value is considered a slight increase,
- moderate if the increase is 4-10 times the normal value, while the increase is considered
- high if the CK or AST are 10 times or more the normal value.
Clinical – pathological findings (cont.)
It is important to remember that. . .
… myopathies or neuropathies causing atrophy, weakness, spasm, stiffness or myotonia rarely lead to significant increases in serum enzymes.
Other useful diagnostic tests
- Antibody titres for other infective agents (polymyositis).
- Plasma cholinesterase concentration.
- Analysis of cephalorachidian fluid.
- Acid-base assessment.
- Carnitine concentration (plasma, urine, muscle).
- Eliminate disendocrinopathy.
CT and MRI are valid diagnostic tools for assessing and localising neuromuscular pathologies.
- Electromyography (EMG) assesses abnormal spontaneous activity generated by the muscle fibre. Abnormal spontaneous muscle activity is characterised by acute positive waves, fibrillation and myotonic firing. When the electric potentials are abnormal, the EMG machine usually makes a strange noise.
- Nerve conduction speed tests assess the integrity and function of the peripheral nervous system. Primary demyelinating diseases will lead to marked reduction in conduction speed of the nerve but the axons remain intact and the muscles innervated. This is why spontaneous activity is lacking.
- Repetitive stimulation of the nerve assesses function at the neuromuscular junction.
C. B. Duchenne du Boulogne was a pioneer in neuro-physiology
Electrodiagnostic tests (cont.)
- To be carried out in specialist centres with experience.
- Electromyography, motor conduction velocity (MCV) and sensory (SCV).
- Amplitude of action potential produced by muscle.
- Repetitive stimulation of nerve.
Diagnosis of neuromuscular pathologies
- The main objective of the clinical examination in a subject presenting with myopathy is an accurate diagnosis to work out, where possible, the cause of the disorder.
- To reach a diagnosis, even an etiological one, it may be necessary to do a histomorphological analysisof the muscle tissue by looking at a biopsy sample using the correct procedure for obtaining, conserving and staining the sample and applying the relevant laboratory tests to diagnose the suspected disorder.
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