What is the normal configuration of a motor unit action potential (MUAP)?
- Triphasic (can also get monophasic, biphasic and polyphasic - a few polyphasic potentials occur in normal muscle but shouldn’t exceed 5-15% of the MUAP observed)
What is the normal effect of probe insertion in the muscle
- Short bursts of high amplitude moderate-high frequency electrical activity (shouldn’t last more than 1-2 seconds)
- Prolonged insertional activity can be caused by hyperirritability and instability of the muscle fibre membrane; might be suggestive of early denervation atrophy, myotonic disorders or myositis.
What are the main differentials for an absence of insertional activity
- Muscle fibre fibrosis (complete)
- Functional inexciteability such as with HYPP or familial periodic paralysis
- Faulty electrode
Describe the pathogenesis for polyphasic MUAP (myopathic potentials) and list the differentials for this finding
Pathogenesis: increased frequency but decreased amplitude and duration resulting from an increased number of action potentials for a given strength of contraction. They are due to diffuse loss of muscle fibres hence more motor units are required to perform the work normally done by fewer motor units.
Most common in primary myopathies:
- Myotonia-like syndromes
- Periodic paralysis
- Myositis
- Botulism
- Myasthenia gravis-like syndromes
List the differentials for fibrillation potentials (initial positive deflection) and the use of these for monitoring
- Spontaneous discharge from acetylcholine-hypersensitive denervated muscle fibres
- Inflammation and focal muscle degeneration
- Muscle atrophy
- Denervation (may also have positive sharp waves) before clinical atrophy
- Reinervation (decrease in fibrillation potentials followed by recording of MUAP over time)
List the differentials for positive sharp waves (primary deflection is downward followed by a lower amplitude longer duration negative deflection)
- Myositis, exertional rhabdomyolysis, spinal shock
- Denervated muscle post RER, myotonia, EPM, laryngeal hemiplegia, suprascapular nerve injury, compressive myelopathy
List the differentials for fasciculation potentials and myotonic/high frequency potentials
- Disease of anterior horn cells
- Irritative-type lesions of spinal root or peripheral nerve
Myotonic: associated with hyperexcitability of the muscle cell membrane - LMN diseases, steroid induced myopathy, polymyositis, chronic denervation, HYPP, myotonia congenita and dystrophica.
List the findings on needle EMG for radial and suprascapular nerve injuries
Radial
- Positive sharp waves and fibrillation potentials in the triceps brachii and extensor carpi radialis muscles
Suprascapular
- Positive sharp waves and fibrillation potentials in the supraspinatus and infraspinatus (or could be damage to these muscles)
If there is post-insertional activity in these muscle groups and the lateral head of the triceps it suggests brachial plexus damage.
List the correlation between the waves of BAER and their anatomic generator sites
Wave I = cochlear nerve; Wave II = cochlear nucleus; Wave III = olivary nucleus; Wave IV = lateral lemniscus; Wave V = caudal colliculus
List the most common pathological causes of an abnormal BAER
- THO
- Congenital sensorineural deafness (Paint Horses, particularly lots of white marking on the face and blue iris)
- Multifocal brain disease
- Otitis media/interna
- Sepsis/neonatal encephalopathy/NI/prematurity in foals - this is usually permanent
