Chapter 27 Neurodiagnostics Flashcards
Neuromuscular diseases can be broken down into
- Neuropathy
- Junctionopathy
- Myopathy
- (and neuromyopathies)
List the specific sites of potential disease for each category.
Neuropathy: Neuron (cell body, axon), Schwann cell, myelin
Junctionopathy: Pre-synaptic (i.e. faulty transmitter synthesis or release), synaptic (acetylcholine problem) or post-synaptic (affectign Ach-R e.g. MG)
Myopathy: sarcolemma, transverse tubules, organelles, myofilaments inclusions.
What is a motorneuron unit?
- Motor neuron (cell body, and its peripheral axon) supported by Schwann cells
- NMJ
- Myofibres innervated by motorneuron
What is variability of myofibre type within a motor unit?
Single histochemical myofibre type, not a mixture of myofibre types
(not usually contiguous –> mosaic pattern)
Name the principal signs of myopathic syndrome
- Generalised weakness
- Exercise intolerance
- Stiff/stilted gait
- Tremors
- Muscle atrophy
- Muscle hypertropy
- “Percussion dimple” contracture
- Muscle pain
- Limited joint movement eg contracture
- Regurg
- Flexion of head and neck
- Trismus (=lockjaw)
What are the principal signs of motor neuropathic syndrome
- Flaccid paresis/paralysis of innervated structures
- Neuroenic muscle atrophy
- Reduced reflexes and muscle tone
- Muscle fasciculations
What are the principal signs of sensory neuropathic syndrome
- Decreased response to noxious stimuli
- Proprioceptive defifits
- Abnormal sensation of face /trunks/lims/self mutilation
- Reduced reflexed in absence of muscle atrophy
Name general clinical signs of neuromuscular disorders
Outline 7 step diagnostic workup plan for neuromuscular disease
List 3 neuromuscular disorder ddx for large CK elevation (>10,000 IU/L)
- Muscular dystrophies
- Necrotising myopathies
- Inflammatory myopathies
What specific test can be run in animals suspeted of having mitochondrial myopathy?
Post-exercise plasma pyruvate
Once haematuria has been ruled out, how is myoglobinuria distinguished from hemoglobinuria?
Ammonium sulfate precipitation test
How is masticatory myositis diagnosed?
2M antibody test
2M myofibre = unique antigen present on muscles of mastication
What are two specific tests for neuromuscluar diseases?
- Electrodiagnostic testing
- Muscle/nerve biopsy
Define electromyography
Electromyography: Detection and characterization of electrical activity recorded from patient’s muscles
Allows accurate determination of distribution of muscles affected by neuromuscular disorder.
Which 3 broad muscle locations shoudl be tested with EMG?
Proximal and distal appendicular muscles as well as axial muscles (usually limit testign to unilateral so that biopsies can be harvested from other side)
What is normal EMG of muscle at rest (eg uGA)
Electrically silent
What are the 4 types of EMG traces considered normal spontaneous activity
- Insertional Activity
- Miniature End Plate Potentials
- End Plate Spike
- Motor Unit Action Potential
What are the 3 types of EMG traces considered abnormal spontaneous activity
- Fibs and Sharps
- Complex Repetitive Discharges
- Myotonic Potentials
If selceting muscle for biopsy is it better to take mildly, moderately or severely affected muscle
moderate
What is it that causes M wave on nerve conduction study
Orthodromic (normal direction) propagation of APs along a nerve, ACh release at NMJ and myofibre depolarization.
Comment on this motor nerve conduction velocity study
Abnormal canine fibular motor nerve conduction study.
Note the polyphasia, temporal dispersion, and decrease in amplitude of the M-wave when compared with panel B. Motor nerve conduction velocities calculated in this 6-year-old dog were slowed: 39 m/s (hip to stifle) and 32 m/s (stifle to hock). These abnormalities are consistent with demyelination.
M waves at each site should have similar configuration.
Normal canine fibular motor nerve conduction study. Note that latency of the M-wave increases with greater distance between stimulating and recording electrodes. Motor nerve conduction velocities calculated in this 1-year-old dog were 69 m/s (hip to stifle) and 57 m/s (stifle to hock).
How are motor nerve conduction velocities calculated (formula)?
MNCV (m/s) = Distance (mm) / (Proximal latency - Distal latency (ms))
Latencies are determined for each potential by marking the point at which the baseline deflects in an upward direction.
Amplitudes may be measured between baseline and negative peak (although some investigators use negative-to-positive peak amplitude).
Distances between the tips of the stimulating electrodes are measured to calculate nerve conduction velocities. It is this difference in distance over the difference in latency that determines the motor nerve conduction velocity, expressed in meters per second.
On nerve conduction velocity study graph, how does sensory potential differ from motor (M-wave)
Sensory potential often polyphasic
Somatosensory evoked potential study in a normal dog (B) and a dog with a sensorimotor polyneuropathy (C), following stimulation of the fibular nerve at the distal end of the fourth metatarsus (stimulus site). Somatosensory evoked potentials were recorded from the hock, stifle, hip, and L4-L5 (cord dorsum potential [CDP]). Latency measurements for sensory conduction velocity determinations are represented by T1, T2, T3, and T4. A, Diagram of electrode placement. B, Normal canine somatosensory evoked potential study. Although the somatosensory evoked potentials appear to be of similar amplitude at each recording site, the sensitivity increases from hock to L4-L5. Note that 1000 individual potentials (N = 1000 to right of figure) were averaged to produce these tracings
Comment on this sensory nerve conduction velocity study
Abnormal canine somatosensory evoked potential study.
All sensory nerve action potentials are severely reduced in amplitude, and sensory nerve conduction velocities are slowed. Dispersion of the potentials is difficult to evaluate because potentials are barely distinguishable from background noise. Note that compared with panel B, 4000 individual potentials (N = 4000) were averaged in an attempt to minimize random background noise from the time-locked electrical signal. These abnormalities are consistent with a severe sensory neuropathy.
Describe conduction pathway that generates f wave
Supramaximal stimulation –> antidromic (reverse) motor nerve activation –> additional volley in orthofromic direction
therefore longer latency than m wave and latency longer the more distal the stimulation site (i.e. inverse to m wave latencies)
Purely motor nerve testing
A, Normal canine M-waves (left) and F-waves (right) recorded following fibular nerve stimulation at the hock (32 superimposed individual tracings).
B, Normal canine M-waves (left) and F-waves (right) recorded following fibular nerve stimulation at the hip (32 superimposed individual tracings). Note the longer latency of the M-waves (left) and the reduced latency of the F-waves (right) compared with those in panel A.
Describe h reflex conduction pathway
Electrically elicited stretch reflex (i.e. like patella reflex)
Afferent via sensory nerve efferent via motor nerve
Normal canine H-waves recorded following fibular nerve stimulation at the hip using a low stimulus intensity (0.6 mA). Note that M-waves are not present (left).
D, The same stimulation and recording sites as used in panel C, after a slight increase in stimulus intensity (to 1.0 mA). M-waves now are present (left).
What repetitive nerve stimulationo result is consitent with a junctionopathy?
Decremental response >10%
Repetitive nerve stimulation. at the hock. Note in the data table that M-wave amplitude (Peak amp mV), area under the M-wave (Area mVms), and percentage amplitude decrement (Amp decr %) remain constant with consecutive stimuli.
B, Decremental response in a cat following 3-Hz fibular nerve stimulation at the hock. Note that amplitude and area under the M-wave decrease with consecutive stimuli, which is measured as a percentage amplitude decrement of 17% to 44%. The cat had muscular dystrophy.
C, Facilitation in a dog following 30-Hz fibular nerve stimulation at the hock. Note that both area and amplitude of the M-wave are increased (i.e., the M-wave becomes taller and is more broad based). The increase in amplitude is represented by a negative Amp decr %. A cause could not be determined in this dog presenting with exercise-induced weakness that occurred in warm weather.
D, Pseudofacilitation in a normal dog following 30-Hz fibular nerve stimulation at the hock. Note the increase in amplitude with no change in the area under the curve (i.e., the M-wave becomes taller and more narrow based). As in panel C, the increase in amplitude is represented by a negative Amp decr %.
