What are the stimulus parameters of upper extremity SSEP?
constant current stim cathode at median wrist, anode 2-3cm distal stim rate 4-7/sec duration 200-300 usec stim above threshold for twitch, unilateral
What does absent SSEPs at all levels indicate
Peripheral nerve lesion, rule out technical problems
What does absence of clavicular, cervical and scalp SSEP indicate?
A lesion in the peripheral nerve or brachial plexus unless explained by technical problems
What does a combination of decreased peripheral conduction velocity and increased central conduction time suggest?
Lesions in both the peripheral and central parts of the somatosensory pathway; combination of peripheral nerve or plexus lesion and central defect
What does the absence of cervical SSEP, scalp SSEP, and preservation of clavicular SSEP suggest?
A lesion involving the spinal cord or roots unless due to technical problems
What does the absence of SSEPs at all levels with leg stimulation suggest?
Peripheral nerve lesion; rule out technical problems
What does absent L3 and T12 potentials with a normal popliteal fossa potential and scalp potential absent or delayed suggest?
Suspect lesion between the popliteal fossa and cauda equina
In the case of posterior tibial nerve stimulation, why might you add recording over the popliteal fossa?
To better evaluate peripheral sensory conduction
In a patient with Friedreich's ataxia, what lower SSEP changes will be seen?
Increased central conduction time
What does absent L3, T12 and T6 with delayed or absent scalp SSEPs suggest?
Defect at or above cauda equina or both
How does subacute combined degeneration affect lower extremity SSEP?
Cortical potentials are delayed or abolished
How does spinal cord injury affect lower extremity SSEPs?
Often abnormal; early return or normal responses indicate favorable prognosis
How do radiculopathies affect lower extremity SSEPs?
How do parasagittal cerebral lesions affect lower extremity SSEPs?
Abolish cortical potentials
How does multiple sclerosis affect lower extremity SSEPs?
Increased lumbar and scalp conduction time normal peripheral conduction time
What does an increase of stimulus intensity do to the SSEP?
Does not change latentcy, increases it amplitude to a maximum
In the case of posterior tibial nerve stimulation, why might you add a channel recording over the popliteal fossa?
To evaluate peripheral sensory conduction/stim
What number of responses is averaged for an upper extremity SSEP?
500 to 2000 sweeps
What are the filter settings for an upper extremity SSEP?
LFF - 10 to 30Hz HFF - 2500 to 4000Hz
What is the sweep length for an upper extremity SSEP?
40 ms; 60 to 100 ms for delayed SSEPs
What are the obligate components of an upper extremity SSEP?
Normal peaks: Erb's pt potential (N9), N13, P13/14, N20
What are the criteria for an abnormal upper extremity SSEP?
1. Absence of normal peaks 2. Slow peripheral conduction from stimulating electrode to Erbs Pt 3. Increased central conduction times between Erbs and cervical, between cervical and scalp, between brainstem and scalp, between Erbs and scalp
How does hypothermia affect the SSEP?
Increased latency of SSEPs at all levels
SSEPs are rendered abnormal by lesions that cause the loss of what sensations?
Loss of vibration and position sense rather than lesions that reduce pain and temperature sensation
Where is the defect with an absent N20 and a normal N9-13 conduction time and a normal peripheral nerve conduction velocity?
The defect is above the lower medulla and at or below the somatosensory cortex
Where is the defect with an absent N13 and an absent or delayed N20
The defect is above the brachial plexus and below the somatosensory cortex
Where is the defect with an increased N9-N13 conduction time with normal N13 amplitude and shape, normal peripheral nerve conduction velocity, and normal N13-N20 central conduction time?
The defect is above the brachial plexus and below the lower medulla
What does an increase of the clavicular-scalp and cervical-scalp conduction time with normal clavicular-cervical conduction time and peripheral conduction velocity time?
A lesion above the lower medulla and below the somatosensory cortex
What does an absent scalp SSEP with preserved clavicular and cervical SSEPs suggest?
A lesion above teh lower medulla
What does an increase of clavicular-cervical and clavicular-scalp conduction times combined with normal peripheral nerve conduction velocity?
A lesion of spinal roots or spinal cord below the lower medulla
What does an increased N9 latency with equally increased N13 and N20 and decreased peripheral nerve conduction velocity with normal conduction times indicate?
Peripheral nerve lesion; rule out technical problem
What does an absent N9 with normal N13 and N20 indicate?
What does an increased N13-N20 central conduction time with a normal N9-N13 conduction time and normal nerve conduction velocity indicate?
Defect above the lower medulla at or below the somatosensory cortex
What do peripheral nerve and plexus lesions do to the median nerve SSEPs?
Delay or abolish all SSEP peaks
what do lesions of the cervical roots and cervical cord do to the median nerve SSEPs?
Cervical and scalp delayed or abolished
How do lesions of the brainstem and cerebrum affect the median nerve SSEP?
Delay or abolish the scalp SSEP without interfering with the clavicular and cervical SSEP; the delay causes an increase of the cervical-scalp and clavicular-scalp conduction times.
How do polyneuropathies affect the median nerve SSEP?
1. slow, reduce or abolish SSEP depending on the degree of neuronal damage 2. cervical-scalp N13-N20 conduction time remains intact 3. clavicular N9-N13 conduction time may be increased if dorsal roots are also involved
How does thoracic outlet syndrome affect the median nerve SSEP?
1. delayed N13 2. N9 may be delayed or of low amplitude
How does a thalamic lesion affect the median nerve SSEP?
Absent or delayed N20
How does a subarachnoid hemorrhage affect the median nerve SSEP?
Low amplitude or absent N20
How does a persistent vegetative state affect the median nerve SSEP?
Absent or delayed N20
How does perinatal asphyxia affect the median nerve SSEP?
Absence, low amp, and/or increased latency of N20; degree of abnormality correlates with extent of damage
How do parietal lesions affect the median nerve SSEP?
absence or low amp N20, less delay w cortical than subcortical lesions
How does multiple sclerosis affect the median nerve SSEP?
increased N9-N13 and/or N13-N20 conduction times
How do brachial plexus lesions affect the median nerve SSEP?
Delayed N13; N13-N20 conduction time normal
How does brain death affect the median nerve SSEP?
Absent N20 and later waves
How does brainstem stroke affect the median nerve SSEP?
normal in lateral medullary syndrome; if infarction involves medial lemnisci, absent or delayed N20
How do cervical cord lesions affect the median nerve SSEP?
1. loss of N13 2. N9 normal 3. increased N13-N20 conduction time
What nonpathological factors may affect peripheral conduction velocity?
1. low limb temperature 2. inaccurate measurement of limb length
What does the latency of the lumbar peaks depend on?
1. stimulation and recording sites 2. length and temp of the leg 3. peripheral conduction velocity
Why is bilateral SSEP stimulation not recommended?
Even though it may give a better defined SSEP, bilateral stimulation can obscure unilateral abnormalities in subcortical structures.
What is the stimulus intensity for stimulation of mixed nerves?
Usually set to slightly above motor threshold; sufficient to produce twitch
What is the correct stimulus intensity for stimulation of sensory nerves?
Set to 2.5 to 3 times the sensory threshold, i.e. what the patient can feel
What are the stimulus parameters for the posterior tibial nerve SSEP?
Electrode placement - cathode behind medial malleolus; anode 3 cm distal Stim intensity - above threshold for twitch causing plantar toe flexion Unilateral stimulation - each side should be stimulated separately
What are the stimulus parameters for the common peroneal nerve SSEP?
Electrode placement - cathode in lateral popliteal fossa; anode 3 cm distal Stim intensity - above threshold for muscle twitch causing eversion of the foot Unilateral stim - each side stimulated separately
What is the number of responses averaged for a lower extremity SSEP?
1,000 to 4,000 responses
What are the filter settings for a lower extremity SSEP?
LFF 5-30 Hz HFF 2500 to 4000 Hz
What is the sweep length for lower extremity SSEPs?
PTN: 60 to 80 ms, 100 to 200 ms for delayed SSEPs Peroneal: 40 to 60 ms, 100 to 200 ms for delayed SSEPs
What are the obligate components of the PTN?
Popliteal fossa, L3 and T12 spine potentials, P37 and N45 peaks of scalp SSEP
What are the obligate components of the common peroneal nerve SSEP?
Normal peaks - L3 and T12 and T6 spine potentials, P27 and N35 peaks of scalp SSEP
What are the criteria for abnormality of the lower extremity SSEP?
1. absence of all spine and scalp SSEP in recording including 100-200 ms sweeps 2. Extremely slow peripheral conduction velocity from stimulus cathode to PF peak to L3 peak for PTN stim and from stim cathode to L3 peak for CPN stim 3. Abnormally slow central conduction velocity from L3 and T12 to scalp P37 for PTN and L3, T12 and T6 spine potentials to scalp P27 peak for CPN stim
What causes decreased peripheral conduction velocity to L3 in the lower extremity SSEP?
Peripheral defect between the PF and cauda equina
What causes decreased central conduction velocity in lower SSEP?
Defect above the cauda equina and below or at the somatosensory cortex
What causes absent scalp potentials in the lower extremity SSEP?
Suspect defect above the cauda equina and at or below the somatosensory cortex
What causes decreased nerve conduction velocity and decreased central conduction velocity in the lower extremity SSEP
lesions above and below the cauda equina or a single lesion at the cauda equina or lower spinal cord
Why is the measurement of central conduction time important?
1. largely eliminates the effects of peripheral lesions of changes in temperature 2. reduces the variations due to different limb lengths 3. helps localize lesions to the peripheral and central segments of the somatosensory pathway
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