Late Responses in Neurophysiology.pptx

1. LATE RESPONSES IN
NEUROPHYSIOLOGY:
F WAVE AND H REFLEX

2. ABSTRACT
• Late responses, specifically the F
wave and the H reflex, are pivotal
components in the electrodiagnostic
assessment of peripheral nerve
disorders
• These responses provide invaluable
insights into the functional integrity of
motor neurons, sensory fibers, and
their synaptic connections
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3. INTRODUCTION
• Late responses are fundamental
neurophysiological tests used to
evaluate the integrity of the peripheral
nervous system
• They are particularly instrumental in
diagnosing and monitoring diseases
affecting the motor neurons and
proximal segments of peripheral
nerves.
• Unlike routine nerve conduction
studies that assess the fastest
conducting fibers and primarily reflect
distal segment health, F waves and H
reflexes evaluate the entire length of
the nerve and offer a window into
proximal nerve segment and root
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4. HISTORY: F WAVE
• The F wave was first described by
Magladery and McDougal in 1950.
They observed it while conducting
studies on spinal motor neurons in
cats.
• Initially, it was thought to be a form of
reflex, but later studies revealed its
true nature as a direct response of
motor neurons to antidromic activation
(when a nerve impulse moves
opposite the normal direction)
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5. HISTORY: H REFLEX
• The H reflex was first described by
Paul Hoffmann in 1918, hence often
referred to as the Hoffmann's reflex.
• It was discovered while Hoffmann was
studying the reflex activity in muscles
and is analogous to the monosynaptic
reflex arc seen in the knee-jerk reflex.
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6. F WAVE -
MECHANISM
• The F wave is a late motor response
elicited by supramaximal electrical
stimulation of a motor nerve.
• It is a reflection of antidromic
(opposite to normal direction)
conduction to the spinal cord and
subsequent orthodromic (normal
direction) back-propagation from a
subset of anterior horn cells.
• The F wave is the result of backfiring
of motor neurons following direct
stimulation. It is not a reflex but rather
a direct motor neuron response
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7. F WAVE -
METHODOLOGY
• To elicit an F wave, supramaximal
stimulation is applied to a motor
nerve. The resulting muscle response
is recorded with surface electrodes.
• The F wave appears as a small wave
following the direct M response.
• Unlike the M response, the F wave
varies in latency, amplitude, and
shape due to the variable number of
anterior horn cells discharging and the
asynchronous nature of their firing
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8. F WAVE –
PARAMETERS
ASSESSED
• Persistence: The percentage of F
waves appearing in a series of stimuli.
• Latency**: Time from the stimulus to
the onset of the F wave. It reflects the
conduction time along the entire length
of the nerve.
• Chronodispersion: The variability in
latency of F waves; it represents the
temporal dispersion of the responses.
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9. H REFLEX –
MECHANISM
• The H reflex, analogous to the
monosynaptic reflex arc (e.g., Achilles
reflex), assesses sensory nerve
conduction to the spinal cord and
subsequent motor nerve conduction
back to the muscle.
• It is elicited by electrical stimulation of
a mixed nerve and is the electrical
equivalent of the stretch reflex,
predominantly reflecting S1 root
function when recorded from the
soleus muscle.
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10. H REFLEX –
METHODOLOGY
• The H reflex is usually recorded from
the soleus muscle by stimulating the
tibial nerve in the popliteal fossa.
• The stimulus intensity is gradually
increased to elicit the largest possible
H reflex, after which it progressively
decreases in size due to the
facilitation of motor neuron firing
(Hoffmann's phenomenon)
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11. H REFLEX –
PARAMETERS
ASSESSED
• Latency: The time from the stimulus to the
onset of the H reflex. It reflects the
conduction time through sensory and motor
fibers and synaptic transmission in the
spinal cord
• Amplitude: The peak-to-peak size of the H
reflex
• Threshold: The minimum stimulus intensity
required to elicit the H reflex
• H/M ratio: The ratio of the maximum H
reflex amplitude to the maximum M wave
amplitude, providing a measure of reflex
excitability
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12. CLINICAL
SIGNIFICANCE
The F wave and H reflex are crucial in the diagnosis and
management of various peripheral nervous system
disorders. They are particularly beneficial in assessing:
• Polyneuropathies: Abnormal F wave latency or absence may
indicate generalized nerve dysfunction.
• Radiculopathies: Prolonged F wave latency may suggest
proximal nerve or root involvement. S1 Radiculopathies**: An
absent or prolonged H reflex can be a sensitive indicator.
• Motor Neuron Diseases: Reduced F wave persistence or
increased chronodispersion may be early signs.
• Monitoring Disease Progression: Serial studies can track the
evolution of peripheral neuropathies or radiculopathies
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13. INTERPRETATION
CHALLENGES &
LIMITATION
Technical and Biological Variability
• Both F waves and H reflexes are
subject to technical factors (e.g.,
electrode placement, stimulus
intensity) and biological variability
(e.g., temperature, age).
• Interpretation should be in the context
of comprehensive clinical and
electrophysiological evaluation.
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14. INTERPRETATION
CHALLENGES &
LIMITATION
Anatomical and Physiological
Considerations
• The presence of anatomic variants,
age-related changes, and the influence
of concurrent medical conditions (e.g.,
diabetes) can affect the latency and
amplitude of these responses,
necessitating a careful, individualized
approach to interpretation.
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15. CASE 1: F WAVE -
SUSPECTED
POLYNEUROPATHY
History:
•Patient: 58-year-old male
•Chief Complaint: Gradual onset of numbness and
tingling in both feet, ascending to the lower legs
over the past 6 months.
•Medical History: Diabetes Mellitus Type 2,
Hypertension.
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Neurological Examination:
•Bilateral reduction in pinprick and vibration
sensation up to the mid-shin.
•Mild weakness in toe extension and flexion.
•Normal upper limb strength and sensation.
•Deep tendon reflexes: Reduced ankle jerks
bilaterally.
NCS/EMG Studies:
•F Wave:
• Prolonged latency in lower limb nerves.
• Reduced persistence, especially in the tibial
nerves.
•Nerve Conduction Studies:
• Slowed conduction velocities in the sural and
tibial nerves.
• Reduced amplitudes in sensory and motor
responses.
Diagnosis:
The prolonged F wave latency, reduced
persistence, slowed conduction velocities, and
reduced amplitudes suggest a generalized
neuropathic process. In the context of the patient's
history and clinical findings, the diagnosis is
consistent with diabetic polyneuropathy. The F
wave abnormalities support the involvement of
nerve segments distant from usual stimulation
sites, indicating a widespread process.

16. CASE 2: F WAVE -
SUSPECTED
RADICULOPATHY
History:
Patient: 45-year-old female
Chief Complaint: Persistent right-sided lower back
pain radiating to the right thigh and leg for 3
months.
Medical History: No significant past medical or
surgical history
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Neurological Examination:
Positive straight leg raise test on the right.
Reduced sensation to pinprick in the right L5
dermatome.
Weakness in the right big toe extension.
Deep tendon reflexes: Reduced right ankle jerk
NCS/EMG Studies:
F Wave:
Prolonged latency in the right tibial nerve compared to
the left.
Normal F wave latency and persistence in upper limb
nerves.
Nerve Conduction Studies:
Normal conduction velocities and amplitudes in upper
limbs and left lower limb.
EMG: Signs of acute denervation in the muscles
innervated by the right L5 nerve root.
Diagnosis:
The unilateral prolonged F wave latency in the right
tibial nerve, along with clinical signs of
radiculopathy (pain distribution, sensory loss, and
muscle weakness in the L5 distribution), points
towards a right L5 radiculopathy. F wave studies
help confirm the proximal involvement, consistent
with a root-level lesion.

17. CASE 3: H REFLEX - S1
RADICULOPATHY
History:
Patient: 52-year-old male
Chief Complaint: Left-sided low back pain with
radiation to the left posterior thigh and calf for 4
months.
Medical History: History of lumbar disc herniation
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Neurological Examination:
Positive straight leg raise test on the left.
Reduced sensation in the left S1 dermatome.
Weakness in the left ankle plantar flexion.
Deep tendon reflexes: Absent left ankle jerk.
NCS/EMG Studies:
H Reflex:
Absent H reflex on the left side.
Normal H reflex on the right side.
Nerve Conduction Studies:
Normal sensory and motor studies in the upper
limbs.
EMG: Chronic denervation in the muscles
innervated by the left S1 nerve root.
Diagnosis:
The absent H reflex on the left, combined with
clinical and EMG findings, suggests left S1
radiculopathy. The H reflex is a sensitive indicator
of S1 root function, and its absence is consistent
with the patient's clinical presentation and history of
lumbar disc herniation.

18. CASE 4: H REFLEX - MONITORING
PROGRESSIVE NEUROLOGICAL
DISORDER
History:
Patient: 60-year-old female
Chief Complaint: Progressive weakness in the
legs, difficulty in climbing stairs for the past year.
Medical History: No significant history.
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Neurological Examination:
Bilateral weakness in lower limbs, more
pronounced in proximal muscles.
Normal sensation.
Deep tendon reflexes: Brisk knee and ankle jerks.
NCS/EMG Studies:
H Reflex:
Reduced H/M ratio bilaterally.
Shortened latency of the H reflex.
Nerve Conduction Studies:
Normal sensory and motor conduction velocities
and amplitudes.
EMG: Chronic neurogenic changes with signs of
active denervation in proximal and distal muscles
of the lower limbs.
Diagnosis:
The reduced H/M ratio and shortened H reflex
latency, in conjunction with clinical and EMG
findings, suggest a lower motor neuron disorder.
These findings, particularly in the context of
progressive weakness and EMG evidence of active
denervation, may indicate a condition like Spinal
Muscular Atrophy or a variant of Motor Neuron
Disease. The H reflex helps in monitoring the
disease progression and assessing the extent of
motor neuron involvement.

19. CONCLUSION
• The F wave and H reflex are indispensable tools in the
neurophysiological evaluation of peripheral nerve
disorders.
• Their ability to assess nerve segments distant from the
stimulation site offers unique insights into the health of
motor neurons and proximal nerve structures.
• Mastery of the technical nuances and a thorough
understanding of the underlying neuroanatomy and
physiology are crucial for the accurate interpretation of
these tests.
• As part of a comprehensive neurodiagnostic
evaluation, they provide invaluable information guiding
the diagnosis, management, and prognostication of
various neuropathic conditions.
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