Overview
This lecture provides a focused overview of clinical neurophysiology, introducing key diagnostic techniques, their principles, and interpretation, especially nerve conduction studies, electromyography (EMG), and lesion localization.
Types of Neurophysiology Tests
- Three main groups: nerve conduction studies/EMG (peripheral nervous system), EEG (cortical signals), and evoked potentials (central pathways).
- Nerve conduction studies primarily assess large fiber function.
Nerve Fiber Classification & Symptoms
- Nerve fibers are classified by size (micrometers) and degree of myelination.
- Large fiber dysfunctions cause negative symptoms (loss of function); small fiber dysfunctions cause positive symptoms (extra sensations).
- Small fiber dysfunction often precedes large fiber dysfunction in diseases like diabetes or alcoholism.
Nerve Conduction Study Techniques
- Stimulation and recording methods include ring electrodes for sensory and muscle electrodes for motor studies.
- Sensory studies can be done in orthodromic (usual direction) or antidromic (reverse direction); each has signal quality considerations.
- Sensory nerve action potentiaI (SNAP) and compound motor action potential (CMAP) are the basic recorded signals.
Interpreting Motor & Sensory Studies
- Motor studies stimulate nerves and record from muscles, capturing the total pathway including neuromuscular junction delay.
- F waves assess conduction through the entire motor pathway to the anterior horn cell.
- H reflex is the electrical equivalent of an ankle jerk, useful for evaluating S1 or early Guillain-Barré syndrome.
Pathological Patterns in Neurophysiology
- Axonal neuropathies: reduced amplitude with preserved velocity unless advanced.
- Demyelinating neuropathies: reduced conduction velocity with relatively normal amplitude until severe.
- Temporal dispersion: broader signals due to uneven slowing.
- Conduction block: loss of signal strength across damaged nerve segments.
Lesion Localization
- Multiple-site testing distinguishes between pre-ganglionic (root or anterior horn cell) and post-ganglionic (peripheral) lesions.
- Pre-ganglionic lesions show normal sensory responses, but reduced motor responses and abnormal EMG.
- Post-ganglionic lesions reduce both sensory and motor responses, with corresponding EMG changes.
EMG Principles and Interpretation
- Needle EMG records motor unit action potentials (MUAPs); size, shape, and recruitment patterns help differentiate myopathies from neuropathies.
- Myopathies show small, polyphasic, rapidly recruited MUAPs; neuropathies show large, wide, polyphasic, and reduced recruitment.
- Spontaneous activities like fibrillations, fasciculations, and myotonia provide diagnostic clues.
Integrating Clinical Neurophysiology
- Neurophysiology must be interpreted alongside clinical history and examination.
- Consider causes: congenital, acquired, inflammatory, infectious, metabolic, vascular, trauma, or degenerative.
Key Terms & Definitions
- SNAP — Sensory Nerve Action Potential, the electrical response from sensory nerves.
- CMAP — Compound Motor Action Potential, the response from muscle after motor nerve stimulation.
- Temporal Dispersion — Broadening of signal due to uneven conduction velocities.
- Conduction Block — Failure of signal transmission across a segment of nerve.
- F Wave — A late motor response assessing the entire motor pathway.
- H Reflex — Electrically evoked equivalent of a tendon (ankle) reflex.
- MUAP — Motor Unit Action Potential, recorded from muscle fibers during EMG.
Action Items / Next Steps
- Review related videos on small fiber testing, quantitative EMG, and spontaneous EMG findings.
- Prepare for case-based data interpretation in the upcoming neurology symposium.
- Visit the "Under the Skin" exhibit if possible before closure.