- Negative symptoms are numbness, loss of feeling, perception, and even anesthesia.
- Positive symptoms are paresthesia, pins and needles, tingling, dysesthesia (uncomfortable feeling) or hyperpathia (painful perception of a non-painful stimulus). Inadequate sensory stimuli can result in allodynia.
The type of sensory disturbance gives a clue to the affected fibers. Loss of temperature and pain perception points to small fiber loss, whereas large fiber loss manifests itself in loss of vibration perception and position sense (Table 1).
The distribution of the sensory symptoms can follow a peripheral nerve (mononeuropathy), a single root (radiculopathy) or in most polyneuropathies, a stocking glove distribution. The sensory trigeminal nerve distribution can suggest a lesion of a branch (e.g., numb chin syndrome) or a ganglionopathy. Maps of dermatomes and peripheral nerve distributions can be used to distinguish and classify the patterns found (Fig. 4).
Transient sensory symptoms can be elicited by local pressure on a nerve, resulting in neurapraxia. In patients who have a history of repeated numbness in a mononeuropathic distribution or permanent symptoms, a hereditary neuropathy with pressure palsy has to be considered. Some transient sensory changes are characteristic but difficult to assess, such as perioral sensations in hypocalciemia or hyperventilation.
A characteristic sign of sensory neuropathy is the Tinel's sign, which is a distally radiating sensation spreading in the direction of a percussed nerve. It is believed to be a sign of reinnervation by sensory fibers, but may also occur in a normal peripheral nerve when vigorously tapped.
Quantitative sensory testing includes sensory NCV, testing of small fibers by cooling, and large fibers by vibration threshold.
Burns TM, Taly A, O'Brien PC, et al (2002) Clinical versus quantitative vibration assessment References improving clinical performance. J Peripheral Nervous System 7: 112-117 Dimitrakoudis D, Bril V (2002) Comparison of sensory testing on different toe surfaces; implications for neuropathy screening. Neurology 59: 611-613
Merkies ISJ, Schmitz PIM, van der Meche FGA (2000) Reliability and responsiveness of a graduated tuning fork in immune mediated polyneuropathy. J Neurol Neurosurg Psychiatry 68: 669-671
Montagna P, Liguori R (2000) The motor Tinel's sign: a useful sign in entrapment neurop-athyneuropathy. Muscle Nerve 23: 976-978
Sindrup SH, Gaist D, Johannsen L, et al (2001) Diagnostic yield by testing small fiber function in patients examined for polyneuropathy. J Peripheral Nervous System 6: 219-226
Table 2. Characteristics of dysesthetic and nerve trunk pain
Dysesthetic pain Nerve trunk pain Symptoms
Burning, raw, crawling, drawing, "electric" Aching, "knife like" Distribution
Usually distal, superficial Deep
Time perspective Continuous, with waxing and
Often intermittent, shooting, lancinating waning
Small fiber neuropathy, causalgia Root compression, plexopathy
Myalgia and pain Myalgia (muscle pain) occurs in neuromuscular diseases in several settings. It can occur at rest (polymyositis), and may be the leading symptom in polymyalgia rheumatica. Focal muscle pain in association with exercise-induced ischemia is observed in occlusive vascular disease. Local, often severe, pain is the hallmark of a compartment syndrome occuring after exercise or ischemia. Exercise-induced muscle pain in association with muscle cramps can be seen in metabolic disease.
Neuropathic pain Neuropath ic pain can result from a damaged peripheral nerve. It can be divided into dysesthetic or nerve trunk pain (Table 2).
Trigeminal neuralgia, sometimes overlapping with "atypical facial " pain are good examples of neuropathic pain.
Reflex sympathetic dystrophy (RSD) is a burning pain in the extremity associated with autonomic changes, allodynia, and trophic and motor abnormalities. It is associatied with local osteoporosis (Sudeck's atrophy), and the pain causes a reduced range of motion and leads to contractures.
The definition and characterization of neuropathic pain has several implications. Firstly, a possible cause-effect relationship, or "symptomatic" cause needs to be ruled out. Secondly, neuropathic pain needs particular treatment considerations, which include a number of drugs and different mechanisms usually not considered for nociceptive pain.
Reference Chelimsky TC, Mehari E (2002) Neuropathic pain. In: Katirji B, Kaminski HJ, Preston DC,
Ruff RL, Shapiro B (eds) Neuromuscular disorders. Butterworth Heinemann, Boston Oxford, pp1353-1368
Autonomic findings Autonomic findings are often neglected and include orthostatic hypotension, tachyarrhythmias, ileus, urinary retention, impotence, incontinence and pupillary abnormalities. In some polyneuropathies and mononeuropathies the autonomic changes are revealed by skin changes at examination. The dry, anhidrot-ic skin in diabetic neuropathy is a good example. Skin changes in peripheral nerve lesions can include pale, dry, and glossy skin, and changes of the nailbeds. The methods suggested for testing include RR variation testing, the sympathetic skin response, and the Ewing battery.
The gait can be a definite clue to the cause of the neuromuscular disease. Proximal weakness (if symmetric) causes a waddling gait. Unilateral pelvic tilt toward the swinging leg is caused by weakness of contralateral hip abductors.
Hyperextension of the knee may be compensatory for quadriceps weakness. If proximal weakness has progressed, hip flexion can be replaced by circumduction of the hyperextended knee. Distal neuropathies often include weakness of the peroneal muscles, resulting in a steppage gait. Loss of position sense due to large fiber damage results in sensory ataxia, with a broad-based gait and worsening of symptoms with eyes closed (Romberg's sign).
NCV/EMG/ autonomic testing and miscellaneous electrophysiologic tests
Motor NCV are one of the basic investigations in peripheral neurology. A Motor NCV studies peripheral nerve is stimulated at one or more points to record a compound action potential (CMAP) from a muscle innervated by this nerve. The amount of time between the stimulation of a motor nerve and a muscle response (distal latency) includes the conduction time along the unmyelinated axonal endings and the neuromuscular transmission time. The difference in latency between two points of stimulation is used to calculate the nerve conduction velocity in m/sec. The amplitude of the CMAP in the muscle reflects the number of innervated muscle fibers. This method can discriminate between axonal and demyelinating neuropathies, and correlates well with morphological findings.
NCV can be used to locate the site of entrapment in mononeuropathies. Local slowing and local impulse blockade of sensory fibers, and decreased or absent sensory nerve action potentials with stimulation proximal and distal of a lesion can be observed. Several techniques are used to detect these changes, including stimulation at different sites, comparison of conduction properties in adjacent nerves (median/ulnar) and the "inching" technique.
NCV can be used intraoperatively, mainly by orthopedic and neurosurgeons, to facilitate decisions in surgery and nerve surgery.
While the measurement of motor nerves at the extremities is methodologically easy, the measurement of NCVs of proximal nerve segments is problematic. For some proximal motor nerves, like the long thoracic and femoral nerves, only the latencies can be assessed with certainty. Age, height, and temperature are also factors that have to be considered.
Sensory NCV studies Unlike motor conduction, where a terminal branch and synapse contribute to latency, no synapse occurs between the stimulating site and recording site in a sensory nerve. Sensory nerve action potentials (SNAPs) can be measured in both the orthodromic and the antidromic direction. This means that stimulation of the main (mixed) nerve trunk results in a signal at the distal sensory nerve, or conversely stimulation of the distal sensory branch yields a signal at the nerve trunk.
The studies can be done with surface recordings, or recording with needle electrodes using a near-nerve technique. Antidromic techniques with surface recording are commonly used. Near-nerve recordings are time-consuming but are able to pick up even low signals, and allow the assessment of several populations conducting at different velocities (dispersion), which may be necessary for diagnosis in sensory neuropathies.
Sensory nerve studies are more sensitive than motor studies at detecting nerve pathology.
Sensory responses are more sensitive to temperature than motor responses in regard to conduction velocity, but not to nerve action potential amplitude. Correction factors or warming of the extremity must be considered.
Radiculopathies do not affect the sensory potentials, as the dorsal root ganglion, which lies within or outside the neural foramen, is not affected. This can be useful if electrophysiology is needed to distinguish between radiculopathy and plexopathy or neuropathy.
References Kline DG, Hudson AR (1995) Nerve injuries. WB Saunders, Philadelphia
Rivner MH, Swift TR, Malik K (2001) Influence of age and height on nerve conduction. Muscle Nerve 24: 1134-1141
Rutkove SB (2001) Effects of temperature on neuromuscular electrophysiology. Muscle Nerve 24: 867-882
Rutkove SB (2001) Focal cooling improves neuronal conduction block in peroneal neuropathy at the fibular neck. Muscle Nerve 24: 1622-1626
Late responses Late responses (e.g. F wave) are techniques to obtain information about the proximal portions of the nerve and nerve roots. This is important because few studies permit access to proximal parts of the PNS.
- The A wave (axon reflex) is a small amplitude potential of short latency (10-20 ms) and high persistence, usually elicited by submaximal stimulation. It is generated by normal or pathologic axon branching. It may occur in neuropathies, possibly due to sprouting.
- The F wave is an antidromic/orthodromic motor response and can be generated from any motor nerve. It has a variable latency and amplitude and can be confused with A waves. It is clinically used to evaluate proximal portions of the nerves.
- The H reflex is an orthodromic sensory/orthodromic motor response and is usually obtained in the L5/S1 portion, evaluating a S1 radiculopathy.
- The blink reflex and the masseteric reflex are used in the evaluation of cranial nerve and brainstem function. The blink reflex has a reflex arc consisting of an orthodromic trigeminal nerve and an orthodromic motor facial nerve loop. Primary and secondary uni- and contralateral responses reveal reflex patterns in the brain stem. The masseteric reflex is induced by tapping on the chin, and results in a response in the masseteric muscle.
Proximal nerve stimulation studies are more difficult than the "standard" NCV Proximal nerve studies. Proximal stimulation can be performed near-nerve with electrical or stimulation studies magnetic stimulation. The proximal parts of nerves like the long thoracic, phrenic, spinal acessory, suprascapular, axillary, musculocutaneous, femoral and sciatic nerves can be evaluated by this method.
Repetitive nerve stimulation is most commonly used to investigate the function Repetitive nerve of the neuromuscular junction. A train of stimuli is given to a peripheral nerve stimulation in a defined frequency. The resulting CMAP amplitudes and areas are recorded and measured. Repetitive nerve stimulation allows a distinction between pre-and postsynaptic transmission disorders. MG is usually detected at low frequency 3 Hz stimulation, whereas high frequency stimulation (20 Hz) leads to an incremental response in the Lambert Eaton Myasthenic syndrome (LEMS). Although this technique is extremely useful, decremental and incremental responses can be observed in other conditions.
Evoked responses, in particular somatosensory evoked responses, allow mea- Evoked responses surement of central structures like the posterior columns, and provide additional insight into peripheral-central conduction properties.
Magnetic stimulation techniques are usually performed with a coil and can be Magnetic stimulation used to measure central conduction time as a parameter for central motor techniques function. Stimulation at the vertebral column and in proximal nerve segments allows measurement of these difficult to approach segments.
Electromyography Electromyography (EMG) is the basic method to study skeletal muscle function.
In Europe, concentric needle electrodes are mainly used, while in the USA monopolar needles in combination with surface reference electrodes are used. The application of surface electrodes for the assessment of muscle function is still a matter of debate.
Three different steps of evaluation of the electrical activity are usually taken:
- Insertional activity is created by small movements of the needle electrode, and results in amorphous discharges with short durations. It is usually increased in neuropathic processes, but is difficult to quantify, and often labeled "irritability". Strictly speaking, pathologic conditions like myotonia, neuromyotonia, myokymia, and complex repetitive discharges (CRDs) belong in that category, but are usually considered spontaneous activity.
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