Electrodiagnostic Evaluation of Ulnar Neuropathy

Anatomy and Physiology

Physiology of Nerve Conduction

Neurons are responsible for receiving, integrating, and propagating the summation of excitatory and inhibitory electrical potentials from other cells. Neurons are composed of dendrites, bodies, and axons. Dendrites receive information from other neurons and serve a critical role in neuroplasticity. The neurons contain the euchromatic nucleus and organelles responsible for producing proteins and chemicals essential for proper neurotransmission at the synapse. Polyribosomes are clustered and can be seen under electron microscopy as Nissl bodies. Axons serve as the conductor and transmit information to other individuals or networks of neurons, glands, and muscles. The axonal fibers terminate at the synapse with the electrochemical activation of a complex and diverse quantity of ligand-gated or G-coupled receptors that vary according to the effector organ.

At rest, neurons have an intracellular resting membrane potential of -70 mV, reflecting a steady-state concentration of sodium (Na+) and potassium (K+) ions intracellularly and extracellularly. This is maintained through the cells' passive and active energy (adenosine triphosphate [ATP]) expending receptors. During depolarization, there is an influx of Na+ ions (given higher concentration extracellularly) via partial voltage-gated sodium channels that open and lead to a propagating stimulation once they reach a positive voltage potential. At the peak of the action potential, voltage-gated Na+ channels close, and voltage-gated potassium K+ channels open, leading to the exit of ions from inside to outside the cell through a concentration gradient. This leads to the hyperpolarization of cells. A Na+/K+ ATPase in the neurons brings the ion gradient back to baseline (to resting potential) by expending energy to expel 3 Na+ out of the cell for 2 K+ inside the cell. The nerve conduction is coordinated with directionality and propagation of depolarization throughout the entire axon, with ultimate activation or neurotransmitters' inhibition into the synaptic cleft. 

Unmyelinated fibers conduct in the range of 1 to 5 m/sec. On the other hand, myelinated motor and sensory nerve axons have conduction velocities up to 150 m/sec. This process is called saltatory conduction. Myelin is produced by Schwann cells, which are concentrically wrapped around axons. These myelin sheaths have gaps called nodes of Ranvier, where action potentials occur and propagate quickly until the next node is met. Therefore, current flows passively and jumps from node to node.

Anatomy of the Ulnar Nerve

The anatomy of the ulnar nerve makes it susceptible to compression. It is the second most common nerve entrapment in the upper extremity after carpal tunnel syndrome. The most common site of entrapment is at the elbow in the cubital tunnel, as the nerve is superficial at this location. Entrapment at the elbow can be caused by leaning on the elbow for prolonged periods, medial epicondyle fractures, chronic subluxation, arthritis within the ulnar groove, and bony deformity. Approximately 75% of medial epicondylar fractures occur in male pediatric patients, with 10% to 15% of those resulting in ulnar nerve dysfunction.[4] Less commonly, ulnar nerve entrapment can also occur at the wrist in the Guyon canal, formed by the pisiform and hook of the hamate. The most common causes of ulnar neuropathy at the wrist are repetitive trauma and ganglion cysts.[5] About 30 to 45% of all cases of Guyon syndrome are due to ganglion cysts.[6] Ulnar nerve compression at the wrist is commonly seen in bikers due to excess pressure from handlebars and desk workers due to prolonged periods of typing.[7]

Martin-Gruber anastomosis 

Martin-Gruber Anastomosis (MGA) is a common variant, almost strictly (with exceptions if the main median trunk crosses over, some sensory innervation can be expected) motor fibers of the median nerve from the anterior interosseous nerve crossover to the ulnar nerve in the forearm. It is thought to run in families in an autosomally dominant fashion. Depending on the variant subtype, it can innervate the first interosseous muscle (FDI), hypothenar, or thenar (adductor pollicis and flexor pollicis brevis) muscles. It can affect the electrodiagnostic study interpretations.

Riche-Cannieu anastomosis 

Riche-Cannieu Anastomosis (RCA), a common anatomical variant, is an anastomosis distally of the median nerve's recurrent branch and the ulnar nerve's deep branch. It does not affect electrodiagnostic study interpretation. In extremely rare cases, the ulnar nerve may innervate all the muscles in the digits of the hand (ulnar nerve) after a traumatic lesion. This could lead to exaggerated weakness in muscles normally innervated by the median nerve or vice-versa. 

Indications

Electrodiagnostic studies can play a vital and ancillary role in diagnosing mono and polyneuropathies, including those that affect the ulnar nerve. We will review basic electrodiagnostic concepts and neuromuscular clinical definitions vital to understanding the changes in ulnar neuropathies.

Nerve Conduction Studies 

Nerve conduction studies (NCS) evaluate large myelinated sensory and motor fibers. Small myelinated fibers in the autonomic and spinothalamic tracts need specialized studies for neuropathy evaluation. They will not be picked up by the NCS (eg, Quantitative Sudomotor Axon Reflex Test [QSART], which is useful for diagnosing type II diabetes mellitus polyneuropathy). NCS should be performed in ideal conditions, including room temperature, affecting latency, duration, and amplitude measurements. NCS is an electrodiagnostic study that measures the summated action potentials of sensory (SNAPs) and motor muscle fibers (CMAPs). It has multiple important measurements to review.[8]

• Conduction velocity: The speed of the fastest conducting motor axon; tends to be prolonged in demyelinating disorders
• Amplitude: Voltage difference from baseline to maximal negative peak with depolarization, a reflection of intact, nondiseased muscle fibers that can depolarize
• Latency: Reflects the speed of neurotransmission; defined as the time from stimulus to initial CMAP deflection from baseline
• Duration: Reflects the synchronous transmission of action potentials; can give a global evaluation of the motor fiber conduction, with many fibers slowing conduction, affecting the duration of action potentials; measured as the time from an initial deflection from baseline to the first crossing.

Electromyogram 

An electromyogram (EMG) measures the integrity of the nerve-muscle connections when electrical stimulation is applied, with an anatomical evaluation of nerves, roots, and plexuses. It predominantly evaluates motor unit action potentials of type 1 muscle fibers and does not pick up type 2 fibers. EMG will evaluate motor units' insertional, spontaneous, and exertional activity.

• Insertional activity: Muscle fiber action potentials burst, provoked by the irritation of the needle electrode
  • Increased activity in neuropathy and myopathic processes
  • Decreased activity in muscle necrosis
• Spontaneous activity: Can be silent (in normal tissue) or show abnormal waves
  • Fibrillations and positive waves: The rhythmic firing of individual muscle fibers representing subacute (7 to 10 days) denervation or muscle inflammation
  • Fasciculations: The irregular popcorn-like firing of muscle fibers representing acute muscle-nerve denervation
  • Complex repetitive discharges: Rhythmic, frequent, complex, and rumbling running motor-like firing of motor unit action potentials (MUAPs)
  • Myotonic discharges: Waxing and waning diver bomber-like firing of muscle fibers
• Exertional activity: MUAP activity, recruitment, firing rate, etc, during muscle contraction 
  • Recruitment: Number of firing motor units firing x force applied during voluntary contraction
    • Early recruitment in myopathy
    • Delayed recruitment in neuropathic and axonal disorders
  • Firing rate: Frequency of discharges during voluntary contraction
  • Innervation of muscle fibers: Polyphasic versus single motor unit innervation
    • Single motor unit innervation can signal reinnervation from chronic neuropathic lesions.
  • Amplitude: Similar to the NCS definition
    • High amplitude can be seen in neuropathic lesions.
  • Duration: Similar NCS definition 
    • The increased duration can be seen in neuropathic/axonal injury.

Neuromuscular Clinical Definitions

• Myopathy: Diseased muscle fibers
• Motor neuron disease: Diseased motor neuron unit within the upper or lower motor spectrum
• Neuromuscular junction disorder: Diseased connection or neurochemical process between the nerve and muscle at the synapse
• Myelopathy: Diseased or damaged spinal cord
• Neuronopathy: Diseased cell body. It can be sensory, motor, sensorimotor, or related to ganglions
• Neuropathy: Diseased cell body, axon, or myelin

Ulnar Neuropathy

Diagnosis of ulnar neuropathy begins with a thorough history and physical examination. In most cases, it can be diagnosed clinically without electrodiagnostic testing. When doubt exists, NCS and EMG can be used to localize and confirm the diagnosis and assess the severity of the neuropathy.[9] If surgical intervention is indicated, electrodiagnostic testing can aid the surgeon in identifying the exact area of nerve entrapment. Electrodiagnostic testing can also differentiate between ulnar neuropathy, C8 radiculopathy, plexopathies, and other forms of mononeuropathy. 

Contraindications

Performing electrodiagnostic studies in patients with ulnar nerve entrapment has few absolute contraindications. Needle EMG is contraindicated in those with severe bleeding disorders. Needles should also never be inserted into areas of active soft tissue infection. Nerve conduction studies are contraindicated in patients with implanted cardiac defibrillators or if connected to external defibrillators. Patients should be screened for pacemakers, and electrical stimulation should not be performed directly on or near the device. 

Equipment

The equipment required for electrodiagnostic evaluation of ulnar neuropathy includes:

• EMG/NCS hardware and software
• Conduction gel
• Measuring tape
• Surface electrodes
• Needle electrodes
• Ring electrodes,
• Alcohol pads for skin sterilization

Personnel

Adequately trained neurodiagnostic personnel are essential for proper evaluation. Familiarity with EMG/NCS hardware and software, electrode placement, and data interpretation of NCS and EMG measurements is indispensable. An interdisciplinary team that includes technicians, nurses, primary/consultant physicians, and neuromuscular specialists is essential to coordinate care and obtain the most accurate and precise data from the NCS and EMG studies.

Preparation

As with all nerve conduction studies, the temperature should be ideally between 32 and 33 degrees Celsius to avoid artifactual measurements. A warming lamp may be used to achieve proper limb temperature. 

Technique or Treatment

A comprehensive review of the patient’s history and clinical course and a complete physical exam must be performed before performing any diagnostic study. The diagnostician will inform the patient at the bedside of the indications and provide an overview of the studies, which will be performed to diagnose ulnar neuropathy properly with electrodiagnostic testing. The diagnostician must thoroughly explain the risks and benefits of the exam to the patient. Consent should also be obtained before initiating testing. For comparison, one should ideally examine at least 2 extremities, performing sensory and motor nerve conduction studies and EMG needle testing in both proximal and distal muscles.

As with all nerve conduction studies, the temperature of the patient’s limbs should be kept warm at 32 ° Celsius. Colder temperatures can cause mistakenly increased amplitudes, prolonged latencies, and slowed conduction velocities on NCS. A notch filter should be used to minimize the electrical interference that can occur while doing the study bedside. All unnecessary machines should be turned off, including unplugging the clinic/hospital bed if possible.

To evaluate for ulnar neuropathy,  electrodes are placed over the muscles innervated by the ulnar nerve (eg, the abductor digiti minimi, quinti, or the first dorsal interosseous muscle) and the tendon of the first or fifth digit. The ulnar nerve is stimulated above or below the elbow and the wrist to help localize the involved sites of entrapment or neuropathies. Short-segment stimulation can increase the sensitivity of the study.

Complications

As with all electrodiagnostic studies in any setting and for any indication, the risk of complications is low. There is always a small risk of bleeding or introducing infection with needle studies. 

Clinical Significance

Sensory Nerve Conduction Studies

In ulnar neuropathy, sensory nerve action potentials (SNAP) are affected as the lesion is distal to the dorsal root ganglion, whereas, in C8 radiculopathy, SNAPs are unaffected as the lesion is proximal to the dorsal root ganglion. In ulnar neuropathy, the amplitude of the ulnar nerve and dorsal ulnar cutaneous SNAP is decreased.

Motor Nerve Conduction Studies

The ulnar nerve can be affected at the level of the epicondylar groove or more distal in the cubital tunnel. Compound muscle action potential (CMAP) conduction velocities in these anatomical locations can help distinguish and localize the neuropathy. In the medial epicondyle, ulnar neuropathy at the elbow shows proximal decreased motor nerve conduction velocity with or without conduction block and decreased amplitude of the ulnar CMAP. On the other hand, a CMAP amplitude 2 cm distal from the medial epicondyle represents an entrapment in the cubital tunnel. A caveat to these findings is the existence of anatomical variants, especially Martin-Gruber anastomosis.

MGA

Knowledge of this variant is important in electrodiagnostic interpretations. A patient without this variant produces mostly equivocal CMAP amplitude either at the wrist or abductor pollicis brevis (thenar eminence) is stimulated. However, in a patient with an MGA, the wrist response is smaller because of the median nerve axonal crossover, consequently leading to a larger response at the elbow level. When the ulnar nerve at the hypothenar eminence (abductor digiti minimi or FDI) is stimulated, the median nerve fibers crossover distally, leading to a larger conduction/amplitude compared to the elbow, which could lead to misinterpretation of a positive conduction block. A difference of less than or equal to 25% between the elbow and wrist would be acceptable in patients with an MGA. Stimulation of the median nerve at the wrist will lead to a smaller response than expected, given the crossover. A difference larger than 25% would raise suspicion for a true ulnar or median nerve neuropathy, respectively. 

Late Responses

F-waves and H-reflexes are not helpful when evaluating ulnar neuropathy due to nonspecificity.

EMG

In electromyography, the most commonly tested hand muscles innervated by the ulnar nerve are the abductor digiti minimi (ADI) and the first dorsal interosseus (FDI). If an axonal lesion is present, these muscles will likely be positive compared to the forearm muscles. If the lesion is distal to the Guyon canal, the FDI will be more involved than the ADM. The flexor carpi ulnaris (FCU) and flexor digitorum profundus (FDP) are spared in ulnar neuropathy at the elbow. If an acute axonal lesion is present, high amplitude, delayed duration, fibrillation potentials, and positive sharp waves may be seen in the intrinsic hand muscles innervated by the ulnar nerve and the FCU. Chronic and severe proximal or distal ulnar nerve axonal injury or entrapment can lead to delayed recruitment and monophasic waves. Cervical paraspinal muscles and C8 can have similar findings and should be tested to rule out C8 radiculopathy.  

Enhancing Healthcare Team Outcomes

Ulnar neuropathy is a relatively common condition seen in the outpatient clinical setting. A thorough history and physical examination are vital in determining the diagnosis. Electrodiagnostic studies can be used by primary care physicians, physiatrists, and neurologists to aid in confirming the diagnosis. If conservative measures, such as anti-inflammatory medications and bracing or splinting, are unsuccessful, orthopedic surgical intervention may be indicated. Ultrasound-guided nerve blocks can also be performed in the ulnar nerve distribution, complete hand blocks, and brachial plexus blocks.[10] 

Physical and occupational therapists can also help patients perform daily living activities and improve functionality. Effective collaboration amongst the various disciplines is necessary to ensure the best possible outcome for patients.