The ulnar nerve is a sensorimotor nerve that arises from the lower trunk of the brachial plexus, converts into the medial cord, further diving into fibers of C8 and T1. It provides motor innervation to the intrinsic hand muscles, except the flexor carpi ulnaris, the flexor digitorum profundus medially, the thenar, and lumbrical muscles. The ulnar nerve also provides sensory innervation to the medial aspect of the forearm, wrist, fourth digit, and the entire fifth digit.
In the clinical setting, the typical patient presentation includes pain, numbness, paresthesia, and the fifth digit and medial aspect of the fourth digit. Often, these symptoms can be exacerbated by flexion of the elbow, especially while sleeping. If the neuropathy is more severe, patients may complain of hand weakness and frequent dropping of objects.
Intrinsic hand muscle atrophy, especially of the first dorsal interosseus, may also be seen. Additionally, a weak abductor digiti minimi and positive Froment sign are indicative of ulnar neuropathy. When testing for the Froment sign, the patient is asked to grasp a piece of paper between the first and second digits while the examiner attempts to pull the paper out from the patient's grasp. The Froment sign is considered positive if there is noticeable hyperflexion at the interphalangeal joint of the flexor pollicis longus, which is a compensatory mechanism for a weakened adductor pollicis longus.
Wartenberg's sign is used to assess the motor weakness of the ulnar nerve. In this test, the patient is instructed to hold their fingers fully adducted with the metacarpophalangeal joint, proximal interphalangeal, and distal interphalangeal joints in full extension. If it is found that the small finger drifts away from the others into abduction, this is known as a positive Wartenberg sign.
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 body of neurons contains 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 transmitter of 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 can vary according to the effector organ.
Let us briefly review the physiology of nerve conduction for completion. 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 passive and active energy (ATP) expending receptors in the cells. During depolarization, there is an influx of Na+ ions (given higher concentration extracellularly) via partial voltage-gated sodium channels that, once they reach a positive voltage potential, open and lead to a propagating stimulation. 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. 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. In fact, about 30 to 45% of all cases of Guyon syndrome are due to ganglion cysts. 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.
Martin-Gruber Anastomosis (MGA)
It is a common variant, almost strictly (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 the flexor pollicis brevis) muscles. It can affect the electrodiagnostic study interpretations.
Riche-Cannieu Anastomosis (RCA)
A common anatomical variant is an anastomosis distally of the recurrent branch of the median nerve and deep branch of the ulnar nerve. 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.
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, as they are vital to understanding the changes seen in ulnar neuropathies.
Nerve conduction studies (NCS) are used to evaluate large myelinated sensory and motor fibers. Small myelinated fibers in the autonomic and spinothalamic tracts need specialized studies for neuropathy evaluation and will not be picked up by the NCS (e.g., Quantitative Sudomotor Axon Reflex Test-QSART, useful to diagnose type II diabetes mellitus polyneuropathy). NCS should be performed in ideal conditions, including room temperature, affecting measurements such as latency, duration, and amplitude. NCS is an electrodiagnostic study that measures the summated actions potentials of sensory (SNAPs) and motor muscle fibers (CMAPs) and has multiple measurements that are important to review.
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 the insertional, spontaneous, and exertional activity of motor units.
Neuromuscular Clinical Definitions
Diagnosis of ulnar neuropathy begins with a thorough history and physical examination. In most cases, it can be diagnosed clinically without the need for electrodiagnostic testing. When doubt exists, NCS and EMG can be used to localize and confirm the diagnosis, as well as assess the severity of the neuropathy. 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.
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 itself.
Electrodiagnostic studies for ulnar neuropathies require EMG/NCS hardware and software, conduction gel, measuring tape, surface electrodes, needle electrodes, ring electrodes, and alcohol pads for skin sterilization.
An adequately trained neurodiagnostic personnel is 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.
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.
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 overview of the studies, which will be performed to properly diagnose ulnar neuropathy 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. One should ideally examine at least 2 extremities, performing both sensory and motor nerve conduction studies, as well as EMG needle testing in both proximal and distal muscles for comparison.
As with all nerve conduction studies, the temperature of the patient’s limbs should be kept warm at 32 degrees Celsius. Colder temperatures can cause mistakenly increased amplitudes, prolonged latencies, and slowed conduction velocities on NCS. To minimize the electrical interference, which can occur while doing the study bedside, a notch filter should be used, and if possible, all unnecessary machines turned off, including unplugging the clinic/hospital bed.
To evaluate for ulnar neuropathy, electrodes are placed over the muscles innervated by the ulnar nerve (e.g., 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 at the level 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.
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.
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. Ulnar neuropathy at the elbow, in the medial epicondyle, shows proximal decreased motor nerve conduction velocity with or without conduction block, decreased amplitude of the ulnar CMAP. On the other hand, a CMAP amplitude that is 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.
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 also leading to a larger response at the level of the elbow. 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 <= to 25% between elbow and wrist would be an acceptable dispersion in patients with an MGA. Stimulation of the median nerve at the wrist will lead to a smaller response than expected given crossover. A difference larger than 25% would raise suspicion for a true ulnar or median nerve neuropathy, respectively.
F-waves and H-reflexes are not helpful when evaluating ulnar neuropathy due to non-specificity.
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, there will be more involvement of the FDI as opposed to the ADM. In ulnar neuropathy at the elbow, the flexor carpi ulnaris (FCU) and flexor digitorum profundus (FDP) are spared. 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.
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 distribution of the ulnar nerve, complete hand blocks, and brachial plexus blocks.
Physical and occupational therapists can also help patients to perform activities of daily living and improve functionality. Effective collaboration amongst the various disciplines is necessary to ensure the best possible outcome for patients.
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