Mononeuropathies

Acute Nerve Compression

When nerve tissues are subjected to mechanical compression, some of the compressed tissues are displaced to sites of lower pressure. This is especially the case for acute compression neuropathies, such as proximal radial neuropathy (“Saturday night palsy”) and neuropathies secondary to tourniquet compression. With acute nerve compression, damage is concentrated at the compression edges. The predominant injury at this level implies that the pressure gradient itself, rather than the absolute pressure, is the critical factor for acute compression neuropathy.

In the setting of experimental acute compression, the earliest histopathologic change seen within just a few hours is an invagination of one paranodal segment into its adjacent paranode. Directed toward the uncompressed tissue, paranodal myelin, tethered to the axon, may be grossly distorted, resulting in invagination on one side and passive stretching on the other side. Longitudinal movement of the axon relative to the Schwann cell accompanies the paranodal myelin alterations. In extreme cases, myelin lamellae may be ruptured. These findings are reminiscent of intussusception of the bowel, suggesting that that the pressure gradient between compressed and uncompressed nerve provides definitive forces causing axoplasm extrusion “similar to toothpaste from a tube.”

The sequential events of acute, focal compression initially include an early combined extrusion of endoneurial fluid (i.e., the fluid between fibers), axonal fluid, and cytoskeletal elements, and subsequently distortion of myelin and Schwann cell elements. A second slower phase is attributed to further endoneurial and axonal fluid extrusion, paranodal disruption, Schwann cell cytoplasm extrusion, and displacement of other tissue elements. Additional damage (e.g., of the cytoskeletal network) may occur at more extreme pressures or with protracted compression. Nodes of Ranvier are frequently obscured or lengthened because of displaced paranodal myelin.

Classic nerve conduction studies provide a means to measure the magnitude of the nerve action potential conduction block as well as focal conduction slowing. These findings correlate with the degree and duration of compression. Focal ischemia may also contribute in some compression neuropathies, particularly in combination with the direct effects of pressure. Transient nerve block, for instance, when a limb “goes to sleep” for a few seconds, may be related to modest external pressures, and/or may be primarily caused by focal ischemia because no recognizable structural nerve pathology has been convincingly demonstrated. For more severe cases of acute compression, nerve fiber remyelination may occur weeks to months after resolution of the acute compression.

Clinical Assessment

Careful history and meticulous neurologic examination are essential for evaluation of mononeuropathies. Initially, one defines the precise motor and sensory deficits and next decides whether this fits an individual peripheral nerve's anatomic distribution. This is relatively easily accomplished with acute nerve trauma, that is, a laceration or gunshot wound. In contrast, most mononeuropathies have a relatively ingravescent course characterized first by intermittent paresthesias initially not producing clinically definable functional loss.

Each peripheral nerve has a unique clinical anatomic signature vis-à-vis motor and sensory deficits when these nerves are compromised. This is illustrated by the seemingly complicated cutaneous sensory distribution of the median, radial, and ulnar nerves in the hand. With this knowledge, clinicians are often able to outline characteristic clinical features of a specific pattern of compromised function appropriate to a mononeuropathy. Frequently, symptoms of a mononeuropathy are stereotyped and sometimes evanescent, such as with the carpal tunnel syndrome.

Occasionally, underlying systemic illnesses predispose to the occurrence of more than one acute mononeuropathy, that is, mononeuritis multiplex. A sudden footdrop secondary to a fibular (peroneal) nerve lesion is followed in days to weeks by another mononeuropathy, such as a wristdrop from an acute radial nerve lesion, and soon thereafter another nerve becomes acutely compromised. Systemic vasculitides, such as occurs in polyarteritis nodosa, are often responsible. Hereditary neuropathies with liability to pressure palsies (HNPP) lead to recurrent multiple neuropathies in a chronic setting (see Plate 5-22 ). Sometimes symptoms of a possible mononeuropathy actually represent an initial sign of a plexus, nerve root, spinal cord, or brain lesion.

Patients with recurrent hand numbness or weakness require consideration for transient cerebral ischemic attacks or, rarely, an intracranial tumor such as a meningioma. Parasagittal cerebral lesions may occasionally manifest primarily with foot weakness. Individuals presenting with hand weakness but no sensory loss or pain may have a deep ulnar motor lesion within their medial palm or even motor neuron disease.

Neck or low back pain usually indicates a protruded or herniated disk (nucleus pulposus) affecting one specific nerve root. Often this discomfort radiates into an arm or a leg and is associated with tingling and numbness (paresthesias), and sometimes weakness is confined to the distribution of a nerve root. However, this clinical picture is not always straightforward enough to allow for simple clinical judgment to make a diagnosis based on history alone. For example, in the common setting of a footdrop, the clinician needs to examine carefully the leg muscles to define whether the affected muscles have only a peroneal nerve distribution with weakness confined to dorsiflexors (tibialis anterior) and evertors (peroneus longus). In contrast, the affected muscles may have an L5 nerve root derivation, that is, tibialis anterior primarily, but also a subtle inability to invert the foot because of weakness of the tibialis posterior. These muscles are innervated by two different peripheral nerves, fibular and tibial, and the weakness is thus compatible with a specific L5 radiculopathy. This distinction is sometimes difficult to make initially, and thus electromyography (EMG) is the first study of choice.

Often, the degree of weakness is more profound in a mononeuropathy than a nerve root lesion because the affected muscles are solely dependent on that nerve, whereas a nerve root lesion does not affect all fibers going to the affected muscles. For example, with a wristdrop where there is concomitant C6 and C7 root supply, if just the C7 root is affected, the muscles continue to have partial innervation from the C6 root, and thus there is not a total paralysis of the wrist and finger extensors. In contrast, if the radial nerve is damaged, there is no overlapping safety feature of multiple innervations as in nerve root disorders. Here the deficit's severity is directly related to how significant the damage is within that nerve itself. Often, total paralysis occurs with acute radial nerve damage. Muscle atrophy develops when there is significant peripheral denervation.

Measuring extremity circumference may document significant side-to-side asymmetries representative of muscle atrophy and, by inference, anterior horn cell, nerve root, or peripheral nerve damage. Patients with brachial or lumbosacral plexus lesions are less likely to have neck or back pain but, rather, pain within the affected extremity. Here the numbness may be more diffuse, and muscles are weakened within the distribution of multiple peripheral nerves/nerve roots.

Numbness rather than pain is much more common with early mononeuropathies. The symptom onset and progression can help in diagnosis. Because sensory examination is the most subjective part of the neurologic examination, occasionally this is difficult to define clearly. Sometimes the patient can provide the most accurate assessment by roughly outlining the area in question using a finger to demonstrate the area of diminished sensation; this is best demonstrated with meralgia paresthetica (see Plates 5-15 and 5-16 ), where the patient outlines an elliptic loss of sensation on the lateral thigh. These assessments often clarify whether the pattern of sensory loss is specific to one peripheral nerve or nerve root dermatome. Meralgia paresthetica best illustrates this with lateral thigh sensory loss secondary to a lateral femoral cutaneous nerve lesion. Often, it is easier for the patient to outline the precise deficit than the clinician.

Percussion over an affected nerve frequently elicits paresthesias within its specific distribution: the Tinel sign . This is best performed using the small head of a percussion hammer; this can be elicited in many mononeuropathies, particularly the median nerve at the wrist, (i.e., carpal tunnel syndrome [CTS]), ulnar nerve at the elbow, radial nerve over the humerus spiral groove, and fibular (peroneal) nerve at the fibular head.

Mononeuropathy: Diagnostic Studies

Sometimes clinical neurologic examination is not precise enough to provide early diagnosis of mononeuropathies. Electrodiagnostic studies are the method of choice for defining the precise anatomic distribution of peripheral nerve damage. This includes nerve conduction studies (NCS) and needle EMG. Thus it is possible to assess the quality of peripheral nerve conduction as well as whether there is damage to muscles specifically innervated by that nerve. NCS allows identification of the site of nerve damage whenever the nerve's myelin is chronically damaged. Examples include chronic ligamentous thickening over the carpal tunnel (see Plate 5-10 ), sudden sustained acute pressure over the radial nerve at the humerus (see Plate 5-13 ), or fibular (peroneal) compression at the knee (see Plate 5-19 ).

Early signs of a carpal tunnel syndrome are best defined by sensory NCS, and later motor NCS, demonstrating prolongation of the time for nerve conduction across the wrist (distal latency). Motor NCS are especially useful for defining more proximal nerve blocks, that is, at the elbow (ulnar nerve), midhumerus (radial nerve), and knee (fibular [peroneal] nerve). This leads to a diminution of the compound muscle action potential (CMAP) just above the site of nerve block (see Plate 5-3 ). Conduction slowing provides another means to identify a nerve block. Here there is focal motor NC slowing (by 30%-40%, i.e., 10-20 m/sec) at the site of anatomic compromise, that is, across the fibular head for the fibular (peroneal) nerve.

Needle EMG also may provide a precise means to specifically define the affected muscles. When the nerve's axon is partially damaged, spontaneous firing of small muscle fibers occurs. These are known as denervation potentials, that is, fibrillation potentials and positive waves. Similarly, when the nerve lesion leads to significant damage, there is a diminution in the number of motor units firing. The healthy remaining motor units (MUPs) attempt to compensate by reinnervation; this results in larger MUPs that are recruited at increased frequency.

Patients with a footdrop secondary to a fibular (peroneal) nerve lesion demonstrate denervation signs confined to fibular-innervated muscles. However, if the footdrop is secondary to an L5 root lesion, signs of denervation are demonstrated not only in fibular (peroneal) muscles but also L5 muscles innervated by both peroneal and posterior tibial nerves. These include the posterior tibial, the gluteus medius, and the lumbosacral paraspinal musculature. Thus by combining NCS and EMG, the electromyographer has the ability to literally map the precise pathoanatomy of the nerve lesion.

Magnetic Resonance Imaging. MRI studies provide an increasingly used means to evaluate for occult tumors or congenital lesions ( Plate 5-4 ). Very rarely, certain congenital or acquired lesions, such as fibrous bands, may entrap the nerve without MRI identification. In this instance, surgical exploration based on the clinical and EMG findings may lead to a diagnosis.

Ultrasound. This modality is gaining an increased presence in some centers for more rapid identification of sites of nerve compression or entrapment.

Skeletal Radiograph. Rarely, bony abnormalities entrap a peripheral nerve. Examples include popliteal fossa bony exostoses entrapping the tibial nerve. The sciatic nerve is rarely entrapped at the pelvic ischium in babies.

Upper Arm

The median nerve is derived from the major cervical nerve roots (C5-8) and a minor first thoracic (T1) nerve root contribution. Within the axilla, various fascicles of these nerve roots join to form the lateral, medial, and posterior cords of the brachial plexus. Subsequently, a significant portion of the lateral and medial cords fuse to form the median nerve adjacent to the axillary artery. As the median nerve travels through the axilla and into the arm, it lies lateral to the brachial artery. Lower, near the coracobrachialis muscle insertion, the nerve moves medially over the brachial artery, descending toward the cubital fossa at the elbow. Occasionally, just above the elbow, the ligament of Struthers, a fibrous band extending from a small supracondylar spur to the medial epicondyle of the humerus, forms the roof of a tunnel for the median nerve and brachial artery to concomitantly pass through as they approach the elbow. Here the median nerve lies posterior to the bicipital aponeurosis (lacertus fibrosis), the intermediate cubital vein and superficial to the insertion of the brachialis muscle at the ulna tuberosity.

When performing venipuncture or arterial puncture, the close proximity of the median nerve to the intermediate basilic vein and brachial artery always must be considered. It is important to perform venipuncture immediately lateral to the bicipital tendon to avoid the brachial artery, which lies just medial to this prominent tendon. There are no significant median nerve motor or sensory branches within the arm.

Forearm

The median nerve enters the forearm between the long and short heads of the biceps muscle. Initially, it innervates the pronator teres (PT) muscle (C6, 7). Subsequently, it innervates three other forearm muscles: flexor carpi radialis (FCR ; C6, 7), palmaris longus (C7, 8, T1), and flexor digitorum superficialis (FDS; C7, 8). It also provides articular twigs to the elbow and proximal radioulnar joints.

Within the very upper forearm, the anterior interosseous nerve (AIN; the primary median nerve motor branch) is derived from the primary trunk of the median nerve. This is a primarily motor branch, coursing distally, superficial to the anterior interosseous ligament, accompanied by its anterior interosseous artery. The AIN innervates the lateral head of the flexor digitorum profundus (FDP; C7, 8) a muscle providing tendons that flex the most distal interphalangeal joints of the index and middle fingers, In addition, the AIN supplies the flexor pollicis longus (FPL; C7, 8, T1), which flexes the distal phalanx of the thumb, and the pronator quadratus (C7, 8, T1), which aids in wrist pronation. Thus the AIN, through its innervation of the FDP and the flexor pollicis longus (FPL), provides the essential means for allowing the most important very fine movements, leading to flexion of the most distal phalange of the index and middle fingers, and for allowing the thumb to make the all important pinch movement possible.

In the lower forearm, the main trunk of the median nerve lies deep to the FDS and superficial to the FDP . Eventually, the primary median nerve trunk becomes more superficial, lying between the tendons of the palmaris longus and the flexor carpi radialis, (FCR; C6, 7). Here the median palmar cutaneous branch originates, arising 3 to 4 cm above the flexor retinaculum and descending over this area to supply the skin of the median palm and the thenar eminence. This is the first and only median sensory branch that is defined before the median nerve enters the hand.

In the forearm, the median and ulnar nerves are occasionally interconnected by fibers passing between these nerves. The most common are the median-ulnar anastomosis (Martin-Gruber syndrome), wherein portions of the median nerve branch off within the forearm to join the ulnar nerve. Typically, when this ulnar nerve variant reaches the hand, these median fibers will subsequently innervate their appropriate median intrinsic muscles. This is important for electromyographers to recognize, especially when looking for ulnar nerve block at the elbow as discussed on the ulnar nerve plates.

Primary Median Trunk

All median nerve function is potentially compromised with very proximal lesions. In contrast to the patient with carpal tunnel syndrome (CTS) experiencing finger paresthesias, median trunk lesions present with combined sensory/motor dysfunction not only affecting the classic 3.5 lateral digits but also diminished palmar sensation because of median palmar cutaneous branch involvement, something not characteristic of CTS. All median-innervated muscles may be affected. Often, these very proximal median lesions are idiopathic, although some may represent another brachial neuritis variant.

Supracondylar humerus fractures leading to nerve compression, entrapment, or total laceration are common causes of proximal median nerve lesions. The primary median nerve trunk is usually damaged; however, occasionally, because of the fascicular characteristics of peripheral nerves, the anterior interosseous nerve may be traumatized in isolation at a proximal site. Elbow dislocations, hyperextension of the arm, shoulder falls, lacerations, blunt nerve trauma, arterial or venous puncture, and repetitive pronation/supination are other mechanisms.

Median nerve entrapment rarely occurs just above the elbow at the ligament of Struthers. This is a fibrous band extending from a small supracondylar spur on the humerus to its medial epicondyle. Here it forms the roof of a tunnel, radiographically identifiable in about 2% of the population. Median entrapment has occurred with a distal humeral osteoid osteoma and congenital fibromuscular bands. Lipofibromas, hamartomas, neurofibromas, hemangiomas, juvenile cutaneous mucinosis, calcified flexor digitorum superficialis tendons, and abscess have also led to proximal median nerve lesions.

Anterior Interosseous Nerve (AIN)

Sometimes clinical involvement of the AIN is referred to as the pronator syndrome; however, this is not a well-accepted terminology because multiple other mechanisms, besides entrapment within the pronator teres, may be operative at this level. These patients present with a characteristic clinical picture, wherein they initially report difficulty with handwriting or placing a key in a lock. This symptom is related to an inability to pinch because concomitant damage to the flexor pollicis longus (FPL) and flexor digitorum profundus (FDP; C8, T1) muscles limit the patient's ability to flex the distal interphalangeal joints of both the index finger and thumb. The third muscle innervated by the AIN, the pronator quadratus (PQ, C8, T1) is clinically silent but provides localizing value with needle electromyography (EMG).

Some instances of idiopathic anterior interosseous neuropathies, having an acute onset and possibly representing an autoimmune process, have also been likened to a partial brachial neuritis variant. One interesting modification is a syndrome of acute median nerve compression by the bicipital aponeurosis at the elbow. Here an acute elbow pain develops during a maximal and vigorous contraction of the biceps brachii muscle. Examination demonstrates severe pain on median nerve palpation at the elbow as well as with triceps contraction when extending the elbow, but there is no neurologic compromise.

Evaluation

EMG is the primary means for identifying these proximal median nerve lesions as discussed above. The role of ultrasound and magnetic resonance imaging (MRI) await prospective analysis, but it is expected that these modalities will be particularly useful for both localization and sometimes identification of the pathology.