Transient damage to the axonal transport system without Wallerian degeneration by acute nerve compression

Lumbar plexus anatomy within the psoas muscle: implications for the transpsoas lateral approach to the L4-L5 disc

BACKGROUND

The transpsoas lateral surgical approach has been advocated as an alternative to direct anterior approaches for less invasive or minimally invasive access to the spine. Postoperative thigh pain, paresthesia, and/or weakness have been described after the use of this surgical approach. The purpose of this cadaveric anatomic study is to provide a description of the lumbar plexus as it relates to the transpsoas lateral surgical approach.

METHODS

Dissection of the lumbar plexus was performed in eighteen cadaveric specimens. Needle markers were placed in the L2-L3, L3-L4, and L4-L5 discs in the midcoronal plane. The anatomic structures were surveyed, and the proximity of the needle to the neural structures was observed.

RESULTS

In thirteen of the eighteen specimens, the femoral nerve received its contributions from the L2 to L4 nerve roots and was formed at the L4-L5 disc space. In all specimens, the femoral nerve passed dorsal to or directly at the midpoint of the disc. In three specimens, the needle displaced or was immediately adjacent to the femoral nerve. The femoral nerve was found between the needle and the posterior aspect of the L4-L5 disc space in thirteen of the eighteen specimens.

CONCLUSIONS

Because of the proximity of the neural elements, in particular the femoral nerve, to the center of the disc space, the transpsoas lateral surgical approach to the L4-L5 disc space will likely cause intraoperative displacement of neural structures from their anatomic course during retractor dilation. Careful attention should be paid to retractor placement and dilation time during transpsoas lateral access surgery, particularly at the L4-L5 disc.

A MILD ACUTE COMPRESSION INDUCES NEURAPRAXIA IN RAT SCIATIC NERVE

The pressure that induces neurapraxia in rat remains unrevealed. To determine the appropriate force to induce neurapraxia, two types of clips were applied to the sciatic nerve and were evaluated with functional, electrophysiological, and histological examinations. With a compression of 60 g/mm2, walking track analysis showed complete sciatic nerve paralysis one day postoperatively, but became normal in 14 days. Electrophysiologically, complete conduction block occurred one day post operatively, whereas the motor conduction velocity (MCV) below the compression site remained normal. Histologically, only limited signs of Wallerian degeneration were seen. The model in this study exhibited the features of neurapraxia.

Rapid recovery of serratus anterior muscle function after microneurolysis of long thoracic nerve injury

Background

Injury to the long thoracic nerve is a common cause of winging scapula. When the serratus anterior muscle is unable to function, patients often lose the ability to raise their arm overhead on the affected side.

Methods

Serratus anterior function was restored through decompression, neurolysis, and tetanic electrical stimulation of the long thoracic nerve. This included partial release of constricting middle scalene fibers and microneurolysis of epineurium and perineurium of the long thoracic nerve under magnification. Abduction angle was measured on the day before and the day following surgery.

Results

In this retrospective study of 13 neurolysis procedures of the long thoracic nerve, abduction is improved by 10% or greater within one day of surgery. The average improvement was 59° (p < 0.00005). Patients had been suffering from winging scapula for 2 months to 12 years. The improvement in abduction is maintained at last follow-up, and winging is also reduced.

Conclusion

In a notable number of cases, decompression and neurolysis of the long thoracic nerve leads to rapid improvements in winging scapula and the associated limitations on shoulder movement. The duration of the injury and the speed of improvement lead us to conclude that axonal channel defects can potentially exist that do not lead to Wallerian degeneration and yet cause a clear decrease in function.

Strain and excursion in the rat sciatic nerve during a modified straight leg raise are altered after traumatic nerve injury

PURPOSE

This study investigated the biomechanics of the sciatic nerve with hind limb positioning in live and euthanized Sprague-Dawley rats after traumatic nerve injury.

METHODS

With radiographic analysis, sciatic nerve excursion and strain were measured in situ during a modified straight leg raise, which included sequential hip flexion and ankle dorsiflexion. Comparisons were made between nerves in uninjured, sham-injured and mild crush-injured rats at the 7-day and 21-day recovery times.

RESULTS

Significant strain and proximal excursion of the sciatic nerve were observed in all groups during hip flexion, and additional increased strain was noted during dorsiflexion. Seven days after nerve injury, strain increased significantly during hip flexion (17.64+/-14.12%; p=0.0091) and dorsiflexion (22.56+/-15.47%; p=0.0082) compared to the sham-injured controls. At 21 days after injury, the strains were similar between the injured and sham-injured groups.

CONCLUSIONS

Nerve bed elongation during straight leg raise causes sciatic nerve strain and excursion towards the moving joint with the greatest movement nearest the moving joint. In the first week after injury, the maximal strain exceeded the level previously shown to impair nerve conduction and circulation.

The progressive nature of Wallerian degeneration in wild-type and slow Wallerian degeneration (WldS) nerves

BACKGROUND

The progressive nature of Wallerian degeneration has long been controversial. Conflicting reports that distal stumps of injured axons degenerate anterogradely, retrogradely, or simultaneously are based on statistical observations at discontinuous locations within the nerve, without observing any single axon at two distant points. As axon degeneration is asynchronous, there are clear advantages to longitudinal studies of individual degenerating axons. We recently validated the study of Wallerian degeneration using yellow fluorescent protein (YFP) in a small, representative population of axons, which greatly improves longitudinal imaging. Here, we apply this method to study the progressive nature of Wallerian degeneration in both wild-type and slow Wallerian degeneration (WldS) mutant mice.

RESULTS

In wild-type nerves, we directly observed partially fragmented axons (average 5.3%) among a majority of fully intact or degenerated axons 37-42 h after transection and 40-44 h after crush injury. Axons exist in this state only transiently, probably for less than one hour. Surprisingly, axons degenerated anterogradely after transection but retrogradely after a crush, but in both cases a sharp boundary separated intact and fragmented regions of individual axons, indicating that Wallerian degeneration progresses as a wave sequentially affecting adjacent regions of the axon. In contrast, most or all WldS axons were partially fragmented 15-25 days after nerve lesion, WldS axons degenerated anterogradely independent of lesion type, and signs of degeneration increased gradually along the nerve instead of abruptly. Furthermore, the first signs of degeneration were short constrictions, not complete breaks.

CONCLUSIONS

We conclude that Wallerian degeneration progresses rapidly along individual wild-type axons after a heterogeneous latent phase. The speed of progression and its ability to travel in either direction challenges earlier models in which clearance of trophic or regulatory factors by axonal transport triggers degeneration. WldS axons, once they finally degenerate, do so by a fundamentally different mechanism, indicated by differences in the rate, direction and abruptness of progression, and by different early morphological signs of degeneration. These observations suggest that WldS axons undergo a slow anterograde decay as axonal components are gradually depleted, and do not simply follow the degeneration pathway of wild-type axons at a slower rate.

MR imaging of entrapment neuropathies at the elbow

MR imaging has a valuable role in the evaluation of compressive neuropathies at the elbow. Specific MR signs in association with clinical findings can supply an accurate diagnosis. A review of normal anatomy, clinical features, and MR assessment of nerve entrapment syndromes at the elbow is presented.

Remyelination and recovery of conduction in cat optic nerve after demyelination by pressure

Pressure has been applied to the optic nerve of cats sufficient to block conduction in the large (Y) nerve fibers. The pressure block produces a mixture of axotomy and demyelination. By means of implanted electrodes, recovery of conduction in these fibers was monitored. There is a short-term recovery starting about 2 weeks after block induction and finishing at about 4 weeks. A later recovery starts at about 6-7 weeks and finishes at about 10-11 weeks. The remyelination has been monitored in the electron microscope by measurement of the myelin thickness and axon diameter of the large fibers. The remyelination follows a time course similar to the late phase of conduction recovery. By reference to the work of others, we surmise that the early recovery of conduction is due to the reorganization of microtubules disorganized by the pressure.

MR myelography of the spine and MR peripheral nerve imaging

MRM is a useful imaging technique that can be incorporated into routine spine MR imaging protocols without a significant increase in imaging time or patient discomfort. When combined with high quality, sequential (no inter-slice gap), axial images of the spine, the authors have found that it decreases the surgeon's need for conventional contrast enhanced myelography by showing the etiology of compression and degree of compressive effect, particularly in cases of degenerative spine disease. In select cases where greater bone detail is desired, an unenhanced CT is often sufficient. MRPNI also is a useful imaging technique that is primarily used in the evaluation of patients with radicular symptoms not explained by spine MR imaging findings. Selective MRPNI is performed based on clinical and electrodiagnostic results to evaluate extra-spinal causes of peripheral neuropathy. It is easily performed using phase array coils and is primarily used to detect traumatic axonal disruption and compressive neuropathy.

Relationship between functional deficiencies and the contribution of myelin nerve fibers derived from L-4, L-5, and L-6 spinolumbar branches in adult rat sciatic nerve

The distribution and relative intrafascicular contribution of myelin fibers derived from spinal segments L-4 to L-6 were analyzed in adult rat sciatic nerve and its main branches, using 200-kDa neurofilament subunit immunodetection in previously injured nerve sections in the L-4 or L-5 spinal branch or both. These branches' functional contribution was evaluated 16 days after the injury, using the method of J. Bain, S. Mackinnon, and D. Hunter (1988, Plast. Reconstr. Surg. 83: 129-136). A common topographic intrafascicular distribution was found in 69% of cases, with notable segregation of L-4 and L-5 fibers and a random distribution for L-6 fibers. At sciatic nerve main branch level, L-4 contributes almost entirely to the peroneal nerve, L-5 to the tibial nerve, and L-6 and other branches to the sural nerve. After injury to L-4, a significant reduction in peroneal nerve functional index (PFI) was observed, as was a reduction in print length (PL). Injury to L-5 caused a significant reduction in the sciatic (SFI) and tibial (TFI) functional nerve indices, an increase in PL, and a reduction in the spread between opposite toes (TS). Finally, transection of both L-4 and L-5 was followed by a significant reduction in all functional indices measured, an increase in PL, and a reduction in intermediate toe (ITS) and opposite toe spread (TS). The results indicate a direct relationship between the distribution and contribution of the spinal nerve fibers forming the sciatic nerve and the alteration in functional indices for sciatic, tibial, and peroneal nerves.

Experimental nerve compression and upregulation of CPON in DRG

Expression of C-terminal flanking peptide of neuropeptide Y (CPON) in DRG and cell proliferation (incorporation of BrdU) in sciatic nerve of rats following chronic nerve compression (silicone tubes with different internal diameters) was studied by immunocytochemistry. An increased number of CPON-positive neurons and cells incorporating BrdU was induced on the compressed side, most pronounced when a tight tube was used, while no cells expressed CPON or BrdU in intact nerves. The increase was transient and declined with time. Nerve compression induces transient cell proliferation in the nerve and expression of CPON in nerve cell bodies, but this is of a lesser magnitude than those following nerve transection.

Pathogenesis of the gradually elongated nerve

The rabbits' sciatic nerves were lengthened by 30 mm in increments of 2.0 mm/day (2-mm group) and 4.0 mm/day (4-mm group). In the 2-mm group, the phosphorylated neurofilament (p-NF) immunoreactivity of axons was similar to that of the control group. However, in the 4-mm group, number of p-NF positive axons decreased. The number of p-NF positive cells at the seventh lumbar dorsal root ganglion cells of the 4-mm group was significantly larger than that of the control group. Abnormal p-NF immunoreactivity in the 4-mm group suggested an impairment of the axonal flow. Leakage of Evans blue-albumin into the endoneurial space, which meant destruction of the blood-nerve barrier function, was clearly evident in the 4-mm group, but minor in the 2-mm group. A speed of 2.0 mm/day, therefore, appears to be critical for safe nerve elongation in this model.

Evaluation of an acute nerve compression injury with magnetic resonance neurography

Although magnetic resonance (MR) imaging is performed routinely, current techniques offer little for evaluation of the peripheral nervous system. An animal model was developed to evaluate the appearance and geographic changes associated with an acute nerve compression injury by MR neurography. Several measurements of signal intensity were made for the contralateral noninjured nerve and each sciatic nerve proximal to the site of compression (PN), at the site of compression (CN), and distal to the site of compression (DN) injury. Mean (+/-SEM) values of the MR nerve/muscle signal intensity ratio were 2.24 +/- 0.08 for normal nerve, 2.29 +/- 0.12 for PN, 3.11 +/- 0.31 for CN, and 4.33 +/- 0.47 for DN. There was a statistically significant geographic variation of nerve/muscle signal intensity ratios along the course of the nerve relative to the site of injury that MR neurography could detect. Magnetic resonance neurography may have significant potential to provide more information about problems such as brachial plexus injuries and peripheral nerve compression.