Regeneration and soma size changes following axotomy of the trochlear nerve

Superior oblique myokymia, a review

PURPOSE OR REVIEW

Superior oblique myokymia is a rare, monocular condition that typically presents in healthy adults. In this article, we review the proposed pathophysiology, describe the typical clinical presentation, and discuss the recommended work up and various treatment options.

RECENT FINDINGS

Treatment for this condition ranges from observation and reassurance to various topical and systemic medications. Both strabismus surgery and neurosurgery are reserved for patients who fail medical therapy or are intolerant to medication secondary to side effects.

SUMMARY

Due to its episodic nature, SOM is often under-diagnosed. We highlight key features in a patient's clinical history and the examination findings that suggest and support the diagnosis, with review of available treatment options.

Use of a Y-Tube Conduit After Facial Nerve Injury Reduces Collateral Axonal Branching at the Lesion Site But Neither Reduces Polyinnervation of Motor Endplates Nor Improves Functional Recovery

BACKGROUND:

Despite increased understanding of peripheral nerve regeneration, functional recovery after surgical repair remains disappointing. A major contributing factor is the extensive collateral branching at the lesion site, which leads to inaccurate axonal navigation and aberrant reinnervation of targets.

OBJECTIVE:

To determine whether the Y tube reconstruction improved axonal regrowth and whether this was associated with improved function.

METHODS:

We used a Y-tube conduit with the aim of improving navigation of regenerating axons after facial nerve transection in rats.

RESULTS:

Retrograde labeling from the zygomatic and buccal branches showed a halving in the number of double-labeled facial motor neurons (15% vs 8%; P < .05) after Y tube reconstruction compared with facial-facial anastomosis coaptation. However, in both surgical groups, the proportion of polyinnervated motor endplates was similar (∼30%; P > .05), and video-based motion analysis of whisking revealed similarly poor function.

CONCLUSION:

Although Y-tube reconstruction decreases axonal branching at the lesion site and improves axonal navigation compared with facial-facial anastomosis coaptation, it fails to promote monoinnervation of motor endplates and confers no functional benefit.

Impaired peripheral nerve regeneration in diabetes mellitus

Diabetes mellitus impairs peripheral nerve regeneration. Regenerative failure likely exacerbates deficits from polyneuropathy or focal neuropathies in patients who might otherwise exhibit spontaneous improvement. Some focal neuropathies, like carpal tunnel syndrome, are common, yet render ongoing disability because of their delayed recovery. Why diabetic nerves fail to regenerate is an interesting question to consider because several mechanisms likely contribute. In this review, we examine a number of these causes. These causes include microangiopathy or disease of small blood vessels, failure to provide proper metabolic support for repair, defects in the entry and actions of inflammatory cells within the injury milieu, less robust support of axons by their Schwann cells, and lack of a full repertoire of trophic factors. A number of the mechanisms that generate neuropathy in the first place also likely contribute to failed regenerative programs, but how they do so is not clear.

Characteristics of regenerating horizontal semicircular canal afferent and efferent fibers in the toadfish, Opsanus tau

The horizontal semicircular canal nerve of the toadfish, Opsanus tau, was transected and allowed to regenerate. The time course, morphometrics, and projection patterns of regenerating afferent and efferent vestibular fibers were determined. Nerve transections were performed both pre- and postganglionically, and regeneration was assessed in afferent and efferent fibers by bulk labeling the peripheral axons of the horizontal semicircular canal nerve with biocytin after nerve regrowth. Afferent fibers regrew through the transection site within 14 days and projected to all vestibular nuclei within 3 weeks. Bouton and branch number, axon length, surface area, volume, fiber diameter, and internodal distance were quantified for afferent fibers from eight sites within the vestibular nuclei, and axon number and soma size was quantified for the efferent fibers. Extensive regeneration was seen within 5 weeks of transection in all nuclei, and most morphometric parameters approached or exceeded control levels within 10 weeks. Regeneration appeared to recapitulate morphogenesis with an initial overproduction of boutons and branch points followed by elimination of presumably superfluous structures. Internodal distance remained significantly shorter in regenerating afferent axons than in control fish throughout the 15-week observation period. Efferent fibers also were observed to regenerate. Efferent axon number, diameter, and soma size were indistinguishable from those in controls from 3 weeks posttransection through week 15. Electrophysiological recordings from the horizontal canal nerve during mechanical stimuli of the canal confirmed that the regenerated axons transmitted normal signals. The return of normal equilibrium and behavior coincided with the projection of afferent fibers into the central vestibular nuclei, indicating that functional connections had been reestablished.

The axonal chemoattractant netrin-1 is also a chemorepellent for trochlear motor axons

Extending axons are guided in part by diffusible chemoattractants that lure them to their targets and by diffusible chemorepellents that keep them away from nontarget regions. Floor plate cells at the ventral midline of the neural tube express a diffusible chemoattractant, netrin-1, that attracts a group of ventrally directed axons. Here we report that floor plate cells also have a long-range repulsive effect on a set of axons, trochlear motor axons, that grow dorsally away from the floor plate in vivo. COS cells secreting recombinant netrin-1 mimic this effect, suggesting that netrin-1 is a bifunctional guidance cue that simultaneously attracts some axons to the floor plate while steering others away. This bifunctionality of netrin-1 in vertebrates mirrors the dual actions of UNC-6, a C. elegans homolog of netrin-1, which is involved in guiding both dorsal and ventral migrations in the nematode.

Increased production of ubiquitin mRNA in motor neurons after axotomy

Ubiquitin targets proteins for attack by certain proteolytic enzymes, but the ubiquitinated cytoplasmic inclusions seen in some chronic neurodegenerative diseases may indicate the occurrence of reparative rather than destructive metabolic events. We have examined the production of ubiquitin in motor neurons of the rat's left hypoglossal nucleus after transection of their axons in circumstances that favour or prevent axonal regeneration. One week after axotomy, in situ hybridization with a radiolabelled cRNA probe revealed a twofold increase in the ubiquitin mRNA content of neurons with regenerating axons (nerve crushed) but not significant change when axonal regeneration had been prevented (nerve transected and ligated). After 2 weeks, ubiquitin mRNA was elevated to about 1.5 times the contralateral control level, regardless of the type of nerve injury, and by 4 weeks there were no longer any differences between the left and right sides. Despite the increased transcription, axotomy was not followed by any change in the quantity of ubiquitin-immunoreactive material in the nuclei or perikarya of hypoglossal neurons as measured by video image analysis of immunohistochemically stained sections. We suggest that ubiquitin is synthesized in neuronal cell bodies and transported into their axons, and that ubiquitin-mediated proteolysis is a metabolic process involved in the elongation of regenerating axons.

Successful outcome following anastomosis of a severed trochlear nerve in the middle fossa

Complete return of function has been obtained following neurosurgical repair of a trochlear nerve inadvertently divided during the clipping of a basilar tip aneurysm. To date this is the second case reported in the literature. The technique of repair and the method of recording the return of function is discussed.

Changes in size and dendritic arborization patterns of adult cat spinal alpha-motoneurons following permanent axotomy

This study was performed to analyse quantitatively the changes in dimensions and dendritic branching patterns of adult cat spinal alpha-motoneurons following permanent axotomy, i.e., in a situation in which the transected motoraxons are prevented from reinnervating their peripheral target muscle. After transection and ligation of the medial gastrocnemius nerve of adult cats, homonymous alpha-motoneurons were intracellularly labelled with horseradish peroxidase and subjected to quantitative light microscopic analyses. The cell bodies and proximal dendrites were studied at 3, 6, and 12 weeks after the axotomy. An initial increase in cell body size at 3 weeks was followed by a gradual return towards normal values. The mean diameter of the stem dendrites was decreased at all time periods studied, and the combined diameter of the stem dendrites was reduced at 12 weeks after the axotomy. Entire dendritic trees were reconstructed at 12 weeks postoperatively, and the regression equations describing the correlations between dendritic stem diameter, on one hand, and the size of the entire dendrite, on the other, were used to calculate the total dendritic length, volume, and membrane area of whole axotomized motoneurons. The dendritic branching patterns were also analysed. In comparison with normal medial gastrocnemius alpha-motoneurons, the dendritic membrane area and volume of the axotomized cells had decreased by 36% and 29%, respectively, at 12 weeks after the axotomy. This reduction in dendritic size was due to a loss of preterminal and terminal dendritic segments. Abnormal dendritic elongations were observed in 2 of 16 completely reconstructed dendrites.