Failure of reinnervation of Pacinian corpuscle after nerve crush. An electron microscopic study

The construct validity and responsiveness of sensory tests in patients with carpal tunnel syndrome

Background and Purpose :

Sensory evaluation is fundamental to evaluation of patients with Carpal Tunnel Syndrome (CTS). The purpose of this study was to determine the construct validity and responsiveness for sensory threshold tests in patients with CTS.

Methods :

Sixty-three patients diagnosed with CTS were evaluated prior to orthotic intervention and again at follow up at 6 and 12 weeks. Sensory tests included touch threshold PSSD (Pressure Specified Sensory Device) and vibration threshold (Vibrometer). Construct validity was assessed by comparing sensory tests to hand function, and dexterity testing using Spearman rho (r_s). Patients were classified as either responders or non-responders to orthotic intervention based on the change score of the Symptom Severity Scale (SSS) of 0.5. Responsiveness of the sensory tools was measured using ROC (receiver operating characteristic) curves, SRM (Standardized Response Mean), and ES (Effect Sizes).

Results :

The PSSD had low to moderate correlations (r_s ≤ 0.32) while Vibrometer scores had moderate correlations (r_s = 0.36 - 0.41) with dexterity scores. The Clinically Important Difference (CID) for the PSSD was estimated at 0.15 g/mm² but was not discriminative. The Vibrometer demonstrated moderate responsiveness, with a SRM = 0.61 and an ES = 0.46 among responders. The PSSD had a SRM = 0.09 and an ES = 0.08 and showed low responsiveness for patients with a clinically important improvement in symptoms.

Conclusion :

Measurement properties suggest that the Vibrometer was preferable to the PSSD because it was more correlated to hand function, and was more responsive. Clinicians may choose use the Vibrometer opposed to the PSSD for determining important change in sensation after orthotic intervention.

Pathophysiology of nerve injury

The response to nerve injury is a complex and often poorly understood mechanism. An in-depth and current command of the relevant neuroanatomy, classifications systems, and responses to injury and regeneration are critical to current clinical success. Continued progress must be made in our current understanding of these varied physiologic mechanisms of neuro-regeneration if any significant progress in clinical treatments or outcome is to be expected in the future. Reconstructive surgeons have in many ways maximized the technical aspects of peripheral nerve repair. However, advances in functional recovery may be seen with improvements in sensory and motor rehabilitation after peripheral nerve surgery and with a combined understanding of the neurobiology and neurophysiology of nerve injury and regeneration.

Multiple axon terminals in reinnervated Pacinian corpuscles of adult rat

The ultrastructure of Pacinian corpuscles localized beneath the crural interosseous membrane was examined two weeks to 18 months after crushing the sciatic nerve in adult rats. The Pacinian inner core and capsule remained preserved during the transient period of denervation. Regenerating axons reached Pacinian corpuscles approximately three weeks after nerve crush. Up to 15 axonal sprouts entered a single corpuscle at the initial stage of reinnervation, but only 1-3 axons increased in size, myelinated and formed axon terminals in the inner core, the excess sprouts being eliminated. Most corpuscles of the crural group were reinnervated by the end of the first month. Three to 19 months after nerve crush, 10% of corpuscles examined were found to be monoaxonal and monoterminal as before the operation; 74% contained multiple terminals; 16% remained denervated. Over half the multiterminal corpuscles were supplied with a single myelinated axon that branched inside the corpuscles; the rest received two or three myelinated axons which formed several terminals. The terminals were distributed at random, usually in the axial region between the lamellae of the inner core. They were cylindrical, with an oval profile; the larger terminals were filled with mitochondria and microtubules at their circumference and contained a core of neurofilaments. Lateral processes of the terminals were filled with vesicles and had membrane specializations as in normal corpuscles. The mean number of terminals in reinnervated corpuscles was 4.07 +/- 0.37 (S.E.M.) at three months, and 3.26 +/- 0.49 (S.E.M.) 6-18 months after nerve crush. This small decrease was apparently the result of degeneration occasionally observed in some axon terminals at later stages of reinnervation. These experiments thus demonstrate that most rat Pacinian corpuscles become reinnervated with multiple terminals after nerve injury and maintain multiterminal innervation permanently.

Reinnervation of mechanoreceptors in the human glabrous skin following peripheral nerve repair

The technique of percutaneous microneurography was used to record single unit activity from 65 reinnervated and 24 normally innervated mechanoreceptors in the glabrous skin of the human hand. The results were obtained from 20 patients and 5 control subjects. The patients had suffered complete traumatic transsection, with subsequent repair, of the median or ulnar nerves. Three types of mechanoreceptors (RA, SAI, SAII) and many unidentified units located in deep tissues were found to become reinnervated. No reinnervated PC units could be identified. Response thresholds, discharge characteristics and receptive field properties of reinnervated receptors were comparable to normal, with the exception that reinnervated SA I units had slower static discharge rates and smaller receptive fields. No evidence was found for multiple peripheral innervation by a single afferent fiber. The reinnervated mechanoreceptors were predominantly located in the palm and the proximal fingers with few in the finger tips, contrary to normal. The locations and frequency of occurrence of the different types of receptors could be correlated with the goodness of sensory recovery. It is suggested that these differences result from misguidance of regenerating fibers and from poor reinnervation, and that they account for reduced sensitivity and poor tactile discrimination in patients with peripheral nerve injuries.

Survival of Pacinian corpuscles after denervation in adult rats

The ultrastructure of Pacinian corpuscles located on the crural interosseous membrane was studied in adult rats 6h to 10 months after transection of the right sciatic nerve. Axon terminals degenerated one day after transection and were engulfed and resorbed by cells of the inner core within one week. The axial space left after removal of the axonal debris was closed by the lamellae of the inner core. The main structural features of the inner core and capsule remained preserved after denervation throughout the period of study. The denervated inner cores, however, became atrophic 10 months after neurotomy, their mean diameter being reduced by 17.5% compared with that of contralateral control corpuscles. The number of capsular lamellae was unaltered, and perineurial pathways of the peripheral nerve stump remained preserved. Schwann cells proliferated and formed Büngner bands during the first month after denervation, but retracted their processes and became atrophic at later stages after neurotomy. Survival of Pacinian corpuscles after long-term denervation in adult rats is in contrast to their rapid degeneration within several days after nerve section in neonates.

Degeneration of mouse digital corpuscles

The sequential changes of the morphology and the histochemically demonstrated, nonspecific cholinesterase (ChE) activity of the denervated digital corpuscle (Meissner corpuscle) of the mouse were observed by light and electron microscopy for 6 months after transection of the sciatic nerve. A fragment of the sciatic nerve, approximately 1 cm long, was removed from the distal stump to prevent reinnervation. Within one day following nerve division, the axon terminals began to manifest degenerative changes: the axoplasm became electron-opaque and some mitochondria became disorganized. The lamellar cell processes (lamellar) also exhibited atrophic changes: they became thinner and more electron opaque. The lamellar cell body became smaller due to a decreased amount of cytoplasm and contained few organelles except for some free ribosomes. These changes advance with time. Around 4-6 months after denervation, the corpuscle was small, consisting of a few thin, shrunken, atrophic lamellar cells. The ChE activity persisted and could be demonstrated in the interlamellar spaces, but its intensity decreased with time so that the corpuscle was hardly identifiable by light microscope histochemistry around 4-6 months after denervation. During this time, the precipitates of the reaction product were scattered as small particles throughout the interlamellar spaces. The enzyme activity in the cisternae of the rough endoplasmic reticulum and the nuclear envelope of the lamellar cell body was no longer demonstrable 5 days or less after denervation. These findings indicate that the lamellar cell is dependent on the axon terminal to maintain its normal morphology and the function of ChE synthesis.

Rapid degeneration of developing rat Pacinian corpuscles after denervation

The effect of denervation upon the development of Pacinian corpuscles was investigated in the crural interosseous membrane of the rat during the early postnatal period. When the sciatic nerve was transected in 1-day-old rats, further development of immature Pacinian corpuscles was arrested and their structure rapidly disintegrated. In the main cluster of corpuscles, Pacinian axon terminals degenerated within 12 h after the operation and were phagocytised by the inner core cells. Subsequently large dense inclusion bodies and vacuoles with dense debris appeared in the inner core and capsule, and cell autolysis and pyknosis ensued from the 2nd day onwards. The debris of degenerated cells was removed by macrophages and the disintegration of individual corpuscles was completed 2-5 days after nerve section. Normal interosseous membranes of 2-6-day-old rats contained 54.9 +/- 1.2 (+/- S.E.) Pacinian corpuscles, as revealed by staining for cholinesterase. The total days after denervation, dropped to 38% on day 3, decreased further to 15% on day 4 and reached zero values on day 5 after nerve section. The experiments demonstrate that the postnatal development and growth of the non-nervous components of Pacinian corpuscles is completely dependent upon the neuronal induction exerted by sensory axon terminals.