Nerve Injury & Effects

Learning Outcomes At the end of this lecture the student would be able to;- Describe functional anatomy of nerve fiber List the common causes of nerve injury Describe different grades of nerve injury Define the wallerian degeneration and retrograde degeneration Explain the degenerative changes in nerve fiber after injury Explain the regenerative changes in nerve fiber after injury and factors affecting the regenerative process Describe complications following nerve injury

Nerve cells The neurons are the basic building blocks of the nervous system, their axons may or may not myelinated. The myelin sheath is produced by the Schwan cells, envelops the axon except at the ends & the nodes of Ranvier The impulse is conducted faster in myelinated than unmyelinated nerves. possesses the capacity of self-regeneration after traumatic injury

Functional Anatomy of Nerve fiber axons are bundled together into fascicles. individual axons are surrounded by the endoneurium. entire fascicle is ensheathed by the perineurium, which forms the blood–nerve barrier. epineurium surrounds the entire nerve, holding these structures together.

Some critical Points Neuron: The primary cell of the PNS, which is polarized and has dendrites to receive information and axons to transmit information Nerves: Bundles of axons enclosed in specialized connective sheaths Schwann cell: Glial cell that supports and myelinates PNS axons Endoneurium: Collection of collagenous tissue that surrounds each individual axons and their associated Schwann cell Perineurium: The blood nerve barrier, which is composed of concentric layers of fibroblasts that form a sheath around each fascicle Epineurium: The collagenous connective tissue layer that surrounds the outer limits of the peripheral nerve, protecting it from external stress

Some critical Points Monofascicular: Nerve section composed of one large fascicle Oligofascicular: Nerve section composed of a few fascicles Polyfascicular: Nerve section composed of many fascicles of varying sizes Neurapraxia: Nerve injury characterized by a reduction or complete block of conduction across a nerve lesion Axonotmesis: Nerve injury characterized by conduction block and interruption of axonal continuity Neurotmesis: Nerve injury characterized by conduction block, interruption of axonal continuity, and connective tissue damage

Common causes of nerve injury Transection of nerve fibre (through and through cut) Crushing of nerve fibre Local injection of toxic substances Ischemia due to obstruction of blood flow Effects of hyperpyrexia on neuron

Different grades of Nerve injury Sunderland classified nerve injury based on cause & extent of damage of nerve fibre to quantify the severity of nerve injury First degree injury Second degree of injury Third degree of injury Forth degree of injury Fifth degree of injury

Different grades of Nerve injury First degree injury Involve only transient loss of nerve function Nerve function return within few hours after the causative factor is removed. Cause is mild pressure on the nerve leading to local anoxia due to ischemia following obstruction of blood flow Second degree of injury Prolonged pressure on the nerve causing severe damage of nerve fibre but endoneural tube remain intact. Damage of nerve followed by degenerative changes Regeneration is possible with restoration of normal function due to intact endoneural tube

Different grades of Nerve injury Third degree of injury Severe damage of nerve fibre with interruption of endoneural tube Most severe degree of nerve injury Damage followed by degenerative process Regeneration process is slow and subjective depends on favourable conditions Forth degree of injury Most severe degree of nerve injury with interrupted endoneural tube and disturbed organisation of fasciculi Fifth degree of injury Seen in complete transection of nerve fibre Highest degree of nerve injury Degenerative changes appears rapidly following injury

Nerve Injury Injury to axon of nerve- causes of series of changes occur at Distal to injury Proximal to injury Cell body Repair of injury also initiates following the injury called regenerative process, provided conditions are favourable Both degenerative and regenerative changes depends on degree and type of injury

Sequence of events following nerve injury

Wallerian degeneration Degenerative changes which occur in the part of axon distal to injury. Name derived after the scientists “A Waller-1862” who discover and identified theses changes. Defined as process of degeneration of a nerve fiber that has been separated from its nutritive center by injury or disease, characterized by segmentation of the myelin and resulting in atrophy and destruction of the axon. Also called orthograde degeneration, secondary degeneration

Wallerian degeneration Occur within few hours after the injury and continue for about 3 month Axon becomes swollen and irregular in shape Breaks up in to small segments The neurofibrills breaks down in to granular debris Myelin sheath start slowly to disintegrates from 8th day after the injury and finally in to fat droplets Schwan cell begins to multiply and gradually occupies the empty neurilemmal space, Schwan cell’s cytoplasm also proliferates rapidly to produce series of membranes which help to form numerous tubes

Retrograde degeneration Degenerative changes occur in the axon of neuron, proximal to injury includes cell body and axon proximal to injury Begins within 48 hours following injury and continue up 15-20 days Defined as Pathologic changes that occur in the axon and cell body of a neuron proximal to an axonal lesion. The process is characterized by central chromatolysis which features flattening and displacement of the nucleus, loss of Nissl bodies, and cellular edema. Central chromatolysis primarily occurs in lower motor neurons.

Retrograde degeneration changes in axon are similar to changes seen in distal part (wallerian degeneration), following injury to axon near to first or second node of Ranvier. Nissl granules of cell body undergo disintegration and finally results in chromatolysis. Process of chromatolysis starts around nucleus and gradually spreads to periphery. Golgi apparatus, mitochondria and neurofibrills fragmented and eventually disappear. Cell body draws more fluid, enlarges and becomes spherical. Nucleus displaced to periphery or extruded out of the cell leading to neuronal atrophy or finally disappear.

Factors affecting regeneration Presence of Neurilemma- must for regeneration. Axon of CNS never regenerate as they do not have neurolemma. Presence of Nucleus- must for regeneration to begin. In condition where Nucleus extruded out, process of regeneration does not occur. Types of injury-process of regeneration occur rapidly in crush injury as compared to sharp cut of neuron. Gap between injured fiber- should not increased more than 3 mm. if the gap between cut end of nerve fibers is less, then chances of regeneration are increased considerably Rapid regeneration process occur if the cut end remains in the same line,not moved away from each other

Regenerative Changes Followed by degeneration depends on favourable condition. Occur in 5 stages Stage of fiber formation; Axis cylinder from proximal cut end of axon elongates and gives out fibrils upto 100 in numbers in all directions fibrils grow in to connective tissue at site of injury and attempt to reach to distal end of axon. Stages of entry of fibrils in to endoneural tube Strands of Schwan cell guide the growing fibrils to enter in to endoneural tube of axon, and grows rapidly at faster rate The fibrils which are unable to enter the endoneural tube, degenerate

Regenerative Changes Stage of active growth Stage of myelination Growing axonal fibril enlarges and establishes the contact with appropriate end organ Process of myelination not yet begin, fiber is ummyelinated Stage of myelination Process of myelination begins slowly and completes in one year Progressive increases in diameter occur Regenerated fibre attain 80% of its original diameter Changes in Cell body Nissl granules followed by Golgi apparatus appears Looses excess cytoplasm to regain its size Nu occupies central position

Complications following nerve injury Complete atrophy of nerve fibre Functional complications in the form of misinterpretation of sensation due to abnormal connection during the process of regeneration Neuroma formation – whorls like mass which is very painful. It is due to abnormal regeneration process where gap between the cut end of nerve fibre is too large, thus leading to formation of fibers turn around for search of endoneural tube and form a mass. Common example is phantom limb