Chapter 55 - Peripheral Nerve Injuries Flashcards
What does the peripheral nervous system (PNS) connect?
It connects the periphery of the body with the central nervous system (CNS).
How many pairs of spinal nerves originate from the spinal cord?
There are 42 to 43 pairs of spinal nerves.
What are the two types of roots that attach spinal nerves to the spinal cord?
Afferent dorsal roots and efferent ventral roots.
What structures do spinal nerves exit through?
Lateral vertebral foramina or intervertebral foramina.
What is formed when the ventral rami of neighboring spinal nerves connect?
Nerve plexuses such as the cervical, brachial, and lumbosacral plexus.
Which cervical nerves form the phrenic nerve?
Cervical nerves 5, 6, and 7.
What is the primary function of the phrenic nerve?
Motor innervation of the diaphragm and sensory supply to thoracic structures.
Where is the brachial plexus located?
Immediately cranial to the first rib, between the two parts of the scalenus muscle.
What spinal nerves contribute to the brachial plexus?
Spinal nerves C6 to T2.
Which spinal nerves form the lumbosacral plexus?
Spinal nerves L4 to S2.
What major nerves arise from the lumbosacral plexus?
The femoral, obturator, sciatic, tibial, and peroneal nerves.
What components make up a nerve?
Numerous nerve fibers and their supporting soft tissues.
What does the term “nerve fiber” refer to?
The axon and its enveloping Schwann cells, myelin sheath, and surrounding endoneurium.
What is the role of the endoneurium?
It is the connective tissue layer surrounding individual nerve fibers.
What structure envelopes a bundle of nerve fibers?
The perineurium.
What surrounds fascicles of nerve fibers?
The interfascicular epineurium.
How does the epineurium function in a nerve?
It encircles the entire nerve, providing structural support.
What is neurapraxia?
A temporary conduction block due to blunt trauma without axonal damage.
Define axonotmesis.
An injury where the axon is damaged but the connective tissue layers remain largely intact.
What is the expected recovery time for neurapraxia?
Complete recovery can take up to 3 months.
What happens during third-degree axonotmesis injuries?
Disruption of endoneurial sheaths leads to disorganized axonal regrowth.
What occurs in fourth-degree peripheral nerve injuries?
Complete disruption of internal nerve structure, with scar tissue blocking regenerating axons.
What characterizes a second-degree axonotmesis injury?
The connective tissue is intact, allowing for optimal axonal regrowth.
What is the prognosis for recovery from a third-degree injury?
Rarely more than 60% to 80% of normal function is regained.
What is “neuroma-in-continuity”?
A mass of nerve fibers formed due to axonal sprouts and scar tissue.
Which types of injuries are not expected to recover without surgery?
Fourth- and fifth-degree (neurotmesis) lesions.
What is the primary limitation of Seddon and Sunderland classification systems?
They require histological examination to determine the degree of injury.
What critical concept is emphasized by both classification systems?
Axonal degeneration occurs if the axon’s continuity is affected.
What is Wallerian degeneration?
The degeneration of the distal axonal stump after axonal injury.
Which type of injury does not result in Wallerian degeneration?
First-degree injuries (neurapraxia).
What is the clinical significance of the endoneurium in nerve injury?
It provides an optimal environment for axonal regrowth in less severe injuries.
How does connective tissue damage affect axonal regrowth?
Increased damage leads to disorganized regrowth and poor recovery outcomes.
What role do Schwann cells play in peripheral nerves?
They support the axons and form the myelin sheath.
What are the implications of a nerve injury on the target organ?
If axons degenerate, the target organ loses its nerve supply, affecting function.
How do you differentiate between the degrees of peripheral nerve injury?
Based on the extent of axonal and connective tissue damage.
Why is understanding nerve anatomy important in veterinary medicine?
It aids in diagnosing and treating peripheral nerve injuries.
What is a common cause of peripheral nerve injuries in horses?
Blunt trauma or compressive injuries.
How does the anatomy of the brachial plexus relate to clinical signs of injury?
Injury can lead to weakness or paralysis of the thoracic limb muscles.
What type of nerve fibers are primarily affected in axonotmesis?
The axonal fibers, while connective tissue layers remain intact.
Why might a horse show chronic signs of nerve injury?
Due to incomplete recovery or compensation for lost function from other nerves.
What are the potential causes of acute peripheral nerve injury?
Blunt or penetrating trauma, extreme temperatures, noxious substances, electricity, ionizing radiation, ischemia, or infection/inflammation.
What is the recommended treatment for trauma-induced peripheral nerve injury in humans?
Surgery is generally considered the best option.
Surgery is generally considered Why is complete recovery after surgical intervention for peripheral nerve injury rare?
The complexity of nerve regeneration and the extent of damage often impede full recovery.
What type of nerve damage can occur as a complication of surgical procedures in horses?
Temporary and permanent damage to peripheral nerves.
What are entrapment neuropathies?
Focal compression injuries of nerve segments due to mechanical irritation.
What anatomical structures predispose the deep branch of the lateral plantar nerve to entrapment in horses?
The third metatarsal bone, vestigial metatarsal bones, and plantar fascia.
What percentage of equine neoplasms are peripheral nerve tumors?
3% to 4%.
Which types of peripheral nerve tumors are most commonly diagnosed?
Schwannomas, neurofibromas, and malignant peripheral nerve sheath tumors.
What factors contribute to postanesthetic peripheral neuropathies?
Increasing bodyweight, prolonged anesthesia, inadequate padding, malpositioning.
What complications can arise from general anesthesia in adult horses?
Postanesthetic peripheral neuropathies (PAPN) and muscle damage.
What two processes occur for target organ reinnervation after peripheral nerve injury?
Collateral sprouting of intact axons and axonal regeneration.
How long does collateral sprouting take to occur?
Over 2 to 6 months.
What is the recovery expectation if more than 90% of the axons are damaged?
Axonal regeneration becomes the primary means of reinnervation.
What is the rate of axonal regrowth after injury?
Approximately 1 mm per day.
What happens to muscle tissue if reinnervation does not occur within 12 to 18 months?
Irreversible muscle atrophy and fibrosis develop.
How long can sensory receptors regain some function after injury?
Even years after the injury.
What is the role of Schwann cells in peripheral nerve injury recovery?
They must remain intact for effective axonal regeneration.
What are common clinical signs of complete paralysis of a main motor nerve?
Abnormal posture, gait abnormalities, and muscle atrophy.
How quickly can neurogenic atrophy occur after a nerve injury?
Within 2 to 4 weeks post-injury.
What is the typical muscle mass loss in the first 14 days following denervation?
Approximately 50% of muscle mass.
What type of sensory changes might accompany muscle paralysis?
Regional cutaneous hypalgesia or analgesia.
What is electromyography (EMG) used for in diagnosing peripheral nerve injuries?
To record the electrical activity of muscles.
What is the sequence of dysfunction in focal neuropathies?
Loss of proprioception, paresis, paralysis, loss of skin sensation, and loss of pain perception.
What are autonomous zones in relation to nerve injury assessment?
Specific skin areas supplied by a single nerve used to test nerve function.
Which nerve fibers are most susceptible to compression injury?
Larger myelinated fibers, followed by motor fibers and smaller sensory axons.
What does nerve conduction studies (NCS) measure?
The response of an innervated muscle to electrical stimulation of a peripheral nerve.
What imaging techniques can aid in the localization of peripheral nerve injuries?
Ultrasonography, computed tomography, and magnetic resonance imaging.
How can NCS differentiate between neurapraxia and axonal degeneration?
Neurapraxia maintains distal nerve excitability; axonal degeneration does not.
What is the primary function of the facial nerve (VII)?
It innervates the superficial muscles of the head and controls facial expressions.
Figure 55-3. End-to-end suture repair. (A) Preplacement of stay-sutures at 9-o’clock and 3-o’clock on a transected nerve (top) and a “neuroma in continuity” (bottom). (B) The nerve ends are trimmed back to a level where healthy epineurium is present and about 75% of the fascicular architecture appears intact. (C) The uppermost epineurium is sutured in a simple continuous or interrupted pattern, bringing the nerve ends into loose contact. The stay-sutures are now used to rotate the nerve to facilitate circumferential suturing.
Figure 55-2. Autonomous zones of innervation for major peripheral nerves of the equine limbs. a, Ulnar nerve; b, musculocutaneous nerve; c, median nerve; d, femoral nerve; e, peroneal nerve; f, tibial nerve.
