Peripheral Nerve Disorders Flashcards
Peripheral Nerve Disorders
Subtypes
Peripheral nerve ⇒ neuropathy
Nerve root ⇒ radiculopathy
Lumbar or brachial plexus ⇒ plexopathy
Anterior horn ⇒ motor neuron disease
Peripheral Nervous System
Basic Anatomy
- Consists of nerves and ganglia outside of the brain and spinal cord
- Peripheral nerves connected to spinal cord by two roots:
- Ventral root ⇒ motor
- Dorsal root ⇒ sensory
- Interconnects all other tissues with the CNS
- Innervates all voluntary muscles
- Transmits the sensory impulses to the whole body
- Individual nerve fibers ⇒ vary widely in diameter and may be heavily myelinated, lightly myelinated or unmyelinated
- Amount of myelin ∝ conduction speed
- Largely under conscious (cerebral) control
- Injury results in both sensory and motor loss
Sensory Innervation
- A sensory nerve (afferent nerve) carries sensory information toward the CNS
- It is a bundle of nerve fibers coming from sensory receptors in the PNS
- Afferent nerve fibers leave the sensory neuron from the dorsal root ganglia of the spinal cord
- Large myelinated sensory fibers ⇒ vibration and proprioception
- Small myelinated and unmyelinated sensory fibers ⇒ pain and temperature
Motor Innervation
-
Upper Motor Neuron
- Neuron in the motor cortex
- Travels down and decussates at the level of the medulla
- Travels down the spinal cord
-
Lower Motor Neuron
- Neuron in the spinal cord or cranial nerve of brainstem
- Travel to the skeletal muscle
Motor Unit
A motor neuron and all the skeletal fibers it innervates.
- Groups of motor units work together to coordinate contraction of a single muscle
-
It is a functional unit made up of:
- Lower motor neuron
- Motor axon
- NMJs
- Muscle fibers innervated by the motor neuron
Upper Motor Neuron
Signs
- Weakness
- Hyperreflexia
- Spasticity
- Stiffness
- Babinski sign
- Brisk jaw jerk
Lower Motor Neuron
Signs
- Fasciculations
- Atrophy/muscle wasting
- Weakness
- Muscle cramps
- Loss of reflexes
Disease Localization
Differentiating PNS from CNS disease
-
Weakness or sensory loss confined to one limb ⇒ peripheral nerve problem
- Associated pain in the extremity ⇒ ↑ likelihood of a peripheral process
- Localization less obvious when weakness/sensory loss involves > 1 extremity or when there is no associated pain or discomfort
- The patient approach will include taking a detailed hx including family hx of similar sx, time course of sx, and comorbidities (ex. DM)
- Complete neurologic exam will be extremely helpful
Peripheral Nerve
Composition
Peripheral nerve is composed of:
- Motor axon/neuron
- Sensory axon/neuron
- Autonomic fibers
- Myelin
- Other supporting structures
Peripheral Nerve
Characteristics
- Individual nerve fibers ⇒ vary widely in diameter and may be heavily myelinated, lightly myelinated or unmyelinated
- Amount of myelin ∝ conduction speed
- Myelin made by Schwann cells
- Distance b/t nodes of Ranvier determines conduction rate
- Largely under conscious (cerebral) control
- Injury results in both sensory and motor loss
Peripheral Nerve Damage
-
Segmental demyelination
- Diseases primarily affect Schwann cells ⇒ loss of myelin
-
Nerve injury/transection
- Primary destruction of the axon ⇒ secondary disintegration of its myelin sheath
- Results in axonal degeneration
- Distal portion ⇒ Wallerian degeneration
- Proximal portion ⇒ retrograde degeneration
- Begins in the periphery and proceeds back toward the cell body ⇒ “dying back“
- Cell body may become chromatolytic
- If process reversed early phases, some regeneration may be possible
Axonal Injury
Traumatic transection of a nerve
-
Distal portion of the nerve fiber degenerates ⇒ Wallerian degeneration
- Axons begin to break down
- Schwann cells begin to catabolize myelin and later engulf axon fragments, forming myelin ovoids
- MΦ recruited and phagocytose axonal and myelin-derived debris
-
Proximal portion of severed nerve degenerates ⇒ retrograde degeneration
- Degenerative changes involving only the most distal two or three internodes
- Proximal stump then undergoes regenerative activity
- Regenerated fiber has shorter internodes
Segmental Demyelination
- Selective loss of individual myelin internodes with preservation of the axon
- Process affects some Schwann cells and their corresponding internodes while sparing others
- Disintegrating myelin is engulfed initially by Schwann cells and later by MΦ
- After segmental demyelination, Schwann cells can proliferate and remyelinate
- Example: Guillain-Barré Syndrome
Regenerative Processes
-
Axonal Sprouting
- After axonal damage, the axon may sprout from the distal end
- Regenerating axons re-grow at ~ 1 mm/day
-
Remyelination stimulated by denuded axon
- Segmentally demyelinated axons may remyelinate ⇒ shorter internodes = slower conduction
- Axonal sprouts will be re-myelinated by surviving Schwann cells
- Remyelination may restore some function
- Repeated episodes of demyelination and remyelination → accumulation of layers of Schwann cell processes
- Onion Bulb: a single thinly myelinated axon surrounded by concentrically arranged Schwann Cells and redundant BM
Neurodiagnostic Testing
Electromyography (EMG) and Nerve conduction studies (NCS)
- Most common diagnostic tests for dx of nerve and muscle disease after hx and exam
- Both are complementary and are always performed together during the same setting
Nerve Conduction Studies (NCS)
Measure the ability of sensory and motor fibers to conduct an action potential
Used mainly for eval of paresthesias and/or weakness of arms and legs
-
Axonal injury/neuropathy
- ↓ amplitude of AP
- Mild slowing
- Can see mix of both
-
Demyelination neuropathy
- Conduction block (failure of AP to propogate)
- Slow conduction
- Amplitude of AP may be normal or reduced
Electromyography (EMG)
Measures muscle response or electrical activity in response to nerve stimulation of the muscle
Used to determine NMJ abnormalities, myopathy, and denervation
-
Increased insertion activity
- Denervation ⇒ will fire spontaneously as a “Fibrillation potential”
- Polymyositis
-
Decreased insertional activity
-
Myopathy ⇒ loss of response of motor units
- Small motor units
- Early recruitment of other motor units
- Muscle is replaced by fat
-
Myopathy ⇒ loss of response of motor units
- Muscle fibers s/p denervation and reinnervated ⇒ very large due to reorganization and formation of large motor units
Peripheral Nerve Biopsy
- Used when the pathology is in question and when therapy is contemplated
- Superficial sensory branches of the sural or radial nerves are often sampled and examined pathologically
Peripheral Neuropathy Patterns
Patterns of peripheral neuropathy aids in clinical dx and finding underlying pathologic processes:
- Mononeuropathy
- Mononeuropathy Multiplex
- Polyneuropathy
Focal Neuropathies
(Mononeuropathies)
- Typically have damage to a single nerve
- Etiologic agents are focal ⇒ process tends to be asymmetric
- Classic cause is vasculitis ⇒ produces discrete infarctions in nerve trunks
- NO distal-proximal gradient b/c an individual nerve trunk is affected
Median Nerve
Responsible for:
- Flexion of the thumb, index, and middle fingers
- Wrist flexion
- Thumb opposition
- Forearm protonation
- Hand abduction
Median Nerve Entrapment
(Carpal Tunnel Syndrome)
Secondary to compression of the median nerve as it passes through the carpal tunnel
Most common nerve entrapment
- Due to any condition that causes decreased space within the carpal tunnel
- Tissue edema, pregnancy, inflammatory arthritis, hypothyroidism, amyloidosis acromegaly, DM, and excessive repetitive motions of the wrist
- Pain primarily at the wrist, may radiate to forearm, arm, or shoulder
-
Sensory findings: paresthesia and sensory loss in the thumb, index and lateral half of ring fingers of affected hand
- Often noted more at night
- Motor findings: Weakness of thumb abduction and atrophy of the thenar eminence
-
Diagnostic signs:
- Phalen’s Sign: elicited when wrist flexion produces paresthesias in the median nerve distribution
- Tinel’s Sign: present when tapping over the median nerve at the wrist elicits paresthesias in the median nerve distribution
- NCS: focal slowing or conduction block of median nerve fibers
- Treatment: wrist splints in mild cases and surgical decompression in severe cases
Ulnar Nerve
Responsible for:
- Wrist flexion
- Flexion of 4th and 5th digits
- Thumb adduction
- Finger abduction and adduction
Ulnar Neuropathy
(Cubital Tunnel Syndrome)
Nerve entrapment due to compression of the ulnar nerve in the cubital tunnel at the elbow.
- Sensory signs: numbness of the little finger and half of the ring finger
- Motor signs: decreased grip and problems with finger dexterity
- NCS: Slow conduction velocity at the elbow, or conduction block
- Treatment: avoidance of behaviors involving leaning on elbows, elbow pads or surgical decompression in severe cases
Radial Nerve
Responsible for:
- Supination of forearm
- Brachioradialis reflex
- Extension of fingers, thumb, wrist, and elbow
Radial neuropathy
“Saturday Night Palsy”
Results from compression of the radial nerve at the radial/spiral groove.
- Classic case seen with sleeping with arm draped over a chair
- Can also be seen with humerus fracture or other insult
- Sensory sx: sensory loss over lateral dorsal hand
- Motor sx: wrist drop, finger drop, weakness of supination, mild elbow flexion weakness
- Treatment is supportive with generally spontaneous recovery