27. Peripheral Nervous System Disorders

What category of disorders primarily affects motor neurons and causes weakness?

Motor neuron diseases (MND)

Which motor neurons are typically affected in MND?

Lower motor neurons (LMN), sometimes also upper motor neurons (UMN)

What is the most common form of motor neuron disease?

Amyotrophic lateral sclerosis (ALS)

What is the epidemiology of ALS?

1–2 per 100,000; 10% genetic, mostly sporadic

What is the pathophysiology of ALS?

Protein aggregation in UMN and LMN cell bodies leads to neuronal death

What are the clinical features of ALS?

UMN (hyperreflexia, spasticity) + LMN signs (atrophy, fasciculations), progressive cranial/extremity weakness, spares eyes/sphincters

What is the prognosis of ALS?

Death in 3–5 years from respiratory failure

What causes poliomyelitis?

Poliovirus transmitted fecal-orally

How common is poliomyelitis in the US?

Not active due to aggressive immunization

What is the pathophysiology of poliomyelitis?

Virus invades anterior horn → LMN destruction

What are the clinical features of poliomyelitis?

Flaccid paralysis (limbs or diaphragm)

What is the treatment for poliomyelitis?

Immunization

What is X-linked bulbar spinal atrophy also called?

Kennedy Disease

What is the epidemiology of Kennedy Disease?

1:150,000; X-linked, affects XY individuals

What causes Kennedy Disease?

Androgen receptor CAG repeat mutation → LMN + endocrine effects

What are clinical features of Kennedy Disease?

LMN weakness (bulbar/axial), gynecomastia, testicular atrophy

What is the progression of Kennedy Disease?

Slowly

What causes Spinal Muscular Atrophy (SMA)?

Deletion of the SMN1 gene

What is the result of SMN1 deletion in SMA?

Death of lower motor neurons

How common is SMA?

1 in 10,000 births

What determines the severity of SMA?

Number of working SMN2 gene copies

What is the presentation of SMA type I?

Floppy baby with early respiratory failure

What is the presentation of SMA type II?

Onset at 6–18 months

What is the presentation of SMA type III?

Later onset with milder symptoms

What is one gene therapy option for SMA?

Viral vector insertion of SMN1 gene

What is a second treatment option for SMA?

Nusinersen, an antisense mRNA to SMN2

What does radiculopathy affect?

Nerve roots

What symptoms does radiculopathy cause?

Weakness in a myotome and numbness in a dermatome

What is a common cause of radiculopathy?

Degenerative disc disease

What causes symptoms in degenerative disc disease?

Compression of ventral and dorsal nerve roots

What is the typical cause of disc compression in radiculopathy?

Trauma or chronic degeneration of the intervertebral disc

What is a common clinical feature of radiculopathy?

Pain in the neck or lower back

What type of pain radiates in radiculopathy?

Pain radiating along a dermatome

What often precipitates radiculopathy symptoms?

Lifting a heavy object

Which nerve root controls shoulder abduction and elbow flexion?

C5–C6

Which nerve root controls elbow extension and wrist flexion?

C6–C7

Which nerve root controls wrist and finger extension?

C7–C8

Which nerve root controls grip and intrinsic hand muscles?

C8–T1

Which nerve root controls hip flexion and knee extension?

L2–L4

Which nerve root controls ankle dorsiflexion?

L4–L5

Which nerve root controls ankle plantarflexion and knee flexion?

L5–S1

Which nerve root controls hip extension?

S1–S2

Why is thoracic disc disease uncommon?

The thoracic spine is less mobile due to the rib cage

What is the conservative treatment for radiculopathy?

Physical therapy and epidural spinal injections

What is the surgical treatment for radiculopathy?

Surgical decompression

Where does cauda equina syndrome occur?

Lumbosacral spine below L1–L2

What causes cauda equina syndrome?

Posterior disc extrusion compresses descending nerve roots

What is a hallmark symptom of cauda equina syndrome?

Saddle anesthesia

What dermatomes are involved in saddle anesthesia?

S2–S4

What motor symptoms occur in cauda equina syndrome?

Leg weakness and incontinence

What is the treatment for cauda equina syndrome?

Emergency surgical decompression

What virus causes shingles-related radiculopathy?

Varicella zoster virus (VZV)

Where does VZV remain dormant?

Dorsal root ganglia

What reactivates VZV in shingles?

Latency breaks, often years after chickenpox

What symptom is characteristic of shingles radiculopathy?

Painful rash in a dermatomal distribution

What is the treatment for shingles radiculopathy?

Valacyclovir

What does plexopathy affect?

The nerve plexuses

What symptoms does plexopathy cause?

Weakness and numbness in multiple nerve root or peripheral nerve distributions

What are common patterns of plexopathy?

Upper plexus (shoulder/upper arm) or lower plexus (forearm/hand, knee/foot) involvement

What causes brachial neuritis?

Post-infection or injury triggers an inflammatory reaction in the brachial plexus

What type of injury is brachial neuritis?

Monophasic autoinflammatory injury to the brachial plexus

When does brachial neuritis typically occur?

About 5 days after immunization, trauma, or surgery

What is the initial symptom of brachial neuritis?

Severe shoulder pain

What follows shoulder pain in brachial neuritis?

Weakness and numbness, usually in the upper plexus

What part of the body is commonly affected in brachial neuritis?

Shoulder and upper arm

What is a possible treatment for brachial neuritis?

Steroids

What is the recovery pattern of brachial neuritis?

Slow improvement over time

What causes Erb’s or Klumpke’s palsy?

Birth injury due to shoulder dystocia or breech delivery

What structure is stretched in Erb’s or Klumpke’s palsy?

The brachial plexus

What part of the plexus is injured in Erb’s palsy?

Upper plexus

What part of the plexus is injured in Klumpke’s palsy?

Lower plexus

What does peripheral neuropathy affect?

Peripheral nerves

What symptoms does peripheral neuropathy cause?

Weakness and numbness in peripheral nerve distribution

What is mononeuropathy?

Injury to a single nerve

What is polyneuropathy?

Injury to multiple nerves

What are the types of neuropathy based on function?

Sensory, motor, or sensorimotor

What is axonal neuropathy?

Injury to the axon

What is demyelinating neuropathy?

Injury to the myelin

What is the most common cause of generalized polyneuropathy?

Diabetes mellitus

What causes diabetic polyneuropathy?

Chronic microvascular inflammation and ischemia of nerves

Why are long nerves more affected in diabetic neuropathy?

They are harder to maintain and die first

What type of neuropathy does diabetes cause?

Sensory-predominant axonal polyneuropathy

What are other causes of polyneuropathy besides diabetes?

B12 deficiency, HIV, multiple myeloma, chemotherapy

What is the typical progression of diabetic polyneuropathy?

Months of worsening symmetric numbness in feet

Where does diabetic neuropathy begin?

Feet, progressing to ankles, knees, and fingers

What sensory symptom is common in diabetic neuropathy?

Painful numbness

What motor symptom can occur in diabetic neuropathy?

Distal foot weakness

What balance issue is associated with diabetic neuropathy?

Gait unsteadiness from proprioception loss

What is the treatment for diabetic neuropathy?

Treat the underlying condition

What causes Guillain-Barré syndrome (AIDP)?

Post-infectious autoinflammatory injury to peripheral nerve myelin

What is the typical trigger for Guillain-Barré?

Campylobacter jejuni infection

What is the typical presentation of Guillain-Barré syndrome?

Weakness and numbness starting in legs

How quickly does Guillain-Barré progress?

Over 2 days

What is the progression of weakness in Guillain-Barré?

Legs → arms → respiratory muscles

What reflex finding is typical in Guillain-Barré?

Absent reflexes

What is CIDP?

Chronic Inflammatory Demyelinating Polyneuropathy

How is CIDP different from AIDP?

Slower progression over 8+ weeks

What does a nerve conduction study show in Guillain-Barré or CIDP?

Patches of demyelination in multiple nerves

What does CSF show in Guillain-Barré?

High protein with normal WBC (albuminocytologic dissociation)

What is the treatment for Guillain-Barré?

IV immunoglobulin (IVIG)