PNS Pathology Flashcards
What are the three general classes of peripheral nerves? What information does each transmit?
- large myelinated: motor, proprioception, fine touch, vibration, 2-point discrimination
- small myelinated: pain and temperature
- small unmyelinated: also pain and temperature
What is the temporal evolution of most peripheral neuropathies?
- most are slow and insidious; chronic
- in cases with acute or subacute development (2 months or less): suspect GBS, vasculitis, infection (Lyme disease), etc.
How can we determine whether or not a neuropathy is targeting the myelin or the actual axon?
- use nerve conduction studies (NCS) and an electromyography (EMG)
- in demyelination: conduction velocity will decrease, latency period will increase, and there will be no fibrillations
- in axonal damage: amplitude of the potentials will decrease, and there will be fibrillations
What are the most common causes of small-fiber (loss of pain and temp) peripheral neuropathy?
- diabetes mellitus / glucose intolerance
- amyloidosis
- (diabetes is the most common cause of peripheral neuropathy in the developed world)
What is mononeuropathy multiplex? What are the most common causes?
- mononeuropathy multiplex is when two or more separate nerves are affected
- vasculitis, hepatitis, Lyme disease, HIV
What are the two conditions where nerve biopsy is indicated?
- in amyloid neuropathy and in vasculitis
- otherwise, it is rarely indicated
Compare radiculopathy, plexopathy, mononeuropathy, and polyneuropathy. What are the most common causes of each?
- radiculopathy: spinal root is affected (herniated disk, Lyme disease)
- plexopathy: brachial or lumbosacral plexus is affected (trauma, tumor invasion, compression, viral infection)
- mononeuropathy: solitary peripheral nerve affected (trauma, compression, entrapment)
- polyneuropathy: multiple nerves simultaneously affected; usually is length-dependent (glove and stocking), affecting lower limbs before upper limbs (GBS, diabetes)
What are common causes of acute polyneuropathy? Chronic polyneuropathy?
- acute: Guillian-Barre syndrome
- chronic: inflammatory (CIDP, vasculitis), infection (leprosy, HCV, HIV), inherited (Charcot-Marie Tooth), metabolic (diabetes, thyroid disease, liver failure), toxic (alcohol), nutrition (vit B12 deficiency), systemic (RA, sarcoidosis, Sjogren’s syndrome), malignancy
What are the most prevalent causes of chronic polyneuropathy?
- diabetes, HIV, uremia, and alcohol
How common are changes in mental status in peripheral neuropathy?
- changes in mental status are rare in peripheral neuropathy (exceptions are with alcohol, uremia, or in the setting of hyperosmolar hypoglycemia in diabetics)
What is peripheral neuropathy? How do patients commonly present?
- any disorder that affects the nerve structures beyond the spinal cord
- most common presentation: sensory change (loss if destructive, paresthesias if irritative)
- motor symptoms (gait and balance instability) are less common; muscle weakness is a late sign
- (this is because smaller fibers, which deal with sensation, are usually more affected than larger fibers, which control movement)
What is Charcot-Marie-Tooth disease? What deformity is commonly associated with this disease? What gene is mutated?
- a group of inherited disorders affecting the peripheral nervous system (most are A.D.)
- look for muscle wasting and sensory loss
- many patients have pes cavus (a high-arched foot with flexion of the small toes, called “hammer toes”)
- results from mutations in the PMP22 gene (this codes a protein in the myelin sheath)
Diabetes is associated with several different patterns of neuropathy - what are they? Which is the most common?
- most common pattern: distal and symmetrical, with sensory-greater-than-motor neuropathy (AKA distal symmetrical sensorimotor polyneuropathy)
- others: painful, asymmetric, proximal-to-distal pattern (AKA lumbosacral radiculopathy); mononeuropathy multiplex pattern; pure autonomic neuropathy pattern (AKA autonomic neuropathy)
What primary investigations should we order for patients presenting with signs of peripheral neuropathy?
- nerve conduction studies (NCS) and electromyography (EMG)
- CBC, electrolytes
- fasting blood glucose, HbA1c (diabetes)
- ESR, CRP (inflammation)
- TSH, B12, folate (metabolic, nutritional)
- rheumatoid factor, auto-antibodies
- LFTs, urinalysis (liver and kidney failure)
What is Guillian-Barre Syndrome? How do patients classically present? How do we treat it?
- a relatively common life-threatening disease of the PNS; autoimmune destruction of Schwann cells
- rapidly progressing acute demyelinating disorder of motor neurons; due to an autoimmune response (usually following an infection) via molecular mimicry (antibodies target ganglioside cells)
- classic presentation: ascending weakness and paralysis (motor deficits are greater than sensory deficits)
- patients can die from respiratory failure once these muscles are affected (30% will require ventilation)
- treat with plasmapheresis and IVIG (however, in patients that are IgA deficient or who have renal failure, only give plasmapheresis)
