3.2 Principles of Neuromuscular Disease

Question 2

What is the neuromuscular functional unit?

Answer 2

the neuromuscular functional unit is comprised of:
- LMN
- NMJ
- muscle

NM disease can therefore be a:
- neuropathy (LMN)
- junctionopathy
- myopathy

the main point of an exam is to localize it to the neuromuscular system (as opposed to brain or spinal cord); from THERE, further localization is very difficult and usually the job of referral hospitals

the lecturer wants us to understand factors that can push us in the direction of myopathy vs neropathy vs junctionopathy to shorten the differential list

Question 3

What are the cardinal signs of neuromuscular (NM) disease?

Answer 3

NM disease is characterized by weakness WITHOUT ataxia

• can be seen as exercise intollerance
• can be seen as an extreme, short, stiff gait: the paw moves centimeters only and the joints do not bend

these are not even close to all of the possible clinical signs (NM system is throughout the entire body), however they are the “cardinal” or most likely signs

Question 4

When should neuromuscular disease be expected?

Answer 4

(1) if you see the cadinal signs

• weakness without ataxia (including short stride / stiff muscles, and exercise intollerance)

(2) if the history includes:

• dysphonia
• regurgitation (megaesophagus)
• dyspnea (laryngeal paralysis, arpiration pneumonia)
• note: cranial nerves can be affected^

(3) if on physical exam, you see:

• marked muscle atrophy (generalized or focal: e.g., bilateral trigenimal neuropathy)
• muscle hypertrophy in specific conditions
• myalgia (painful mucles on palpation)

Question 5

You suspect neuromuscular disease: what are the signs that can push you in the direction of a neuropathy?
Spinal reflexes?

Answer 5

• reduced to absent spinal reflexes
• plantigrade/palmigrade stance
• reduced to absent muscle tone

+/- neurogenic muscle atrophy

Question 6

You suspect neuromuscular disease: what are the signs that can push you in the direction of a junctionopathy?
Spinal reflexes?

Answer 6

• normal to reduced spinal reflexes
• exercise intolerance (may be normal on exam prior to exercise)

Question 7

You suspect neuromuscular disease: what are the signs that can push you in the direction of a myopathy? Spinal reflexes?

Answer 7

most important and specific: increased muscle enzymes (CK, AST, ALT), as well as myoglobinuria

• often normal spinal reflexes (unless severe and specific conditions)
• exercise intolerance
• cervical ventroflexion (cats ONLY), or low head carriage

+/- myalgia
+/- mucle atrophy/hypertrophy

Question 8

What are common electrodiagnostics?

Answer 8

both are referral techniques to further localize the neuromuscular disease to a neuropathy, junctionopathy, or myopathy

(1) electromyography:
- needle in muscle and see electrical activity
- distinguishes between disuse atrophy and neurogenic atrophy
- cannot determine etiology

(2) nerve conduction studies:
- stimulate nerve at different points and detect signal in muscle
- more specific
- e.g., normal = myopathy; decreased response with repetition = myasthenia gravis (junctionopathy); decreased amplitude = axonal disease (neuropathy)

Question 9

Once you have distinguished if a NM disease is a neuropathy, junctionopathy, or myopathy, what is then most crucial to prioritize the differential list (and localize the lesion)?

Answer 9

onset and progression from the 5 finger rule are the most important

Question 10

What diagnostic testing must be done for all NM disease suspect cases?

Answer 10

• hematology
• serum biochemistry
• urinalysis

additional testing is extremely varied (genetic, endocrine, CSF, etc.), and depends on the most likely differentials

Question 11

What is acute polyradiculoneuritis?

Answer 11

coonhound paralysis:
- an immune-mediated neuropathy: reaction to raccoon saliva, vaccination, bacteria, or other antigen is suspected
- clinical signs often develop 7–14 days after a raccoon bite or scratch
- common in dogs and rare in cats
- presumptive diagnosis via electrodynamics and CSF analysis
- acute onset -> progressive neuromuscular weakness (days) -> plateus (weeks) -> gradual recovery without specific treatment (weeks-months)

it can also occur in dogs who have not encountered a raccoon (idiopathic): cause is unknown

treat with physical rehabilitation or IV human immunoglobulins in severe cases

Question 12

What is centronuclear myopathy?

Answer 12

“hereditary myopathy of labradors” or “labrador myopathy” – now named for the typical histopathology finding of internalized myofibre nuclei:

• hereditary myopathy
• resulting from a mutation in the protein tyrosine phosphatase-like member A gene (PTPLA) necessary for muscle homeostasis
• noticed at 3-4 months old, worse with stress, cold weather, exercise
• diagnosis by muscle biopsy
• managed with dietary supplements (Vit B, L-carnitine, etc.)
• usually stabilized around 6-12 months with mild disabilities