Neuromuscular Disorders

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    Source: "The Little ICU Book" by Paul Marino (2008)

     

    Myasthenia Gravis

    Pathophysiology

    • Autoimmune disease caused by antibody-mediated destruction of acetylcholine receptors on post-synaptic neuromuscular junctions.
    • Often triggered by something:
      • Major surgery
      • Illness
      • Thymic tumors (20% of cases)
      • Hyperthyroidism (5%)
      • Drugs (aminoglycosides, ciprofloxacin, cardiac drugs [B-blockers, lidocaine, procainamide, quinidine])

     

    Clinical Features

    • #1 symptom: Weakness!!!
      • Worse with activity, improves with rest
    • Classically:
      • Starts in the eyelides and extra-ocular muscles.
      • Progresses to limbs
      • Can progress to chest wall  + diaphragm --> respiratory failure --> "myasthenic crisis" (15-25% of pts)
    • Initial Focal onset:
      • 53% Ocular (ptosis, diplopia)
        • Often with reading
      • 20% Facial Limb
        • Masseter fatigue, nasal speech
      • 17% Bulbar, respiratory (dysphagia/arthria)
      • 10% Generalized
    • 85% become generalized
    • Limbs: Proximal, diurnal variation.
    • Eyes: Vertical Diplopia
      • Ice cube test: Ptosis goes away with ice.
      • (Enzyme to break down acetylcholine not as effective?)

     

    Diagnosis

    • Diagnosis

      1.  Characteristic pattern of muscle weakness (eyelid/extra-ocular --> worse with repeated use)

      2.  Increased muscle strength after edrophonium (Tensilon) - an acetylcholineesterase inhibitor

      3.  Acetylcholine receptor antibodies (85% of MG - presence secures dx)

     

    Treartment:

    • Acetylcholinesterase inhibitors
      • Pyridostigmine (aka Mestinon) 60mg q6hrs (can increase to 120mg q6h)
        • Can give IV as well  in myesthenic crisis: 1/30th of oral dose.
      • Careful, increases secretions (If cannot clear, can cause problems)
    • Immunothreapy
      • Three options:
        • Prednisone 1-1.5 mg/kg/day (slow increase to 60-80mg/day - 50% get worse initially)
        • Azathioprine 1-3 mg/kg/day
          • Steroid sparing (2.5 mg/kg/d)
          • Skin cancer risk is higher (if >10y)
          • Takes a long time to kick in (3-6mo)
        • Cyclosporine 2.5 mg/kg BID
          • Careful with cystitis
      • Surgical thymectomy (if < 60yo to reduce need for long-term immunosuppression)
    • Plasma Exchange OR IVIG (both qually effective, plasma exchange = faster response)
      • Plasma exchange  (responds in days)
        • used for severe cases that require mechanical ventilation
      • IVIG
        • Immunoglobulin G 0.4-2g/kg/day IV for 2-5d)

     

    Critical Illness Polyneuropathy & Myopathy

     

    Critical Illness Polyneuropathy

    • Diffuse sensory & motor axonal neuropathy
    • Reported in 50% of severe sepsis
    • Presents 2 days to few weeks after illness onset
    • Diagnosis:
      • Nerve Conduction Studies (NCS) necessary for diagnosis
    • Treatment:
      • No treatment...
      • Treat precipitating condition.
      • Complete recovery in 50% (can take months)

     

    Critical Illness Myopathy

    • CIM --> spectrum of muscle disorders.
    • Predisposing conditions:
      • Systemic inflammation
      • Prolongued steroid use
      • Prolongued neuromuscular blocker use
    • Diagnosis:
      • Electromyography (shows myopathy)
      • Muscle biopsy (shows atrophy and loss of myosin filaments)
    • Treatment:
      • No treatment (treat precipitating factors)
      • Most recover fully in few months.

     

    Lambert-Eaton Syndrome

    • Myasthenic syndrome
    • 50% will have Small-Cell Lung Ca (but 3% of SCLCa will get Lambert-Eaton Syndrome)
    • Rare neuromuscular junction disorder caused by antibodies to presynaptic voltage-gated P/Q-type calcium channel
    • Symptoms:
      • Progressive proximal limb weakness
      • Dysautonomia (dry eyes, dry mouth, constipation, erectile dysfunction)
      • Absent Deep Tendon reflexes
        • Facilitation (improvement after brief isometric exercise)
      • Suspect Lambert-Eaton Myasthenic Syndrome in:

        1.  Proximal limb weakness

        2.  Absent deep tendon reflexes

    • Diagnosis:
      • EMG testing + Positive assay for P/Q-type Ca++ channel antibodies

     

    ALS

    • Fatal disease involving motoneurons
    • Symptoms:
      • Progressive weakness & muscle wasting
      • Focal Weakness, muscle twitches, cramping
      • Difficulties with speech & swallowing
      • --> In Advanced --> Respiratory Failure and Death
    • Physical Exam
      • Upper Motor Neuron (Hyperreflexia, spasticity, extensor plantar response)
        AND
      • Lower Motor Neuron (atrophy, fasciculation)
      • NO Sensory deficits (IMPORTANT!!)
      • Frontotemporal Dementia is sometimes present
    • Diagnosis:
      • Clinical Findings
      • EMG Results
      • MRI brain & spine --> May show T2 bright spinal bilateral corticospinal pathways (ALS feature)
        • Exclude other Dx like stroke!
    • Treatment:
      • Supportive!
      • Riluzole (Glutamate Release Inhibitor) - may slow progression by 3mo
        • Must monitor liver function
      • G-Tube prolongs survival
      • Respiratory Support - BiPAP (once CO2 goes up) if within goals of care
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