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A neuromuscular or a myopathic disorder differs from a neurological disorder in at least one important aspect. The patient in a persistent vegetative state, who is unable to breathe or cough effectively due to a neurological defect is unaware of his limitations. While he may retain a cough or gag reflex, he lacks cortex involvement; in a way, there is no real SOB in this patient because there is no real awareness of dyspnea.   

On the other hand, the person with a neuromuscular or myopathic disorder exists in a hellish reality in which she is completely aware of her dyspnea, but unable to do anything about it. She is aware; she has SOB, but lacks the muscle strength to even create the retractions, or nasal flaring that would signal to the rest of us that she is in serious respiratory distress.  

In many neuromuscular or myopathic disorders, the patient retains sensory impulses, but lacks the motor neurons or the muscles these motor neurons approach fail to respond to the commands. 

PATHOPHYSIOLOGY OF NEUROMUSCULAR/ DISORDERS  

In the patient with a neurological disorder, the cortex, brainstem, spinal cord & sometimes upper motor neurons are affected, but in the neuromuscular patient the damage is usually in the spinal cord or in the peripheral nervous system. Some disorders damage the lower motor neurons such as the anterior horn cells, other damage the peripheral neurons, or the neuromuscular junction.

 

Due to the patient’s muscle weakness or discoordination, there can be problems with some or all of the following: 

1.    Inability to protect the airway which can result in aspiration of oral secretions, and increased risk of aspiration pneumonia—

·      if not actual upper airway occlusion from soft-tissue obstruction which may be worse during supine positioning

·      dis-coordination of the vocal cords to work properly during the cough

2.    Inability to cough and deep breathe which leads to increased secretions, peripheral micro-atelectasis and recurrent bronchopneumonia. It is no surprise that the most common cause of death for many types of neuromuscular patients is infection.

·      the accessory muscles of inspiration fail to give the patient the IC he needs for inspiration phase & the breath-hold

·      the muscles of the vocal cords may fail to close to create the increased pressure of the compression phase

·      the accessory muscles of exhalation fail to create the positive pressure needed to cough

3.    At worst, the muscle weakness decreases the Vt so that there is hypoventilation, which can lead to hypoxemic hypoxia, hypercapnia, decreased V/Q, and increased Vd/Vt as the alveolar ventilation drops.

4.    As the alveolar ventilation drops the critical volume drops so that the lung compliance gets lower and increased WOB is created just at a time when the patient cannot manage normal WOB

5.    In the long-term progressive paralysis disorders, the chronically low PA02 leads to cor pulmonale

6.    In the long-term progressive paralysis disorders,  chronically high PaC02 leads to compensated respiratory acidosis and a person who may have a hypoxic drive

7.    In other, more rapidly moving disorders, the patient’s ABG may be perfect right up to the moment of sudden respiratory failure.

8.    Patients with significant paralysis are at risk for positional hypotension due to the inability of the venous blood to return to the thorax by muscle action on the veins of the lower extremities.

9.    Immobilized patients may also be at increased risk for deep vein thrombosis which could result in pulmonary emboli 

 

S/S OF ALL NEUROMUSCULAR DISEASES

 

1.    during interview, in the early stages of the progressive disorders, the patient may c/o dyspnea on exertion or that he is easily fatigued

2.     a more serious sign is when the patient c/o dyspnea at rest

3.     the patient may c/o orthopnea

4.     if he c/o excessive daytime somnolence, particularly if coupled with s/s of cor pulmonale, the possibility of sleep apnea complicates the situation

5.    difficulty with eating, and swallowing is a serious sign of potential need for intubation and mechanical ventilation. This patient may cough after swallowing

6.    the RCP might note the patient speaking with low-volume voice & he may have staccato breathing [has to take a breath between each word]

·      an easy bedside test of a patient’s IC, is to ask him to inhale to max and count out loud to 50. The normal person can do this; the patient with severe defect will count to less than 15.

7.    on observation, the RCP may note that the patient has rapid, shallow breathing

8.    on observation, the RCP may notice paradoxical abdominal movement which is inward movement  on inspiration. This is a sign of significant bilateral diaphragmatic weakness. Rather than observation, employ palpation of these muscles to assess them.

9.    there can be weakness of the inspiratory muscles and of the accessory muscles of exhalation, but any muscles the patient can use, he will, so there can be increased use of accessory muscles

10. on auscultation, there will be decreased BBS in the lower lobes & crackles with atelectasis

11. On percussion there can be dullness over areas of consolidation

12. bedside PFT's are the most sensitive marker for neuromuscular patients:

·      this is a restrictive defect so we know that the volumes & capacities will be decreased

o VC of less than 30 ml/kg is associated with weak cough & atelectasis

o VC of less than 15 ml/kg is an indication for mechanical ventilation

·      the Fev₁ is decreased due to inability to use accessory muscles of exhalation to force out the air

·      but oddly the RV may be normal even elevated in some because the patient can not force an exhalation

·      if the VC and the Fev₁ both decrease 20-25% between sitting and lying, there is diaphragmatic weakness

·      the patient will not be able to generate both inspiratory and expiratory pressures due to weakness. In fact, these pressures may be decreased 50% before the VC or Fev₁ decreases

o inspiratory max pressure: measures inspiratory chest wall muscles and diaphragm. a need for mechanical ventilation is seen with a [P_Imax] less -30 cmH20

o  expiratory max pressure: measures weakness of the abdominal muscles. a need for mechanical ventilation is seen with a [P_Emax] less than + 40 cmH20

o be aware that facial weakness can result in false values for these two figures if the patient cannot seal properly—needless to say, that alone tells us we have problems  

 

 

Remember! The 20/30/40 rule

for intubation of neuromuscular patients:

VC less than 20,

Inspiratory pressure less than -30

Expiratory pressure less than + 40.

 

1. ABG:

   • It is important to understand that with some of the rapidly progressing neuromuscular problems, ABGs can stay WNL right up to the moment of respiratory failure.

 

• in the slower progressive paralysis patients, at first there can be respiratory alkalosis due to the rapid, shallow breathing, but later as Vt decreases, acute respiratory acidosis results.

 

• with the chronic, long-term progressive neuromuscular disorders, the patient may have compensated respiratory acidosis with moderate hypoxemia as a baseline.

 

2. constant pulse-oximetry may need to be placed with alarm settings to alert us to the possibility of significant nocturnal desaturations

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Because these disorders are so different from one another, we need to discuss the ones the RCP sees most often one by one.

 

1.     anterior horn cells

a.     Amyotrophic lateral sclerosis or ALS [Lou Gehrig’s Disease] is an aggressive paralysis of upper & lower neurons with an onset in the 50s. The patient is usually dead within 5 years of diagnosis.

 

·      because this disease is relentless, the patient may opt to refuse intubation and ventilation

·      non-invasive mechanical ventilation is an option as long as the patient can protect his airway, but most patient will end up with tracheostomy & PPV

 

b.     poliomyelitis [polio] an acute viral infection that leaves varying degrees of paralysis.

·      At worst the patient has lost control of his respiratory muscles & required mechanical ventilation-- sometimes for years. This disease is easily prevented by vaccination.

·      Because of the limited technology available in the 1950s, it is well-established that these patients did quite well with negative pressure ventilation by iron lung.

·      In the last few years, it has been seen that many of the middle-aged patients who survived the last great epidemics of the 1950s as children are starting to lose function in their muscles. ‘Postpoliomyelitis’ is a new disorder for which the new generation of RCP will have to deal.

 

2.     the peripheral neurons

a.    Guillain-Barré Syndrome is a peripheral neuritis that starts in the lower extremities & rises toward the head. It can stop at any point, but when it involves the nerves of the chest wall and diaphragm, the patient will go into respiratory failure.

 

b.    Guillain-Barré Syndrome is an auto-immune defect in which the body’s antibodies attacking the patient’s own myelin sheath around the nerves.  

 

c.   This reaction seems to be triggered by viral illness such as the flu, or viral GI infections, but it has been associated with some vaccines such as rabies shots and the bivalent flu shot.

 

d.    S/S of GBS

·          the cerebral spinal fluid [CSF] has increased protein levels

·      nerve conduction studies are abnormal

 

·      the patient will have loss of deep tendon reflexes often before there is loss of muscle strength  

 

·      there is a progressive, bilateral ascending motor weakness starting at the feet and moving up.

 

·      the patient may c/o tingling, ‘pins and needles’ and tenderness to these limbs 

 

·      this progression can happen quite rapidly, with a patient unable to walk at breakfast time & requiring mechanical ventilation by supper time

 

e.    This is often an acute illness that is self-limiting. 50% of patients will peak at 2 weeks & 80% will be over this disorder in 40 days.

 

f.     Children fare much better than adults with only 10-15% of kids having permanent weakness.  

 

g.    As many as 20% of adults will have significant weakness

 

h.    Many of the patients require mechanical ventilation for a few weeks, but some have stayed on mechanical ventilation for a year. 

 

i.     One of the more disturbing features of GB is that some of the neurons that can be attacked are autonomic nerves so that blood pressures can become quite unstable.

 

j.     Even short disconnections from mechanical ventilation can cause blood pressures to drop to seriously low levels. 

 

k.    These patients are also subject to sudden bradycardia or bronchorrhea from the autonomic malfunctioning

 

l.      treatment includes  

·      Plasmapheresis to remove these antibodies from the blood serum

·      IV gamma globulin injections started within the first 2 weeks is more successful with kids than with adults, who sometimes rebound

·      We are aggressive with initiation of mechanical ventilation as soon as it becomes clear the patient cannot protect his airway or manage his secretions-even if his VC is still good

·      If intubation lasts longer than 2 weeks, tracheotomy is recommended

·      Once the patient can achieve a VC of 18 ml/kg and a [P_Imax] of more than 30 cm H₂O, you can consider weaning him from mechanical ventilation 

 

m.  Unilateral Paralysis of the diaphragm due to phrenic nerve damage is possible with open heart surgery or other chest trauma. This is usually unilateral damage & if the nerve is just pinched, it comes back in 6 weeks, if not there is permanent unilateral paralysis.

·      the effected hemi-diaphragm will be seen on x-ray as higher than normal and if an exhalation film is done, the damaged hemi-diaphragm will be seen to not move, a live action-ray called a fluoroscope will show only motion on the unaffected side

·      the healthy adult may only have a 15-20% drop in VC and TLC, while the infant will fare less well.

 

n.    Bilateral paralysis of the diaphragm due to Spinal cord injury between C2-C5 can damage one or both phrenic nerves.  Immediate treatment with IV steroids is suggested with all spinal cord injuries 

 

 

 

3.     neuromuscular junction

Myasthenia Gravis [MG] is a disorder characterized by exacerbations [MG crisis] triggered by physical or emotional trauma.

·      At increased risk are young women (3-4.5x more likely than males)

·      The young woman with MG may present with droopy eyes and might have more upper airway occlusion than expected with relatively strong chest and abdominal muscles

·      The weakness progresses through the day and is made worse by exercise

·      It is an auto-immune defect in which the patient creates antibodies against acetylcholine. These may be noted when blood is drawn

·      When the patient is given an anti-cholinesterase inhibitor, the Tensilon Test, there is a transient recovery of muscle strength

·      Repeated nerve conduction studies show a fading away effect

·      10% of MG patients have abnormal thymus glands

·      10-15% of these women who have babies can have infants with a transient MG that recedes within 18 days. It is felt this is due to the mothers antibodies in the fetus and the baby is not at increased risk of developing MG

o    Treatment of MG includes

1.    administration of anti-cholinesterase

2.    sometimes we can use plasmapheresis to remove these antibodies from the blood serum

3.    a newborn with severe transient MG can get an exchange transfusion to remove mom’s antibodies

4.    removal of thymus is helpful in some adult patients but is strongly recommended in the early stages during childhood MG

5.    steroids might be helpful in children, but steroids also cause myopathy

6.    we avoid pulmonary rehabilitation in this patient because exercise exacerbates the weakness

7.    we intubate and ventilate when the bedside parameters show that the patient is at risk of respiratory failure, but remember this patient might need intubation for the upper airway problems before she needs ventilation.

8.    Non-invasive mechanical ventilation is contra-indicated if she cannot protect her airways 

 

PATHOPHYSIOLOGY OF MYOPATHIC  DISORDERS 

Some disorders limit action by the disorders damage to the muscle itself.

1.     Duchenne’s Muscular Dystrophy: genetic defect on the X chromosome that affects males only, although their mothers & sisters can be carriers of the gene.

 

a.   the structural protein dystrophin is missing in skeletal muscles

b.   the child may have some gait problems between 3-5 years of age

c.   patient will be wheel-chair bound by 12-16 years of age

d.   when the muscle weakness reaches the chest wall, the patient will die of   respiratory failure by the time he is 20 years of age

e.   there will be more respiratory problems at night and some will fear bedtime

f.    a few patients may have some cardiomyopathy and scarring of the left ventricle

g.   due to the differences in the different muscle groups, boney deformities such as scoliosis are common with the MD patient

h.    starting these patients on prophylactic NIPPV has not helped, we need to wait for them to show s/s of requiring mechanical ventilation

i.     patients with this disorder may opt to defer intubation, but many have been maintained for years on NIPPV and with negative pressure ventilation.

j.     sometimes the patient starts out sleeping on PPV, then as the disease progresses, will be on the ventilator all the time

 

2.    Myotonic dystrophy: is a common progressive muscle weakness in adults that can be manifested by cardiac conduction problems, and endocrine dysfunction

a.    This is due to a defect on chromosome 19

b.    Persons with Myotonic dystrophy are abnormally sensitive to sedation and paralytic drugs & during the post-op period,  they need to be weaned slowly with close monitoring

c.    They do not always have serious enough problems to have respiratory distress

d.    Sleep apnea is common with these patients due to collapse of soft tissue in the upper airways during sleep

e.    These patient respond quite well to nocturnal nasal CPAP to keep the airways open  

S/S OF MYOPATHIC DISEASES, in addition to the other s/s of neuromuscular disorders

 

1.    on interview, there could be a family history of myopathic illness

2.    abnormal skeletal muscle biopsy usually done on the upper leg

a.    in DMD, there will be absent levels of dystrophin

b.    in polymyositis, there will be elevated muscle enzymes such as serum creatine kinase

3.    on observation, one may notice some scoliosis as some muscle groups are affected more than others  

Neuromuscular/myopathy associated with long-term ventilation associated with sepsis/ARDS/multiple organ failure

In the last few years, a phenomenon has been noted in the MICU. After several days of mechanical ventilation, some patients have shown signs of a neuropathy or of myopathic changes not only in ventilatory muscles but in the extremities. Nerve conduction studies have displayed neuropathy and muscle biopsies have demonstrated the muscle atrophy. Some patients have one, others have both problems. There is no loss of sensation; the patient seems to feel the pain, but the muscle weakness prevents the patient from pulling back from the stimuli. 

 

  While little is known about these two syndromes, there are a few facts known.

 

• We know that mice placed in Control mode for a mere 18 hours will show wasting of the diaphragm & that PEEP seems to make that worse; this fact is one of the reasons we have moved to SIMV and PS modes in the last few years and why we try to wean the patient as soon as possible.

 

• We know that patients who are in the ICU for sepsis, ARDS and status –post organ transplants seem to be at increased risk of myopathy.

 

• We know that the patients who are in the ICU for sepsis, SIRS [systemic inflammatory response syndrome] & multiple organ failure are at increased risk for neuropathy.

 

• We know that a few asthmatic patients who were intubated, ventilated and placed on high levels of paralysis and steroids demonstrated some s/s of acute myopathy

 

• We know that elevated serum creatine kinase shows up quite early in course of the myopathy

 

• While little is sure, research seems to show that high risk patients for getting either of these weakness disorders seems to be getting a combination of these situations:

1.    patient who is getting high levels of steroids

2.    patient who is hyperglycemic

3.    patient who has been paralyzed by neuromuscular blocking agent such as tubocurarine or pancuronium [Pavulon]

• Research is being done to study the effects of treating critically ill ICU patients with insulin to keep the glucose levels down.

• Other studies are looking at increasing the periods of time that the patient spends off the paralytic agents

• Other studies are looking at the ability of steroids to increase the glucose levels

• We must also remember:

o   that some disorder such as Myasthenia Gravis can be exacerbated by massive insult 

o   Guillain-Barré  Syndrome can be triggered by massive insult

o   patients with Myotonic dystrophy are abnormally sensitive to sedation and paralytic drugs

 

 Treatment of all patients who suffer partial or full paralysis of muscles of ventilation.

 

1.     protect the airway with timely intubation

 

2.     assist deep breathing and coughing

a.    hyperinflation by IPPB once it  has been established that the patient’s IC is less than 10 ml/kg

b.    maximal insufflations can be achieved by stacking a few breaths via manual resuscitator to the mask. This should be followed by a cough once the chest wall is expanded and elastic recoil is at a peak

c.    patient who can ‘frog breath’ by gulping air can also stack breaths