A 55-year-old female with no reported comorbid conditions has been admitted unexpectedly to your tertiary intensive care unit from the operating theatre. She had an uneventful general anaesthetic for a total hip replacement.
Forty-five minutes later, she was found unrousable by the attending nurse. An arterial blood gas showed the following:
Normal Adult Range
7.35 – 7.45
110 mmHg (14.5 kPa)
35 – 45 (4.6 – 6.0)
100 mmHg (13.1 kPa) (15 L/min oxygen via non-rebreather mask)
-2 – +2
She was urgently reintubated and transferred to your ICU. She is hemodynamically stable.
Give the most likely differential diagnoses.
This is taken from the chapter on hypercapnoea
|Decreased minute ventilation||
Central nervous system
Metabolic, endocrine and environmental
|Increased dead space||
Increased anatomical dead space
Increased alveolar dead space (i.e. ventilated but not perfused)
|Increased CO2 production||
Increased metabolic rate
How will you assess the patient?
- Short fat neck of OSA
- Cushingoid appearance (OSA, but also suspicious of long term steroids for some sort of autoimmune condition, or COPD)
- Wasting and cachexia of severe CCF, end-stage COPD or cancer
- Abnormal breathig pattern (eg. the abdominal breathing of a C-spine quad)
Start with the hands.
- Clubbing (suggestive of chronicity)
- Unilateral small muscle wasting (lung mass invading brachial plexus)
- Pulse (collapsing pulse of AR?)
Axillae and neck
- Lymph nodes
- JVP (cardiac causes of ventilation failure)
- Dissection scars from lymph node clearance; radiotherapy tattoos
Face and cranial nerves
- Plethoric "mitral facies"
- Droop, cranial nerve signs of stroke
- Horner's syndrome (malignancy or stroke)
- Temporalis wasting (malnutrition)
- Abnormal chest wall movement (eg. flail segment or unilateral phrenic nerve paralysis)
- Subcutaneous emphysema on palpation, suggestive of pneumothorax
- Percussion findings (eg. dullness of an effusion)
- Auscultation findings of wheeze or creps (spasm or APO)
- Recent abdominal wounds (is pain or infection preventing diaphragm excursion?)
- Distension (Gas? Poop? Ascites?)
- Oedema of CCF or prolonged bed stay
- Muscle wasting of quads (another feature of malnutrition)
On your first examination of thsi new admission, you find that the chest is clear and the cardiovascular examination is unremarkabkle. The patient is now awake. There is generalised weakness, 3/5 power. Reflexes are normal. There is bilateral ptosis and diplopia.
What are the possible explanations for this?
A massive list of differentials can be generated.
Ideally, the trainee will organise these in groups, or at least in some sort of order where they start centrally and move peripherally.
A suggested list of differentials:
- Motor neuron disease
- Spinal cord disease
- Compression by epidural haematoma or abscess
- Spinal cord infarction
- Peripheral nerve disease
- Vitamin B 12 deficiency
- Heavy metal poisoning
- Neuromuscular junction
- Myasthenia gravis
- Eaton-Lambert syndrome
- Incomplete reversal of muscle relaxant
- Electrolyte disturbance
A neurology consult is solicited by the orthopaedic team (without your knowledge). The neurology trainee lists Guillain-Barre syndrome at the top of their list of differentials. What are the cardinal features of Guillain-Barre syndrome?
- Antecedent viral illness; usually with diarrhoea
- Campylobacter jejuni
- Generalised and ascending
- Not fatiguable
- Weakness of the limbs is mainly distal
- Weakness is progressive
Cranial nerve involvement
A CK level was sent as part of the routine work-up. The level is elevated, at 3500 IU/L. Your registrar asks whether myopathy might account for this presentation.
What features might you expect from a myopathy, as compared to a neuropathy, if it were the cause of this weakness?
Features expected of myopathy:
- The sensory supply should be preserved
- The reflexes should be preserved
- Weakness should be proximal - that is where the bigger muscles are, and the weakness there will be more obvious.
- There should be no fasciculations
- There may be myocardial involvement (skeletal myopathies tend to be associated with cadiomyopathy)
- The muscles involved may be painful and tender(as in myositis)
- There may be muscle contractures, requiring splints
The neurology team insists that Guillain Barre syndrome be excluded by objective means. What investigations might help?
- CSF protein is elevated
- GQ1b antibodies (Miller Fischer variant)
- Nerve conduction studies: Marked slowing, conduction block
- Electromyography: Abnormal spontaneous activity, reduced recruitment, normal MUPs (early in disease).
A lumbar puncture is performed, which is normal. Nerve conduction studies reveal a progressive decline in the CMAP amplitude, and the electromyography is reported as "abnormal jitter".
Myasthenia gravis is implicated. What further testing can confirm this condition?
- Ice pack test: neuromuscular transmission should improve with cold; ptosis should be reversed when the eyelid is cooled with an icepack.
- Edrophonium challenge: 10mg of edrophonium is given to transiently antagonise acetylcholinesterase. The myasthenic patient should immediately regain their muscle strength.
- Acetylcholine receptor antibodies: (AChr-Ab)
- Muscle specific tyrosine kinase antibodies (MuSK-Ab)
Acetylcholine receptor antibody titer is high, supporting the diagnosis of myasthenia gravis. What management strategies are available?
Emergent crisis management:
- Escalate steroids: eg. prednisolone 1mg/kg/d
- Acetylcholinesterase inhibitors: pyridostigmine as IV preparation
- Only useful as a short-term treatment
- Only applicable in myasthenic crisis
- Useful as a bridge to slower-acting immunosuppressants
- Useful preoperatively before thymectomy
- No real difference in outcomes when compared to IVIG (Gajdos et al, 2002)
- Intravenous immunoglobulin
- Like plasmapheresis, only useful as a short-term treatment and only applicable in myasthenic crisis
- Usually given as 2g/kg over 5 days (i.e. 0.4g/kg per day)
- A single dose of 1g/kg is probably equally effective (Gajdos, 2005)
- Acetylcholinesterase inhibitors: pyridostigmine is the mainstay
- Surgery (thymectomy) is a clear-cut indication in thymoma
- In absence of thymoma, likelihood of remission is still twice as high if you get your thymus removed (Gronseth et al, 2000)
- The college answer suggests that thymectomy is "not recommended routinely for age>60", which probably refers to the recent British guidelines (Sussman et al, 2015). They recommend thymectomy for under-45s within 2 years of diagnosis.
The patient undergoes five sessions of plasmapheresis, and slowly improves with steroids and pyridostigmine. She is discharged from the ICU. She is planned for thymectomy. What advice can you offer to the anaesthetist taking her to theatre?
This comes from the excellent article by Abel and Eisenkraft (2002)
- Assess bulbar function pre-operatively to determine ability to cough and clear secretions
- Collect data about respirqtory function (formal pulmonary function tests)
- Assess the size of thymoma (i.e. a large one can obstruct the airway)
- Omit the anticholinesterase inhibitor on the morning of the procedure
- Use stress dose steroids periperatively
- Expect the patient to have abnormally high sensitivity to non-depolarising neuromuscular junction blockers, and a resistance to depolarising agents.
- Rule of thumb is, you need about 50% of the normal rocuronium dose, and about 250% of the normal suxamethonium dose
- Avoid long-acting blockers (eg. pancuronium)
- Avoid mivacurium, as pyridostigmine inhibits its clearance and increases the duration of effect
- Cholinesterase is depleted by plasmapheresis; prolonged blockade from suxamethonium or mivacurium may result
- Regional techniques are generally held to be safe
- A post-operative ICU bed should be organised