FRCA Notes

Myotonic dystrophy

Resources

  • Myotonic dystrophy is an inherited multisystem condition characterised by muscle weakness and muscle loss
  • Although it was identified by its unique effects on skeletal muscle, it has direct effects on most organ systems
  • The disease has two genetic forms, both of which demonstrate autosomal dominant inheritance
  • Incidence 2.4 - 5.5/100,000 in the UK and Europe
  1. Myotonic dystrophy 1 (Steinert disease)
    • Can present at any age though if childhood disease is associated with increased difficult airways
    • Abnormal DMPK or CNBP genes on chromosome 19

  2. Myotonic dystrophy 2 (Proximal myotonic myopathy a.k.a. PROMM)
    • Typically presents in adults with less severe symptoms
    • Abnormal ZNF9 gene on chromosome 3

Pathophysiology

  • In both subtypes there are abnormalities of Na+ or Cl- channels
  • This leads to muscles being in an abnormal, hyperexcitable state
  • Repetitive action potentials arise causing sustained muscle contraction
  • This manifests clinically as the inability to relax muscle groups

Musculoskeletal

  • Frontal balding
  • Muscle wasting and weakness, particularly:
    • Facial muscles
    • Muscles of mastication
    • Sternocleidomastoid
    • Levator palpebrae superioris → bilateral ptosis

  • Myotonias
    • Myotonic contractures occur due to permanent sodium influx/chloride efflux, rendering the muscle membrane hyperexcitable
    • There is incomplete muscle relaxation i.e. 'can't let go' after a hand grip

    Factors inducing myotonia
    Suxamethonium
    Anti-cholinesterases i.e. neostigmine
    Acidosis
    Hypothermia / shivering
    Mechanical or electrical stimulus e.g. ToF monitor
    Opioids
    Excess potassium

Airway & Respiratory

  • Pharyngeal muscle weakness predisposes to aspiration
  • No inherent features which predispose to difficult airway, although facial muscle myotonia may cause difficult mask ventilation

  • Restrictive lung disease
  • Central sleep apnoea ± OSA
  • Respiratory muscle weakness
  • Arterial hypoxaemia
  • Diminished ventilatory response to both hypoxia and hypercapnoea; predisposes to post-operative ventilatory insufficiency
  • Poor cough

Cardiovascular

  • Cardiac conduction defects inc. heart block
    • May lead to sudden death
    • Should have an ECG at least annually
    • May have a pacemaker in situ
  • Cardiomyopathy ± congestive heart failure
  • Structural cardiac abnormalities

Neurological

  • Central hypersomnia due to narcolepsy-like central hypersomnia, OSA and restless leg syndromes
  • Susceptibility to analgesics/sedatives
  • Behavioural problems
  • Cognitive impairment/decline

Gastrointestinal

  • Bulbar weakness and dysphagia
  • Delayed gastric emptying

Endocrine

  • Thyroid impairment
  • Adrenal impairment
  • T2DM and deranged glycaemic control
  • Testicular atrophy

Perioperative considerations for a patient with myotonic dystrophy


  • Patients may present for procedures such as:
    • Pacemaker implantation
    • Muscle biopsies ± orthopaedic surgery
    • Cataract
    • General surgery, esp. cholecystectomy

History and examination

  • Full history and examination, including respiratory, cardiovascular and neurological systems
  • Assess degree of muscular impairment
  • There is a high incidence of respiratory complications owing to difficult airways, or the disease affecting pharyngeal and respiratory muscles
  • Cardiac disease may be hidden as the patient cannot stress the myocardium to a significant degree

Investigations

  • Bloods: FBC | U&E
  • Baseline CK
  • Baseline ɣGT ± other liver function tests

  • CXR
  • Pulmonary function tests
  • Sleep studies or overnight oximetry to check for OSA
  • ± baseline ABG

  • ECG
  • TTE

Optimisation

  • Pre-operative liaison with Neurology
  • Consider Cardiology referral for elective pacemaker insertion prior to major surgery if significant infranodal conduction delay (PR>200ms, QRS >100ms)

  • Consider pre-admission for optimisation
  • Minimise fasting times
  • Administer glucose-containing IVI when fasting

  • Consider prokinetic and antacid premedication as autonomic disturbance may delay gastric emptying
  • Avoid benzodiazepine premedication as high susceptibility to respiratory depressant effects

  • Arrange for HDU bed ± facility for overnight CPAP

Monitoring and access

  • AAGBI
  • Awake arterial line as significant cardiovascular disease
  • High risk of intra-operative arrhythmia - apply defibrillator pads and pacing
  • Peri-operative blood glucose monitoring

Regional anaesthesia

  • Regional anaesthesia may be preferable, although it may:
    • Mask rapidly progressing disease
    • Exacerbate autonomic dysfunction
    • Worsen mitochondrial disease
    • Trigger myotonia through epidural-induced shivering
    • Cause respiratory depression through neuraxial opioid use

General anaesthesia

  • Bulbar palsy mandates intubation
  • High risk of aspiration so use RSI technique

  • Propofol, thiopentone and etomidate are all safe
    • Propofol associated with less post-operative ventilation need than others
    • Adding lidocaine to propofol can reduce the incidence of pain-induced myotonia
    • Etomidate may trigger myotonia through painful injection or its pro-convulsant properties
  • Judicious use of induction agent as higher risk of cardiorespiratory depression

  • Avoid muscle relaxants entirely if possible
    • Suxamethonium can trigger myotonic contracture
    • Increased sensitivity to non-depolarising agents; reduce dose 10 - 20% if used
    • Avoid neostigmine as associated with myotonia
    • Sugammadex is safe

  • Volatile agents are overall safe; there is no increased risk of MH in myotonic dystrophy
    • They may exacerbate cardiomyopathy, however
  • Propofol/remifentanil TCI may be preferential

Myotonias

  • Avoid factors which can precipitate myotonia

  • If myotonias occur:
    • Give sodium channel antagonists e.g. local anaesthetics, phenytoin or quinidine IV
      • Local anaesthetics can also be given directly into the muscle
    • NB spinal anaesthesia does not reduce muscle spasticity
    • NB dantrolene does not reduce muscle spasticity

Care bundle

  • Normothermia to avoid hypothermia-induced myotonia
  • Consider using normal saline as potassium-containing fluids can induce hyperkalaemia and thus myotonia
  • Monitor glucose as prone to hypoglycaemia

  • Unsuitable for day-case surgery and should be planned for an overnight stay
  • Up to 8% of patients suffer a complication, the majority of which are respiratory in nature

  • Probably best managed in an HDU setting with greater levels of monitoring, analgesia and ready access to respiratory support
    • There is an increased risk of delayed-onset apnoea and death for 24hrs after surgery so patients should be monitored for this long
    • If prolonged post-operative ventilatory support is required, it should be weaned rapidly and prolonged immobilisation avoided

Analgesia

  • Use multi-modal analgesia to avoid excessive opioids, as patients have increased sensitivity to opioids and existing respiratory disease
    • Simple analgesics
    • Local anaesthetics
    • Use low-doses short-acting opioids