Distinguishing features of lower motor neuron syndromes

The mysterious whole-body weakness is a favourite topic of the CICM examiners. In fact, for some reason the college perpetually want the candidates to compare various "weakening" disease states to Guillain-Barre Syndrome. This has now come up so many times that the number of SAQs now merits an unordered list:

  • Question 3 from the second paper of 2018: compare GBS with acute transverse myelitis
  • Question 20 from the first paper of 2016: compare GBS with myasthenia gravis. 
  • Question 10 from the first paper of 2013:  compare GBS with those of critical illness polyneuropathy.

Therefore, from a cynical exam-oriented standpoint, it would be worth knowing a lot about GBS because in such questions one stands a reasonable chance of scoring at least half the marks. The other condition does not matter quite so much; although having said this, transverse myelitis has appeared in Question 10 from the second paper of 2019  GBS itself is discussed in greater detail elsewhere. It also receives a thorough LITFL treatment. In this chapter, only comparisons of GBS with other conditions will be listed. For those wishing to have a more thorough and official resource, Garg et al (2017) offer an excellent description of the lower motor neuron syndromes.

Differential diagnosis of lower motor neuron syndromes

This list of plausible differentials was generated with the aid of this excellent article by Garg et al (2017). No effert was expended to make this list comprehensive or definitive.

  • Vascular causes
    • spinal cord infarction
  • Infectious causes
    • Polyomyelitis
    • Epidural abscess
    • Herpes zoster
  • Neoplastic causes
    • Spinal canal tumour
  • Drug-related causes
    • B12 deficiency
    • Steroid-induced myopathy
  • Hereditary distal neuropathies
    • Spinobulbar muscular atrophy (Kennedy's disease)
    • genetically heterogeneous distal hereditary motor neuropathies
  • Autoimmune/ immune-mediated diseases
    • Guillain-Barré syndrome
    • multifocal motor neuropathy (MMN)
    • Chronic inflammatory demyelinating polyneuropathy (CIDP)
    • Multiple sclerosis
    • Myasthenia gravis
    • Eaton-Lambert syndrome
    • Myositis and dermatomyositis
  • Iatrogenic causes
    • Critical illness polyneuromyopathy

Guillain-Barre vs CIPN

In brief:

  • CIPN is associated with a severe illness, whereas GBS is associated with a mild one.
  • CIPN does not have autonomic dysfunction, whereas GBS does.
  • CIPN will not have any specific CSF findings, whereas GBS will have raised CSF protein and perhaps even a monocytosis
  • CIPN nerve conduction studies will not show any decrease in conduction velocity, whereas in GBS the cardinal feature is decreased conduction velocity.

The college answer is presented as a table.

A much better table is presented in the 2006 article by Upinder Dhand (Table 3, pp. 1034).

If one were to cut and paste this table into one's SAQ answer, the college examiners would have been very pleased. So impressive is it, that I have taken the liberty of reproducing it here with minimal modification, without any permission but with the best of intentions.

Differences between GBS and CIP

 GBS CIP History and Examination

Recent GI or resp illness. Progressive bilateral symmetric paralysis. Subtypes can be more localized e.g. MF opthalmoplegia and ataxia. Sensory involvement is common. Areflexic. Autonomic involvement may be present

Always occurs in association with a critical illness in particular severe sepsis. May have an association encephalopathy in early stages. It is a symmetrical weakness. May have muscle tenderness, hyporeflexic, diminished distal sensation Not associated with autonomic involvement


Albuminocytologic dissociation in CSF. Identification of infection with campylobacter, mycoplasma, EBV,Varicella, CMV. Elevated csf IGG levels and possible serum antiganglioside antibodies

Elevated CK which may be transient.

Nerve conduction studies and EMG

When the demyelinating form is present, you get a reduction in conduction velocity as well as reduction in CMAP. In axonal forms however it is only the distribution of the findings that helps determine the diagnosis.

Clinical Features and Laboratory Findings
Critical-Illness Polyneuropathy, Guillain-Barre´ Syndrome, and Critical Illness Myopathy

Critical illness polyneuropathy


Critical Illness Myopathy

Clinical setting

Multiorgan failure

Septic encephalopathy

Antecedent viral illness


Campylobacter jejuni


Neuromuscular blocking drugs



Organ transplant

Motor weakness

Generalised and distal

Generalised and ascending

Generalised and proximal




Preserved but weak

Cranial nerve palsy





Normal autonomic function

Frequent dysautonomia

Normal autonomic function

Sensory deficit


Normal sensation, or slightly altered

Normal sensation

CK level




Nerve conduction

Reduced CMAP and SNAP amplitude

(compound muscle action potentials and sensory nerve action potentials)

Marked slowing, conduction block

Reduced CMAP amplitude
Normal SNAP amplitude

Needle EMG

Abnormal spontaneous activity

Reduced recruitment

Large polyphasic motor unit potentials (MUPs)

Abnormal spontaneous activity

Reduced recruitment

Normal MUPs (early in disease)

Minimal spontaneous activity

Early recruitment

Small polyphasic MUPs;

Direct muscle stimulation



Absent or reduced

Muscle biopsy

Neuropathic changes

Neuropathic changes

Myopathic changes

Thick myosin filament loss

Muscle fiber necrosis

Guillain-Barre vs. myasthenia gravis

It would be possible but pointless to replicate the table above with myasthenia gravis plugged into the CIPN slot. Instead, a more directed approach will be taken, where clinical features, investigations and treatment are discussed in a tabulated form, best suited to answer Question 20 from the first paper of 2016.


Clinical Features of Guillain Barre Syndrome vs. Myasthenia Gravis
Features Guillain-Barre Myasthenia Gravis
Clinical setting 
  • Antecedent viral illness;
    usually with diarrhoea
  • Surgery
  • Campylobacter jejuni
  • HIV
  • Autoimmune diseases, eg. SLE
  • Thyroid disease
  • Pregnancy
  • Thymoma
Motor weakness
  • Generalised and ascending
  • Symmetrical
  • Not fatiguable
  • Weakness of the limbs is mainly distal
  • Weakness is progressive
  • Eye muscles are the first to go
  • Ptosis and diplopia are early features
  • Fatiguability (weakness worse after sustained exercise)
  • Neck extensor and flexor muscles
  • Weakness of the limbs is mainly proximal
  • Weakness fluctuates
Sensory loss
  • Mild or absent
  • No sensory loss
  • Absent
  • Reflexes are usually normal
Cranial nerve involvement
  • Common
  • Earliest manifestations are cranial nerve signs (ptosis and diplopia)
  • Facial muscles become involved early; patients become expresisonless
  • Common
  • Uncommon
Investigations of Guillain Barre Syndrome vs. Myasthenia Gravis
Guillain Barre Syndrome
  • 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).
Myasthenia Gravis
  • 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)
  • Repetitive nerve stimulation: the characteristic finding is a progressive decline in the CMAP amplitude.
  • Electromyography: the characteristic finding is "abnormal jitter".
Management of Guillain Barre Syndrome vs. Myasthenia Gravis
Guillain Barre Syndrome
  • Intubation and ventilation
  • Corticosteroids are counterproductive.
  • Plasmapheresis works: 4 exchanges of 1-2 plasma volumes, over 1-2 weeks.
    Plasma exchange for Guillain-Barre syndrome aims to clear the aetiological autoantibody from the bloodstream. In essence, we say "we have no idea which antibody is causing the demyelination, so we will get rid of all of them". The evidence seems to support a 5-treatment regimen; it seems that six treatments are no better than four. Because there is no missing proteins to replace, the exchanged plasma can be FFP or albumin - it does not seem to matter to the resolution of disease. However, because FFP has a slightly higher risk of transfusion reactions, so in general albumin is the recommended replacement solution, unless there are specific reasons to replace blood proteins.
  • IV immunoglobulin is at least as effective as plasmapheresis. Dose is 2g/kg, over 5 days. The college answer mentions a Cochrane review, probably referring to Hughes et al (2014) who demonstrated that in severe disease IVIG within the first 2 weeks "hastens recovery as much as plasma exchange".
Myasthenia Gravis
  • Thymectomy
    • 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 answersuggests 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.
  • Maintenance therapy:
    • Acetylcholinesterase inhibitors:  pyridostigmine is the mainstay
    • Immunosuppressants:
      • Corticosteroids
      • Azathioprine
      • Mycophenolate
      • Cyclosporine
  • Crisis therapy:
    • Intubation and ventilation
    • Escalate steroids: eg. prednisolone 1mg/kg/d
    • Acetylcholinesterase inhibitors:  pyridostigmine as IV preparation
    • Plasmapheresis
      • 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)

GBS vs. acute transverse myelitis

This was the topic of Question 3 from the second paper of 2018. Judging by the pass rate of ~60%, many did not have much of an idea about the latter disease. In case future exam candidates wish to explore the topic further, they will find it well-reviewed in the paywalled UpToDate article by Krishnan et al (2016). Another inaccessible but excellent article is the 2004 paper by Harzheim et al, trapped in the dungeons of Elsevier. The freegan may avail oneself of the LITFL summary page and this excellent review paper by West (2013) which Chris Nickson links. 

The college question wanted the two compared in terms of "relevant history, the clinical features, and the relevant investigation findings". The official college answer to this SAQ was actually quite extensive, and likely represents an earnest effort from the examiners. It would be difficult to improve on such an answer. One can merely rearrange the categories and add some irrelevant tidbits here and there. The product appears below.

Guillian-Barre versus Acute Transverse Myelitis
Guillian-Barré Acute Transverse Myelitis

Acute inflammatory demyelinating peripheral neuropathy, associated with infection such as:

  • Antecedent viral illness; usually with diarrhoea
  • EBV
  • HSV
  • Campylobacter jejuni
  • HIV

Autoimmune inflammation of the spinal cord; may be idiopathic or associated with other illnesses:

  • Usually occurs as a postinfectious complication
  • Can fall within the spectrum of coexisting MS
  • Can coexist with acute disseminated encephalomyelitis
  • Autoimmune diseases are associated (eg. SLE, scleroderma, etc)
Typical features of history
  • Sub-acute onset
  • Ascending pattern of clinical signs
  • Often, very rapidly progressing
  • Weakness nadir is achieved within 4 hours in some cases (though some take as long as 21 days)
  • Bilaterally decreased
  • Symmetrical
  • Weakness ascends over time
  • Bilaterally decreased
  • Symmetrical
  • Weakness remains at and below the level of the lesion
  • "Pyramidal" preference: flexors of the legs and the extensors of the arms 
  • Flaccid 
  • Later, remains flaccid
  • Initially flaccid
  • Later, hypertonic spasticity
  • Diminished or absent
  • Later, remain diminished
  • Depressed initially
  • Hyperreflexia subsequently
Cranial nerves
  • Usually, not involved
  • Miller Fischer variant involves (usually, medullary) cranial nerves
  • Usually, not involved
  • When it forms a part of the MS spectrum, there may be optic neuritis
Autonomic features
  • Usually present
  • Not usually involved, unless the level of the lesion is high
  • High lesions may present with spinal shock
Sensory findings
  • Sensation usually preserved or oly mildly affected
  • Sensation is usually absent 
  • There is usually a distinct symmetrical sensory level
CSF features
  • Raised protein
  • Usually no white cells
  • Antibodies (Anti-GM1) or GQ1b antibodies in the Miller Fischer variant
  • Raised protein
  • Lymphocytosis
  • Specific immunology (NMO, ADEM)
  • Nothing specific
  • Aquaporin-4 IgG antibodies
Nerve conduction studies
  • Marked slowing, conduction block
  • Reduced amplitude sensory nerve action potential (SNAP)
  • Pathological F-wave responses
  • Decreased conduction velocity of motor and sensory nerves.
  • Abnormal spontaneous activity, reduced recruitment, normal MUPs (early in disease). Later, reduced
  • Reduced amplitude of motor (MUP) action potentials
  • Noncontrast MRI is essentially normal
  • Gadolinium reveals surface thickening and contrast enhancement on the conus medullaris and the nerve roots of the cauda equina
  • Noncontrast MRI reveals cord oedema at the level of the lesion (but in 40%, looks totally normal)
  • Gadolinium-enhancing signal abnormality extending over one or more cord segments.
  • Lesions ccupy most of the transverse diameter of the cord (2/3rds)
  • Corticosteroids are counterproductive.
  • Plasmapheresis works: 4 exchanges of 1-2 plasma volumes, over 1-2 weeks.
  • IV immunoglobulin is at least as effective as plasmapheresis. Dose is 2g/kg. 
  • High dose steroids arre first line (methylpred, 1g/day for 3-7 days)
  • Plasmaphersesis is second-line a(5 exchanges over 10 days)
  • Cyclophosphamide is occasionally recommended
  • IV immunoglobulin is usually not recommended (anecdotal evidence only)

Electrophysiology findings in Guillain-Barre Syndrome

The nerve conduction findings of GBS have been asked about specifically in Question 2 from the first paper of 2019. The following were derived from a guideline by Asbury & Cornblath (1990):

  • Nerve conduction slowing in 80% of cases at some point during the illness (velocity is usually less than 60% of normal), which is consistent with demyelination
  • Specific characteristic findings:
    • Partial conduction block
    • Decreased M-responses
    • Temporal dispersion
    • Prolonged distal latencies
    • Prolonged or absent F waves and H-reflexes
  • 20% of cases have normal conduction studies
  • Sensory conduction studies are occasionally abnormal (reduction in evoked amplitude)
  • Axonal disease features absent or severely reduced compound muscle action potential amplitudes, with preserved conduction velocities


Oh's Intensive Care manual:

Chapter   57   (pp. 617)  Neuromuscular  diseases  in  intensive  care by George  Skowronski  and  Manoj  K  Saxena

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