In Miller Fisher syndrome (MFS), the triad of ophthalmoplegia, ataxia, and areflexia is associated with anti-GQ1b antibodies. GQ1b ganglioside is particularly concentrated at which anatomical location explaining the ophthalmoplegia?
- A Neuromuscular junctions of all six extraocular muscles
- B Myelin sheaths of the medial longitudinal fasciculus
- C Paranodal region of cranial nerve III, IV, and VI, and muscle spindle afferents in these nerves ✓
- D Presynaptic terminals of acetylcholine-releasing neurons at the ciliary ganglion
Explanation
GQ1b ganglioside is highly expressed at paranodal regions of myelinated nerves, particularly at cranial nerves III, IV, and VI (explaining ophthalmoplegia), in Ia muscle spindle afferents (explaining areflexia from disruption of monosynaptic reflex arcs), and in the cerebellum (explaining ataxia). Anti-GQ1b IgG antibodies cross-react with these gangliosides following molecular mimicry with Campylobacter jejuni lipooligosaccharide. At the nerve terminal level, anti-GQ1b antibodies activate complement and cause dysfunction of voltage-gated calcium channels, impairing neurotransmitter release at the NMJ of extraocular muscles. MFS has an excellent prognosis — most cases resolve spontaneously within weeks to months. IVIG may shorten duration.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.