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Visual-evoked potential evidence of chiasmal hypoplasia
D A Thompson1, A Kriss, K Chong
1Ophthalmology Department, Great Ormond Street Hospital NHS Trust, London, England. d.thompson@vissci.ion.ucl.ac.uk
Insights
Chiasmal hypoplasia in infants is linked to developmental gene anomalies, not isolated defects. This study highlights associated brain abnormalities and optic nerve findings in affected infants.
Area of Science:
- Neuroscience
- Developmental Biology
- Ophthalmology
Background:
- Chiasmal hypoplasia/aplasia can be part of a broader developmental anomaly.
- Understanding associated clinical findings is crucial for diagnosis and management.
Observation:
- Five infants with electrophysiologically confirmed chiasmal hypoplasia were studied.
- Evaluations included electroretinography, visual-evoked potentials (VEPs), ophthalmologic exams, and MRI brain scans.
Findings:
- All patients exhibited crossed asymmetry in VEPs, indicating reduced optic chiasm fiber crossing.
- MRI revealed varying degrees of chiasmal hypoplasia and other midline brain abnormalities.
- Optic disc appearance ranged from normal to hypoplastic or colobomatous.
Implications:
- Chiasmal hypoplasia may be a phenotypic manifestation of underlying genetic developmental disorders.
- Findings suggest potential parallels with genetic achiasmia observed in animal models.
- Further research into genetic causes of chiasmal anomalies is warranted.
Purpose:
To show that chiasmal hypoplasia or aplasia need not be an isolated developmental anomaly and to examine the spectrum of associated clinical findings to explore the possibility that these patients may represent a phenotypic manifestation of a developmental gene anomaly.
Design:
An observational case series.
Participants:
Five infants, between several weeks and 7 months of age, in whom the electrophysiologic characteristic of chiasmal hypoplasia had been noted were included.
Methods:
Flash electroretinography and flash and pattern visual-evoked potentials (VEPs) were elicited from all patients. Clinical ophthalmologic examinations, including funduscopy, were performed, and all patients had magnetic resonance imaging (MRI) brain scans.
Main Outcome Measures:
The occipital distribution of monocular VEP response peaks was studied. The symmetry of lateral channel responses was compared for monocular stimulation.
Results:
All five patients had a crossed asymmetry in the monocular VEP occipital distribution, which is consistent with a paucity of fibers crossing at the chiasm. The MRI findings supported this electrophysiologic observation, illustrating degrees of chiasmal hypoplasia and variable coincidence of other midline abnormalities of the brain. Optic disc appearances varied from normal to hypoplastic and colobomatous.
Conclusions:
The ophthalmologic and MRI findings of five patients who showed a crossed asymmetry in monocular flash VEPs are consistent with a paucity of axons crossing at the chiasm. The similarities between achiasmia in humans and mice due to a Pax2 gene anomaly are discussed.