The achiasmia spectrum: congenitally reduced chiasmal decussation

Abstract

Aim: To describe the clinical spectrum of achiasmia, a congenital disorder of reduced relative decussation at the optic chiasm. Methods: A retrospective case note and patient review of nine children (four boys). Achiasmia was defined by the combination of a characteristic asymmetry of the monocular visual evoked potential (VEP) response to flash and neuroimaging showing reduced chiasmal size. Results: Three of the children had an associated skull base encephalocele with agenesis of the corpus callosum. In two patients achiasmia was associated with septo-optic dysplasia. Three patients had no neuroimaging abnormalities other than reduced chiasmal size and have no known pituitary dysfunction. One child had multiple physical deformities but the only brain imaging abnormality was reduced chiasmal size. Conclusions: Some children with disorders of midline central nervous system development, including septo-optic dysplasia and skull base encephaloceles, have congenitally reduced chiasmal decussation. Reduced relative decussation may co-exist with overall chiasmal hypoplasia. Children with an apparently isolated chiasmal decussation deficit may have other subtle neurological findings, but our clinical impression is that most of these children function well.

VEP characteristics in children with achiasmia, in comparison to albino and healthy children

“…The ophthalmological findings in the present study relating to these children with achiasmia and albinism are in agreement with previous studies [12,17]. In these four children with achiasmia, their ophthalmological findings are presented in detail [47].…”

“…Therefore, the lack of uniformity in VEP asymmetries shown previously has also been found in the present study. In the majority of previous studies, ipsilateral asymmetry was observed with flash and pattern onset stimulation [3][4][5][6][7], while crossed asymmetry and crossed contralateral positivity have also been associated with achiasmia [8][9][10][11][12]36]. In the present study, the ipsilateral asymmetry defines the absent optic-nervefibre decussation at the chiasm for both of the eyes.…”

“…Colour vision was tested with Ishihara plates, while visual field testing was performed with Goldmann perimetry and static perimetry (Octopus perimeter, G2 top programme). Table 1 gives the ophthalmological findings of the children with achiasmia (children 1-4) and with ocular albinism (children [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. All of these children had infantile nystagmus, normal near vision and normal colour vision (analphabetic or Ishihara colour plates).…”

“…In achiasmia, a number of VEP asymmetries have been described, with all of them defining absence of or reduced optic-nerve-fibre decussation at the chiasm. From each eye, the optic nerve fibres that mainly or solely project into the ipsilateral hemisphere produce VEP distributions that have been defined as: ipsilateral asymmetry [3][4][5][6][7], crossed asymmetry [8][9][10][11] and crossed contralateral positivity [12]. In all forms of albinism, VEP asymmetries have been described as contralateral asymmetry [13,14] and albino crossed asymmetry [15][16][17].…”

Developmental abnormalities of the optic nerve and chiasm

“…They may also have developmental delay and optic disc anomalies. 72 Three types have been suggested: 72 1. Type A: reduced decussation, with basically normal appearing optic nerves on clinical examination.…”