Non-decussating retinal-fugal fiber syndrome

Non-decussating retinal-fugal fiber syndrome (NRFFS), also referred to as achiasma, is the isolated congenital absence of optic chiasm with unremarkable rest of the optic pathway and midline structures. Clinically, it typically presents as seesaw nystagmus since childhood. Diagnosis is usually made out by MRI which shows an isolated absence of optic chiasm. It is important for radiologists to be aware of this entity as the pathology may be overlooked if only thick sections are evaluated and attention is not paid to the optic pathway.

Epidemiology

NRFFS is a rare cause of congenital nystagmus. Achiasma or hypochiasma has been reported in patients with congenital anophthalmos, midline anomalies like septo-optic dysplasia and in patients with albinism. However, the isolated absence of optic chiasm is rare, with only a few cases reported previously.

Clinical presentation

Clinically congenital seesaw nystagmus, ‘mirror reversal’ of visual field representation and interocular ipsilateral asymmetry on monocular VEP point toward achiasma and warrant further evaluation with MRI. Seesaw nystagmus is a characteristic, vertical-torsional eye movement disorder wherein there is intorsion and elevation of one eye and simultaneous extortion and depression of another eye. It could have either pendular waveform (due to midline bilaterally compressing meso-diencephalic mass) or jerk-waveform (due to unilateral lesion).

Pathology

It is a congenital disorder characterized by failure of the crossing of optic nerve fibers at the optic chiasm.

Radiographic features

MRI

MRI is the investigation of choice to evaluate a patient suspected to have achiasma. Typical imaging findings include optic nerves transitioning into optic tracts without normal optic chiasm. No other midline abnormalities are noted. Thin sections or a three-dimensional sequence may be of great help. Functional MRI confirms electrophysiology observations of crossed asymmetry with each visual cortex receiving complete but monocular visual field.

Treatment and prognosis

Treatment includes correction of strabismus that usually accompanies. In spite of such large functional abnormalities, vision is typically preserved in most regards because of the reorganization of intracortical connections. 

History and etymology 

The condition was first described in the early 1990s in Belgian sheep dogs by Williams et al.