a 4-year - old female child presented to the ophthalmology outpatient department of the hospital with chief complaint of unresponsiveness toward external stimuli since birth aggravated in preceding few days . there was associated history of near sightedness with child bringing objects very close to the eyes to see them . there was no history of abnormal head movements , torticollis , or any significant family history . the patient did not have any abnormal head posture , face turn , or chin lift . there were no limitations in ocular motility , and cover - uncover test did not reveal any strabismus . her slit - lamp biomicroscopy showed a normal anterior segment , with sluggishly reacting pupils . a dilated fundoscopic examination with indirect ophthalmoscopy showed temporal disc pallor in both eyes , with small symmetrical discs ( however , there was no evidence of optic nerve hypoplasia ) . torchlight examination revealed bilateral , symmetric , conjugate horizontal nystagmus with intermittent disconjugate vertical nystagmus . visual evoked potential ( vep ) could not be performed due to lack of patient cooperation and availability of limited resources . the patient was unable to appreciate the elevated letters , despite repeating the test thrice ; demonstrating the absence of gross stereopsis . she was referred to us for mri brain for evaluation of optic pathway , midline congenital and pituitary anomalies . mri revealed the isolated absence of optic chiasm ( achiasma ) with normal optic nerves and optic tracts [ figs . 1 - 3 ] . the midline structures , like corpus callosum ( thick white arrow ) , pituitary infundibulum ( thin black arrows ) etc . there is mild prominence of infundibular recess of third ventricle sequential coronal sections ( a - c ) of t1-weighted inversion recovery sequence of magnetic resonance imaging brain showing transition of optic nerve ( arrows in 2a ) into optic tracts ( arrows in 2c ) without normal optic chiasm ( asterisk ) . normal pituitary gland and infundibulum are noted curved planar reformatted axial section of heavily t2-weighted image of magnetic resonance imaging brain showing isolated absence of optic chiasm with unremarkable optic nerves ( black arrows ) and optic tracts ( dashed white arrows ) eye movement disorders can present in the pediatric age group with characteristic rhythmic , oscillatory movements of the eye . see - saw nystagmus is a characteristic , vertical - torsional eye movement disorder wherein there are intorsion and elevation of one eye and simultaneous extortion and depression of other eye . it could have either pendular waveform ( due to midline bilaterally compressing meso - diencephalic mass ) or jerk - waveform ( due to unilateral lesion ) . see - saw nystagmus is an important sign of structural anomalies such as achiasma or hemichiasm and thus warrants an evaluation using mri . nrffs or achiasma is a rare cause of nystagmus . in the early 1990s , williams et al . reported a new abnormality i.e. canine achiasma in a belgian sheep dogs who presented with see - saw nystagmus and showed the failure of crossing of optic nerve fibers at optic chiasm . electrophysiology reveals  crossed asymmetry  wherein the right cortex receives right eye 's visually evoked response and vice versa . however , this condition was ruled out in our patient since there was no abnormal vertical deviation . ocular tilt reaction , another rare cause of see - saw nystagmus was ruled out in our patient since there was no evidence of abnormal head posturing , skew deviations or vertical strabismus . it is imperative to highlight several important differences between achiasma and congenital nystagmus . unlike achiasma , congenital nystagmus may not present at birth apart from a few hereditary cases [ table 1 ] another important differentiating feature between the two entities is the presence of see - saw nystagmus in achiasma , which is very rare in congenital nystagmus . rarely , outpatient clinical observation of eye movements may not confidently differentiate between these conditions , unless an increasing velocity exponential of slow phase of the nystagmus is demonstrated on ocular movement recordings . hence , a detailed imaging evaluation is essential in all patients presenting with nystagmus in young age . etiology of congenital nystagmus achiasma or hypochiasma has been reported in patients with congenital anopthalmos , midline anomalies like septo - optic dysplasia and in patients with albinism . however , isolated absence of optic chiasm is rare , with only a few cases reported previously . in albinism , abnormal erroneous decussation of temporal retinal - fugal projections occurs at the optic chiasm and these misrouted temporal retinal fibers misalign with normally decussating nasal retinal fibers of the same eye . therefore , the medial segments of lateral geniculate nucleus layers representing a substantial portion of the ipsilateral visual field are mistakenly aligned with lateral geniculate nucleus layers of opposite , contralateral field . this results in a partial  mirror reversal  of the left and right half visual space coordinates . in achiasma , there is disruption of retinal - fugal projections , organization and function throughout the visual pathways . as against albinism , wherein there is erroneous decussation of temporal retinal fibers at chiasma ; the achiasmatic syndrome is characterized by the absence of optic chiasma , and thus all nasal fibers fail to decussate , thereby misprojecting and misaligning with projections of the temporal retina of the same eye . the visual field representation reveals  mirror reversaly  of left and right half visual space coordinates in a more complete fashion . monocular vep reveals interocular ipsilateral asymmetry differentiating it from albinism wherein pathognomonic interocular contralateral asymmetry is noted . in spite of early reorganization and plasticity with multiple inter and intracortical connections , typical imaging findings of achiasma include optic nerves transitioning into optic tracts without normal optic chiasm . functional mri confirms electrophysiology observations of crossed asymmetry with each ocular cortex receiving complete but monocular visual field . in spite of such large functional abnormalities we wish to highlight the typical clinical and imaging features of a case of nrffs or achiasma and the need of investigation by mri of a patient presenting with see - saw nystagmus for evaluation of optic chiasm . achiasma needs to be considered as a rare condition presenting with nystagmus in early childhood that may be overlooked unless an mri is done .
we report the clinical details and imaging findings for a case of nondecussating retinal - fugal fiber syndrome or isolated achiasma in a 4-year - old female child . 
 findings included the isolated absence of optic chiasm with unremarkable rest of the optic pathway and midline structures in a child presenting clinically with see - saw nystagmus . 
 clinically congenital see - saw nystagmus ,  mirror reversal  of visual field representation and interocular ipsilateral asymmetry on monocular visual evoked potential point toward achiasma and warrant further evaluation with magnetic resonance imaging ( mri ) . 
 isolated achiasma is a rare condition that may remain undiagnosed unless mri is done .