a 4-year - old boy developed exotropia after undergoing a decompressive craniectomy for brain hemorrhage because he fell over and bumped his head against the desk , 18 months prior to presentation at our hospital . brain computed tomography and magnetic resonance imaging performed at that time showed hypodense changes in the right middle cerebral artery territory and both anterior cerebral artery territories , as well as subfalcian herniation due to swelling of the right hemisphere ( fig . when he was fixing with the right eye , alternate prism cover test showed more than 50 prism diopters ( pd ) of the left exotropia was noted . 30 pd exotropia of the right eye was noted when he was fixing with the left eye ( fig . unfortunately , we could not determine if the patient had stereopsis , due to his lack of cooperation . the forced duction test under general anesthesia , muscle tightening or loosening was not noted . we treated the patient with left lateral rectus muscle recession of 7.0 mm and left medial rectus muscle resection of 3.5 mm . alternate prism cover testing showed 8 pd of left exotropia and 8 pd of right esotropia at distance . six months after surgery , alternate prism cover testing showed 4 pd of exotropia in the left eye and 2 pd of esotropia in the right eye at distance . dissociated strabismus is regarded as a marker of early disruption of normal binocular function , and dhd usually manifests as spontaneous unilateral exodeviation of greater magnitude in one eye . in some instances , fixation with one eye evokes an esodeviation of the other eye during prism and alternate cover testing . dhd is usually present in patients who experienced binocular disruption in early infancy , such as infantile esotropes . brodsky   reviewed 28 patients with consecutive exotropia and found that 50% of patients had dhd . tychsen   studied infant monkeys exposed to binocular decorrelation and found that those exposed at 12 weeks and at 24 weeks had dissociated deviation ( dissociated vertical deviation or dhd ) . the visual cortex and visual association cortex lie in the occipital lobe of the brain . the visual cortex ( v1 , striated cortex ) processes visual information from the eyes , while the visual association cortex ( v2-v8 , extrastriated cortex ) processes the more complex data , such as color and movement , and stereopsis . among all the parts of the cortex , stereoscopic vision depends on the magnocellular pathway in the v4 extrastriate cortex which is located in the right parieto - occipital lobe . hence , this case might suggest that dissociated strabismus can develop in patients who have sustained a brain injury . we also question why the patient presented with dhd instead of dissociated vertical deviation ( dvd ) . this may be explained by the difference in brain lesions associated with dhd and dvd , but this possibility needs to be investigated in future studies .
a 4-year - old boy visited the hospital with exotropia after brain hemorrhage caused by trauma . 
 he had undergone decompressive craniectomy and cranioplasty 18 months prior to presentation at our hospital . 
 an alternate prism cover test showed more than 50 prism diopters ( pd ) of left exotropia when he was fixing with the right eye and 30 pd of right exotropia when he was fixing with the left eye at near and far distance . on the hirschberg test 
 , 60 pd of left exotropia was noted in the primary position . 
 brain computerized tomography imaging performed 18 months prior showed hypodense changes in the right middle cerebral artery and anterior cerebral artery territories . 
 subfalcian herniation was also noted secondary to swelling of the right hemisphere . 
 the patient underwent a left lateral rectus muscle recession of 7.0 mm and a left medial rectus muscle resection of 3.5 mm . 
 three weeks after the surgery , the hirschberg test showed orthotropia . on alternate prism cover testing , 
 8 pd of left exotropia and 8 pd of right esotropia were noted at distance . 
 we report a patient who developed dissociated horizontal deviation after right subfalcian subdural hemorrhage caused by trauma .