MRI Proven Isolated Proximal Fascicular Part of Oculomotor Nerve Injury in Head Trauma

Sir, Isolated ON injury along its course in mild head injury (MHI) is very uncommon. Due to non-availability of CT scan and MRI, documentations were based on clinical examinations alone.[1] In the past few years due to advanced imaging techniques we found ample number of reports of isolated ONP in MHI without any signs of demonstrable injury to nerve on MRI.[2,3] We have described a case of MHI with ptosis and direct injury to the fascicle of third nerve on MRI. A 28-year-old woman met with an accident while traveling as pillion rider. On examination, left eye had complete ptosis with eye ball deviated downward and outward. Left side adduction, supra-duction and infra-duction were completely lost [Figure 1a and b]. Left side pupil was dilated and not reacting to the light. MRI brain revealed T2 and FLAIR hyperintensity with mild diffusion restriction in left-sided ventral midbrain near the nuclear zone of ON presumably in proximal fascicle [Figure 2a-c]. This was suggestive of non-hemorrhagic contusion or ischemia of fascicle. Susceptibility weight imaging revealed blooming foci in multiple areas of brain parenchyma along with left anterior genu of corpus callosum which was typical of diffuse axonal injury (DAI). Screening STIR T2 and 3D FIESTA sequences revealed no focal abnormality in orbits.Figure 1: (a) Photograph showing ptosis of left eye due to the involvement of third cranial nerve. (b) Shows loss of all eye movements on left side except lateral movement due to the involvement of the third cranial nerveFigure 2: (a) T2W image showing hyperintensity signal in the dorsal brainstem area near the periaqueductal grey matter area. (b) Diffusion imaging showing restricted diffusion in the dorsal midbrain extending in to the periaqueductal grey matter near third cranial nerve nucleus. (c) FLAIR image shows hyperintense signal involving corpus callosum with evidence of bleed within the lateral ventricleTraumatic isolated ON palsy in minor head injury is rare with overall incidence ranging from 1% to 15%.[4] However, isolated direct ON palsy in minor head injury with radiological signature is even rarer. Our case is unique where MRI showed non-hemorrhagic contusion or ischemia in dorsal midbrain anterior to nuclear complex involving the proximal fascicle of left ON. ON project through interpeduncular fossa in an open “V” shaped manner by multiple rootlets to posterior clinoid process. Our patient had weakness in left side adduction, supra-duction and infra-duction. Right side supra-duction was completely normal and ptosis was unilateral. This suggests that it’s not a nuclear palsy clinically. The “V” shaped anatomical configuration poses ON to shear and stretch from frontal blow, causing a line of force along it. A large tentorial opening may allow free movement of the midbrain and adjacent structures attached to it at the time of impact.[5] The possibility of ONP may occur at the site where the nerve passes through the dura, as during the time of impact the tension and distortion gets transmitted along the dura fold and the structures that passes through it. This explains how stretching of ON at the entry and exit point of cavernous sinus can cause DAI. Shearing strain and angular forces also can directly injure the brain stem with its downward displacement which may cause injury to the ON at its tethering sites like brainstem itself, posterior clinoid ligament, dura of cavernous sinus and superior orbital fissure. In 1969, Heinze concluded that differential movement between brain stem and supratentorial structures may be responsible for root avulsion or distal fascicular damage of ON.[1] So, it is evident that multiple mechanisms are involved at the time of impact for direct injury to ON. In spite of advance imaging techniques positive detection of direct injury to the ON is sparse. We have reviewed the literature with traumatic third nerve palsy which had been depicted in MRI [Table 1]. In 1995, Yamada et al. reported a case of severe head injury with right ONP with features of DAI and midbrain infarct in MRI. The patient died after 50 hours of trauma and her autopsy confirmed midbrain infarct with root avulsion of right ON.[6] Kwuon et al., Ditta et al. and Welling et al. described case reports of complete avulsion of isolated third nerve on MRI.[7-9] Balcer et al., Berti et al., Kim et al., Tajsic et al. and Sartoretti et al. have reported cases with either diffuse enhancement of ON with swelling or with micro-bleed in different MRI sequences.[10-14] Kaido et al. and Takeuchi et al. proposed posterior clinoid ligament as the fulcrum for strain injury of ON in DAI.[15,16] In the present case, MRI revealed T2 and FLAIR hyperintensity and diffusion restriction in the fascicle of left ON. The finding in the present case could have been due to disruption of blood–nerve barrier following DAI.Table 1: Literatures on MRI positive isolated oculomotor nerve palsySWI is best sequence available to diagnose micro-bleed in nerve or elsewhere in brain in the present era.[14] Avulsion injury to the ON nerve can be best detected by constructive interference steady state (CISS) imaging sequences.[9] Recently, use of diffusion tensor imaging (DTI) was reported in head injury to delineate avulsion of ON.[7] Injured ON nerve shows postcontrast enhancement due to disruption of nerve–blood barrier, ischemia or inflammation and delayed enhancement is common. They usually recover in 1-6 months, with diminishing nerve enhancement.[12-14] Recovery of ONP is slow and prolonged and depends on the types of injuries. In one study on isolated ONP with MHI, the prognosis of patients with ptosis, external ophthalmoplegia and internal ophthalmoplegia (IO) were evaluated separately. Recovery rates were 100%, 60%, and 50% respectively.[17] The recovery is very unlikely in complete avulsion of ON.[8-10,14] In shearing or contusion types of injury, recovery is partial or variable and IO is least likely to recover.[7,11,13,15,16] In the present case, after 4 months of follow up, there was partial improvement of ptosis and extraocular movement but no change in IO. Patient still had diplopia and glare. There is no consensus regarding the standard treatment protocol. Some treat conservatively with steroid and claimed improvement.[4,12] Imaging studies of these patients didn’t show any root avulsion. Others have also shown improvement without steroid treatment.[11,13,16] We have treated conservatively with physiotherapy. Strabismus surgery has been proposed when there is no improvement.[8] Isolated ONP is not rare but proximal fascicular injury with MRI signature is being reported for the first time. Advanced imaging techniques are key to diagnose and prognosticate these patients. Consensus regarding definitive management of this condition is still not achieved. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initial s will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.