a 48-year - old man presented with acute lower extremity weakness and back pain developed while he was working in a crawl space hyper extending his back . muscle strength was 4/5 in right iliopsoas and hamstrings , and 0/5 in the remaining muscles of right and all muscles of the left lower limb . bilateral knee and ankle reflexes pinprick sensation was decreased over left leg up to t12 level , and right l3 through s2 dermatomes . vibration and proprioception sensation were normal . at a peripheral hospital , spine mri was unremarkable on day 1 . spine mri on day 2 showed a non - enhancing intra - axial bright t2-weighted signal preferentially involving the anterior part of the cord extending from the conus to t11 ( figure 1a&b ) . on day 4 , ncs showed normal sensory and motor studies except an unrecordable right peroneal motor response ( chronic , post ankle fracture ) . f - waves were not obtainable over the peroneal and tibial nerves ( figure 2a - c ) . cerebrospinal fluid protein , glucose , cell count , cytology and flow cytometry were normal . extensive metabolic , infectious , vasculitis , cardiac , malignancy and thrombophilia workup were unremarkable . he initially received intravenous methylprednisolone and plasma exchange for a presumed transverse myelitis . on day 10 , mri showed smooth enhancement of the ventral nerve roots and conus ( figure 1c&d ) . the diagnosis was revised to conus medullaris infarct based on the distribution and evolution of mri findings . . left peroneal and bilateral tibial compound muscle action potential ( cmap ) amplitudes decreased . left peroneal f waves reappeared with poor persistence ( 6.7% ) and prolonged latency ( 64 ms ) . although cmap amplitudes remained decreased , all f - waves were obtainable with a slightly prolonged latency and normal persistence ( figure 2g - i ) . spine mri axial ( a ) and sagittal ( b ) t2-weighted mri obtained 2 days after symptoms onset demonstrating t2 hyperintense signal within the conus extending to t11 . axial t2-weighted ( c ) and contrast enhanced ( d ) mri obtained 10 days after symptom onset demonstrating contrast enhancement of the anterior part of the conus and nerve roots . f - waves ( a - c ) f responses recorded by stimulating the left peroneal and bilateral tibial nerves at day 4 , ( d - f ) day 18 , and ( g - i ) day 56 after disease onset . m - direct motor response ; ml - minimal latency ; mv - millivolt , nr - no response ; ms - millisecond f - wave is not considered part of the workup in conus medullaris infarct . however , in the absence of upper motor neuron signs , especially with a normal early spine mri and absent f - wave , confusion about the diagnosis may arise . in a previous case report , f waves were absent in the hyperacute stage ( first 4 hours ) of anterior spinal cord infarct.2 in our patient , f waves were absent at the time of the first nerve conduction studies ( ncs ) ( day 4 ) and they may have been absent from the onset . hiersemenzel et al3 described reappearance of f waves after the stage of spinal shock in 12 patients with a traumatic paraplegia above t10 level , and attributed the early absence of f waves to reduced excitability of motor neurons at the stage of spinal shock . in our patient , the mechanism of injury was ischemia ( rather than trauma ) and the injury level was at the conus ( lumbosacral spinal cord segments ) with involvement of the motor nerve roots that showed enhancement on the follow - up mri . recovery of left tibial f waves on day 18 coincided with development of hyperreflexia in the same leg . in contrast , at the time of reappearance of right tibial f waves on day 56 , right ankle reflex was absent . thus , we hypothesize that the absent f - waves in the acute stages of conus medullaris infarct might have been due to temporarily unexcitable ahc as a result of spinal shock , spinal cord edema , and possibly oligemia ( figure 1 ) . we would not expect f waves to be recordable in the setting of a diffuse infarction of all ahc in the spinal cord segments subserving the tested nerve . we are not aware of any reports describing f wave as a prognostic indicator of conus medullaris infarct . we postulate that persistently absent f waves , beyond the stage of spinal shock ( 2 - 4 weeks ) , in conus medullaris infarct indicates severe damage to ahc in the lumbosacral region and poor chance for renervation through collateral sprouting . all f waves had reappeared by day 56 , at least 8 weeks before he regained walking . the area of infarct involved the ventral part of the spinal cord supplied by the anterior spinal artery . we suspect the mechanism of infarct was secondary to a hyperextension injury similar to the non - traumatic spinal cord infarct described in novice surfers ( surfers myelopathy ) . in such cases , the postulated mechanisms of ischemia include transient arterial compression during a prolonged back hyperextension in the setting of poor collaterals , vasospasm or thrombosis of the artery of adamkiewicz , and avulsion of perforating blood vessels.4,5 the prognosis of surfers myelopathy varies from complete recovery to persistent paraparesis.4,5 a normal mri in the first few days does not rule out spinal cord infarct . in reported case series , the earliest changes ( bright t2-weighted signals ) appeared after 1 - 2 days , followed by enhancement appeared within the first week and peaked at 14 -21 days.6,7 cauda equina enhancement post spinal cord infarct has been described in a few case reports and attributed to a possible disruption of blood - nerve barrier.2,8,9 in conclusion , further studies are required to test our hypothesis that reappearance of tibial and peroneal f waves has a prognostic value in predicting walking in patients with conus medullaris infarct .
absent f wave in the stage of spinal shock has been described in cases of traumatic spinal cord injury . 
 the role of f wave in predicting prognosis after conus medullaris infarct has not been described . 
 we describe herein a middle aged - man with a conus medullaris infarct . 
 both tibial and peroneal f waves were absent on day 4 . 
 the left tibial f wave reappeared in the following study on day 18 . 
 all f waves reappeared on day 56 at which time the patient was still wheelchair bound . 
 he regained walking on day 105 . 
 we hypothesize that reappearance of initially absent f waves post conus medullaris infarct is a good prognostic sign for the return of ambulation . 
 the applicability of this observation requires further research . 
 we also discuss clinical and diagnostic caveats in this case .