carbon monoxide poisoning is the fourth most common type of poisoning in china , and is a leading cause of neurological disturbances . the psychiatric and neurological symptoms of carbon monoxide poisoning include headache , muscle weakness , drowsiness , memory disturbances , apraxia , delirium , speech disorder , epileptic seizure , ataxia , and symptoms of parkinsonism . the causes of dyslexia are poorly understood , but have been linked to damage to a reading - specific brain region in the left hemisphere known as the visual speech area . in this study , we describe a case of delayed encephalopathy with digit and letter alexia following carbon monoxide poisoning . general patient information the patient was diagnosed with delayed encephalopathy after acute carbon monoxide poisoning . treatments included hyperbaric oxygen therapy , as well as a low dose of methylprednisolone ( 80 mg a day ) and drugs for neurotrophy . this project was approved by the administrative regulations on medical institution , formulated by the state council of the people 's republic of china . written informed consent was obtained from the patient prior to the study . at admission , the patient complained of visual impairment , as well as digit and letter alexia after acute carbon monoxide poisoning . other common diseases that cause acute focal cortical injury include stroke ( ischemia and hemorrhage ) , trauma , and metabolic diseases ( such as mitochondrial encephalomyopathy ) . li et al reported a case of digit alexia after left parietal lobe hemorrhage . in the current study , cranial mri did not support the diagnosis of stroke and metabolism diseases ( figure 2 ) . the clinical history did not provide information indicating the development of alexia , except after the carbon monoxide poisoning . visual field examination was performed at day 2 when the patient was admitted to the hospital . after 2 weeks of treatment , the patient 's visual deficit had fully recovered ( b ) . brain imaging examination . t2-weighted brain magnetic resonance imaging ( a ) and magnetic resonance angiography ( b ) at 8 days after carbon monoxide intoxication revealed no abnormal findings . when the patient was discharged from the hospital , electroencephalography revealed apparent improvement , and 8  9 hz  wave activity returned as the predominant background rhythm . the patient was asked to write his name , address and occupation . before treatment , treatments included hyperbaric oxygen therapy , as well as a low dose of methylprednisolone ( 80 mg a day ) and drugs for neurotrophy . this project was approved by the administrative regulations on medical institution , formulated by the state council of the people 's republic of china . written informed consent was obtained from the patient prior to the study . at admission , the patient complained of visual impairment , as well as digit and letter alexia after acute carbon monoxide poisoning . other common diseases that cause acute focal cortical injury include stroke ( ischemia and hemorrhage ) , trauma , and metabolic diseases ( such as mitochondrial encephalomyopathy ) . li et al reported a case of digit alexia after left parietal lobe hemorrhage . in the current study , cranial mri did not support the diagnosis of stroke and metabolism diseases ( figure 2 ) . the clinical history did not provide information indicating the development of alexia , except after the carbon monoxide poisoning . visual field examination was performed at day 2 when the patient was admitted to the hospital . after 2 weeks of treatment , the patient 's visual deficit had fully recovered ( b ) . brain imaging examination . t2-weighted brain magnetic resonance imaging ( a ) and magnetic resonance angiography ( b ) at 8 days after carbon monoxide intoxication revealed no abnormal findings . when the patient was discharged from the hospital , electroencephalography revealed apparent improvement , and 8  9 hz  wave activity returned as the predominant background rhythm . the patient was asked to write his name , address and occupation . before treatment , common symptoms of delayed encephalopathy include memory loss , cognitive dysfunction and neuropsychological impairment due to cerebral cortex damage , as well as pyramidal and extrapyramidal syndromes involving the globus pallidus and subcortical white matter . in a number evaluation task , the patient was unable to read eight out of ten formulas or ten out of twenty numbers in the hundreds . in addition , he could not correctly recognize the digits of any numbers in the thousands . most of these errors were not due to simple visual spatial impairment , which is typically exhibited as mistakenly recognizing  9  as  6  , or  13  as  31  . the present results indicated that our patient 's condition was related to an impairment of the connection between visual identification and reading of numbers . the specific brain regions underlying the patient 's impaired letter and number reading ability , but preserved chinese character reading ability , are currently unclear . the cerebral cortex is known to be susceptible to anoxia . previous studies have reported that damage to the cortex , globus pallidus , cerebral deep white matter , putamen , caudate nucleus , thalamus and hippocampus exhibit abnormalities that are observable on magnetic resonance imaging scans in cases of delayed encephalopathy . in the current study , no abnormality was observed on magnetic resonance imaging and magnetic resonance angiography scans , which eliminates the diagnosis of stroke or metabolic diseases . although it may not be possible to determine the nature of the disease by the appearance of the electroencephalographic signal alone , diffuse slow wave activity in a conscious person typically indicates injury to the whole cortex in varying degrees . this measure may be more sensitive than magnetic resonance imaging . in the current patient , a partial defect in the visual field and widespread low amplitude  wave on electroencephalography the cause of the patient 's letter and number alexia , but preserved chinese character reading , remains unclear . this pattern of performance has not been previously reported , although there have been reports of selective alexia such as hangja alexia , japanese alexia , selective alexia and agraphia sparing numbers . some studies have suggested that the processing of ideograms and phonograms may be mediated by different brain regions . in addition , letter and number reading appear to be dependent on dissociable processes . despite the difficulties in localizing the selective alexia exhibited by our patient , we propose that focal cortical dysfunction may account for the findings of the visual field test and electroencephalography . functional mri revealed activation of a wide area including left inferior / middle frontal gyri , bilateral medial frontal gyri , posterior inferior temporal area , bilateral middle occipital / fusiform gyri , and the bilateral cerebellum during chinese letter reading . previous studies have indicated that chinese characters involve more complicated neural connections compared with english letters and numbers . as such , the latter may exhibit more impairment and less compensation after brain injury , because the functional brain area is relatively limited and is more likely to suffer complete damage . recovery of writing skill is reported to be more difficult than the recovery of listening , reading or speaking . this may explain why the patient 's chinese writing ability only partially recovered after treatment , whereas his reading ability completely recovered . a less complicated and less extensive brain network is involved in the reading of numbers and letters compared with that involved in reading chinese words , so less compensation may be seen after injury . this may explain why number and letter reading were more difficult to recover in the present study .
this study examined a 24-year - old patient with delayed encephalopathy , who was admitted to hospital with complaints of headache and visual impairment 1 week after acute carbon monoxide poisoning . 
 the results of a visual field assessment , electroencephalography and head magnetic resonance imaging indicated damage to the cerebral cortex . after a 2-week treatment period 
 , the patient had recovered from the visual impairment , but exhibited digit- and letter - reading difficulty . 
 the chinese aphasia battery and the number and letter battery supplement were conducted . 
 the results revealed that the patient exhibited digit and letter alexia , while the ability to read chinese characters was preserved . 
 in contrast , the patient exhibited a deficit in chinese character writing , while number and letter writing remained intact . 
 following treatment , reading and writing ability was improved and electroencephalographic abnormalities were ameliorated . overall , our experimental findings demonstrated that delayed encephalopathy following acute carbon monoxide poisoning was characterized by digit and letter alexia .