Uncovering the Infection-Associated Auditory Agnosia

Sir, Auditory agnosia (AA) is a rare neuropsychological disorder characterized by isolated deficit in auditory processing despite adequate hearing and preserved language function. AA has three subtypes: 1) generalized, 2) verbal, and 3) nonverbal. In generalized AA, verbal and nonverbal sounds cannot be appreciated. In verbal AA/pure word deafness, a selective decrease occurs in recognition of verbal sounds, whereas nonverbal AA has a selective decrease in recognition of nonverbal sounds. The most common cause of AA is ischemic stroke. Less common causes include head trauma, encephalitis, or tumor. The anatomical site responsible is either the bilateral or unilateral temporal lobe, especially the Heschl’s Gyrus or Primary Auditory Cortex. Infection is a rare cause of AA. Here, we present a rare case of central nervous system (CNS) tuberculoma presenting with AA. A 39-year-old, right-handed man with chronic liver disease (CLD) and tubercular pleural effusion presented with abdominal distension and altered sensorium for 15 days and was admitted for CLD, Ascites, and hepatic encephalopathy. During hospital stay, he had one episode of convulsion (bilateral tonic clonic seizure). Routine blood examination was suggestive of deranged liver function tests. Electroencephalogram (EEG) showed diffuse slowing suggestive of metabolic encephalopathy. Magnetic resonance imaging (MRI) of brain showed a ring enhancing lesion in the left temporal lobe, suggestive of tuberculoma [Figure 1]. A week later, his attendants noticed that whenever they spoke to him, he could not understand them but could understand their sign language. He himself then asked them to write and communicate with him. He could understand all written commands and appreciate nonverbal sounds like music but could not understand when spoken to him. There was no history of headache, vomiting, or features suggestive of cranial nerve palsy or any limb weakness. On examination, his vitals were normal, with Glasgow Coma Scale (GCS) E4V5M6 (on written communication). Cranial nerve examination and motor and sensory examination were normal. He could not understand spoken words but could understand written language. Verbal repetition and writing to written commands were spared. He could hear sounds but had difficulty in identifying words. He could appreciate environmental sounds like music and sounds of laughing/talking and animals. The MMSE score was 4/30 with verbal communication and 30/30 with written communication. Standard pure tone audiometry (PTA) was done, which showed normal findings. Brain evoked response audiometry (BERA) study exhibited normal bilateral latencies [Figure 1]. He was therefore diagnosed as having AA to verbal sounds. He was already on antitubercular medication and managed with speech therapy, and he completely recovered within 2 months.Figure 1: (a) Normal brainstem evoked response audiometry (right ear) (b) Normal brainstem evoked response audiometry (left ear) and MRI brain axial, sagittal, and coronal sections showing (c) T2/FLAIR lesion in left temporal region with surrounding edema (d and e). Ring enhancing on contrast imagesThe research question for this literature review was CNS infection-related AA. The PubMed database was searched from year 1947 to 2024 using MeSH terms “Auditory Agnosia” AND “Tuberculoma” OR “Auditory Agnosia” AND “Infection” OR “Auditory Agnosia” AND “Encephalitis”. Inclusion criteria: Case reports and case series with patients of any age group and sex with AA only in association with central nervous system infection were included. Exclusion criteria: Studies with AA due to causes other than infection including tumor, trauma, stroke, and demyelination were excluded from the review. Texts without availability of full text and text in languages other than English were also excluded. 137 total results were obtained. Two authors (HK and DT) independently screened the titles and abstracts, and eight studies were extracted [Figure 2].Figure 2: PRISMA flow diagramAuditory processing involves perception of both verbal and verbal sounds, which are first processed in the outer and middle ears, and cochlea; then the cochlear nerve is stimulated, which activates the auditory pathway of the brainstem, inferior colliculus of the midbrain, medial geniculate body, auditory radiation, and the auditory cortex in the cerebrum.[1]Figure 3 represents the connections of right and left hemispheres involved in auditory gnosis and lesions affecting these connections. In bilateral lesions, pure AA occurs because left and right cortical and/or subcortical structures are damaged, so auditory stimuli can no longer be processed adequately. AA has been reported following unilateral damage to the primary auditory cortex. AA has been considered as a disconnection syndrome, in which left-hemisphere language areas are anatomically disconnected from both left and the right auditory cortices, because of damage to intrahemispheric pathways and corpus-callosal commissural fibers, respectively. Associative deficits like phonagnosia, musical recognition defects, and apperceptive deficits like amusia and environmental sounds agnosia are seen in lesions affecting the right temporal lobe, while lesions affecting the left temporal lobe may manifest as words meaning deafness and verbal AA. Lesions affecting both temporal lobes lead to global AA.Figure 3: Cerebral connections involved in auditory agnosis and lesions causing auditory agnosia. (The Figure is not adapted or reproduced from any article, constructed by authors)For AA diagnosis, it is necessary to first establish that the patient’s sound-processing deficit is not due to peripheral hearing problems via audiometric testing (PTA), otoacoustic emission testing, BERA, or other audiometric methods. Though AA is classically seen in bilateral lesions, many case reports have been there of AA with unilateral lesions. CNS infection-induced AA occurs either as direct involvement of the brain or as sequalae of infectious processes. Literature [Table 1] depicts AA in association with infections with their outcome. AA agnosia causes profound disability as AA at an early age makes education difficult, leading to communication disorder. In children, communication requires using sign language and they need to attend a special school for the deafness.Table 1: Cases of infection-related auditory agnosia reported in literatureIn our case, AA developed as sequalae of tuberculoma in the left temporal lobe. He responded to antitubercular treatment, and AA recovered. Our case is unique in that among all the cases described so far in literature, there was minimal recovery. In conclusion, regarding the outcome, AA has two subtypes, reversible and irreversible. CNS tuberculosis is a common and treatable infectious entity. To the best of our knowledge, this is probably the first case report with literature review presenting as a reversible infectious cause of AA. Clinical diagnosis of AA can often be missed or misdiagnosed as sensory aphasia. Thorough clinical history, examination, and speech and auditory functions are important in order to make an exact diagnosis of AA. Declaration of patient consent Authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has given his consent for his history, examination, investigations and other clinical information to be reported in the journal. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.