Sudden hearing loss in acoustic neuroma patients.

Patients with acoustic neuroma may have sudden sensorineural hearing loss. Most patients with sudden hearing loss seek medical attention promptly, but the diagnosis of an acoustic neuroma may be delayed for months or years because sudden hearing loss is an unusual initial symptom of an acoustic neuroma. In a retrospective review of 836 cases of sudden hearing loss, we found 13 patients with acoustic neuromas. The prevalence of acoustic neuromas for those screened with auditory brain stem response or magnetic resonance imaging was 2.5%. In addition to these 13 patients, 79 acoustic neuroma patients treated in our clinic had well-documented sudden hearing loss as the initial symptom. Hearing loss in these 92 patients ranged from mild to profound. Associated symptoms of pain, facial paresthesia, or unilateral tinnitus preceding the sudden hearing loss were suggestive of an acoustic neuroma, as was a midfrequency (U-shaped) hearing loss. A history of other diseases or events that might explain the sudden hearing loss, a normal electronystagmogram, or recovery of hearing does not eliminate the possibility of a tumor. Because there are no clinical findings that clearly distinguish those patients with acoustic neuromas from other patients with sudden hearing loss, we recommend either an evaluation with auditory brain stem response or gadolinium-enhanced magnetic resonance imaging for any patient with sudden hearing loss.

Utility of auditory brainstem response audiometry in diagnosis of acoustic neuromas.

Auditory brainstem response (ABR) audiometry is said to be 90% sensitive in the diagnosis of acoustic neuromas. Since gadolinium-DTPA was introduced, magnetic resonance imaging (MRI) is capable of detecting acoustic neuromas as small as 3 mm. Early diagnosis is important, because hearing can frequently be preserved with resection of tumors smaller than 2 cm. At the same time, cost-containment has become a pressing issue in medicine. Auditory brainstem response, although considerably less expensive than MRI, may not be as sensitive as previously thought, particularly for small tumors. Of 753 acoustic neuromas treated at the House Ear Clinic from January 1988 through March 1993, 197 had ABR data available. The 98 males and 99 females ranged in age from 13 to 78 years with a mean of 48 years. The overall sensitivity of ABR was 92.3% using an interaural latency difference for wave V of more than 0.2 ms, and was 81.6% using waveform morphology. There was a statistically significant difference in sensitivity with respect to tumor size. Auditory brainstem response interaural latency difference sensitivity ranged from 100% in tumors larger than 3.0 cm to 83.1% in tumors 1.0 cm or smaller. Waveform morphology was abnormal in 100% of tumors larger than 2.0 cm but in only 76.5% of tumors 1.0 cm or smaller.

Classical conditioning selectively increases AMPA receptor binding in rabbit hippocampus.

The NMDA and AMPA receptors have been shown to play critical roles in various forms of synaptic plasticity (learning and memory, long-term potentiation). The present study investigated the involvement of these two receptors in a well-characterized classical conditioning paradigm. Following classical conditioning of the rabbit nictitating membrane the binding properties of these two subclasses of excitatory amino acid transmitter receptors were analyzed in dorsal hippocampi by quantitative autoradiography. [3H] TCP and [3H] AMPA were used to identify the NMDA and AMPA receptors, respectively. The binding of [3H]TCP to the NMDA receptor remained unchanged in all the experimental groups tested. Paired presentations of the conditioned and unconditioned stimuli resulted in increased [3H] AMPA binding to the AMPA receptor in several subfields of the hippocampus, while unpaired presentations had no significant effects. The increase in binding was due to an increased affinity of the low-affinity component of the AMPA receptor. The results support the hypothesis that changes in glutamate receptors participate in the synaptic plasticity involved in certain forms of learning.