At SpecialtyCare, our mission is to make surgery safer, which is why we focus on providing reliable, high quality intraoperative neuromonitoring (IONM) services to hospitals around the country. IONM comprises a range of modalities that map and monitor electrical signals throughout the brain and spine, acting as an early warning system against neurological injury and offering vital data to inform both diagnostic and intraoperative decision-making.
Auditory Brain Stem Responses (ABRs) provide real-time monitoring of auditory nerve and brainstem function during surgery. Recently, SpecialtyCare team member Cyndi Gregg, MHA/MBA, R.EEG/EP.T, CNIM, CLTM, presented on the importance of ABRs for improved outcomes during a meeting of Arizona ASET, a chapter of the ASET Neurodiagnostic Society. Here are the key points of her presentation.
What Are Auditory Brain Stem Responses (ABRs)?
Auditory Brain Stem Responses (ABRs) assess the sound signals that travel from the ear to the auditory nerve and brainstem. In IONM, they are also referred to as Brainstem Auditory Evoked Potentials (BAEPs) or Brainstem Auditory Evoked Responses (BAERs). In this modality, sound stimulation is translated into measurable electrical responses that IONM technicians and surgeons can monitor throughout the surgical process.
ABRs generate a series of waveform peaks, labeled I through VII, that correspond to different structures along the auditory pathway. The following chart reveals how different incidents impact ABR wave signals:
| Transection of distal auditory nerve. | Loss of Wave I-V. (Distal wave I may be preserved in some cases.) |
| Ischemia of brainstem. | Loss of wave V; loss of III-V; increase in IPL of III-V. |
| Cochlear ischemia (labyrinthine artery). | Rapid increase in absolute latency; ultimately loss of all waveforms (no collateral suppliers). |
| Compression or vascular compromise of brainstem involving CNVIII. | Changes in waves I-V. Use knowledge of anatomy to deduce the site of lesion. |
| Cerebellar retraction, dissection of tumor directly from nerve, transection of nerve. | Loss of (or significant prolongation of) wave V, loss of BAEP response within 20 min. |
| Opening of dura or irrigation. | Latency increase (temperature changes), possibly also amplitude reduction. |
| Fluid in the ear/tubing. Any obstruction in the outer or middle ear. | Increased absolute latency; can also prevent all waves. |
| Drilling of auditory canal. | Loss of BAEP response. May be temporary. |
Why ABR Monitoring Is Important During Cranial Surgery
Cranial surgery is complex, involving small spaces, intricate structures, and neurological function. A patient’s hearing is at risk with procedures that involve the auditory nerve, cerebellopontine angle, brainstem, and skull base. There are two types of hearing loss that can occur, conducive and sensorineural. ABR waveforms can provide insight into both types of hearing loss or damage, helping clinicians detect and troubleshoot changes to preserve auditory function.
Moreover, ABR signals remain relatively stable under anesthesia. Benzodiazepines and ketamine have no impact on wave signals, and though some report certain changes in wave forms related to other drugs, ABRs are a trustworthy source of information that can help prevent a patient’s hearing loss.
How ABRs Detect and Localize Neurologic Changes
ABRs can help clinicians identify important issues because their waveforms are specifically related to electrical output of certain auditory structures. Wave I corresponds to the auditory nerve, Wave III corresponds to the pons, and Wave V corresponds to the midbrain. Additionally, waveform latency and morphology indicate where problems are occurring along the auditory pathway. Because clinicians can quickly pinpoint the source of a signal change, they can also respond quickly to make corrections. As an example, the following chart shows the difference in wave signals when a surgeon entered the tumor capsule during a cranial surgery, with the changes on the left:
IONM professionals can translate these changes to the surgeon and help identify the cause of the change in real time, allowing for adjustments that provide greater accuracy in terms of surgical correction and more safety for the patient.
Partner with SpecialtyCare for IONM and Better Outcomes
SpecialtyCare is the largest provider of IONM in the US for a reason. Our experienced team includes dozens of oversight neurologists and around 700 surgical neurophysiologists and technicians in the OR, all highly trained and CNIM certified. Our team members are employed in roughly 42 states, supporting roughly 750 hospitals nationwide. IONM is becoming a non-negotiable standard of care around the country, reducing liability, preventing surgeon burnout, and improving outcomes for the long-term. If your OR needs IONM support, contact us today to learn more about how our team members can integrate seamlessly into your workflow and help you make complex surgery safer!
