Brain tumors, traumatic head injury and a number of brain and nervous system conditions can cause pressure to build up inside the skull. As intracranial pressure (ICP) rises it can compress the brain and result in swelling of the optic nerves, damaging brain tissue and causing irreversible vision loss.
That’s what nearly happened to a 13-year-old boy who had three weeks of uncontrolled headaches and sudden double vision. His neuro-ophthalmologist at Boston Children’s Hospital, Gena Heidary, MD, PhD, found reduced vision in the right eye, along with poor peripheral vision, an enlarged blind spot and swelling of both optic nerves.
As Heidary suspected, he had idiopathic intracranial hypertension, a condition that can raise ICP both in children and adults. Heidary performed an operation around the optic nerve to relieve the pressure, and vision in the boy’s right eye gradually improved, though not completely. Heidary has had to monitor his ICP ever since to protect his visual system from further irreversible damage.
Unfortunately, such monitoring currently is pretty invasive. For hospitalized patients, ICP is tracked by drilling a hole through the skull and placing a catheter or sensor, which requires a neurosurgical team. For outpatients who are tracked over time, ICP measurement may involve multiple invasive lumbar punctures or spinal taps, which require local anesthesia and can have side effects. “We would like to obviate the need for multiple spinal taps to reduce morbidity for our patients,” says Heidary.
Over the past several years, she’s been testing a simple, noninvasive alternative method to measure children’s ICP – in the ear.
The ear-brain connection
In most people, the inner ear has a direct connection to the cerebrospinal fluid space. Scientists suspected that fluctuations in ICP may indirectly affect ear function, and in the late 1980s and early 1990s looked at displacement of the tympanic membrane as a possible proxy for ICP. While this method at first seemed promising, subsequent studies suggested that it may have too much variability to be truly reliable in patients with disease.
During her neuro-ophthalmology fellowship at the Massachusetts Eye and Ear Infirmary, Heidary and her colleagues began collaborating with an electrical engineer named Susan Voss, PhD, who was interested in principles of hearing and middle ear function. Voss had demonstrated that a certain type of hearing response termed a Distortion Product Otoacoustic Emission (DPOAE) – essentially, an echo of tones sounded into the ear – changed systematically in response to ICP changes in healthy adults. (Last year, she published the first proof-of-concept study using this hearing test in adults with high ICP.)
On arriving at Boston Children’s Hospital, Heidary set out to do the first evaluations in children. Could a simple hearing test be a reliable gauge of changes in ICP in children with neurologic disease? The benefits would be huge: After an initial spinal tap to calibrate the system, subsequent measurements could potentially be done noninvasively. And it would involve equipment already used to test hearing – making it safe enough to use in children, even newborns, in an office environment.
“There are some software differences that allow us to analyze the data better, but the equipment really hasn’t been modified,” says Heidary. “That’s an exciting property – it’s something that could be readily available. “
Hearing is believing
In collaboration with neurologist Michel Fayad, MD, and the spinal tap team at Boston Children’s, Heidary completed a pilot proof-of-concept study in children verifying a relationship between DPOAEs and changes in ICP. She and her colleagues are now narrowing down which of 13 different tone frequencies correlates best with ICP changes. They hope to begin additional collaborations with neurosurgeons and critical care specialists at Boston Children’s to see if DPOAEs can track ICP in children with hydrocephalus, head injury and other conditions.
“The next step is to show what happens with the hearing test on a day-to-day basis,” says Heidary. “How does the hearing test fluctuate with ICP long term? What types of noise are in the system? Would we be able to tell if things are worsening with the hearing test alone? The goal would be for us to do one DPOAE measurement at the beginning of therapy, and in six months, instead of a lumbar puncture, do a hearing test to see if the therapy was effective. With a non-invasive method, our ability to treat and manage our patients would be transformed for the better and this is our ultimate goal.”