Overview of Optic Nerve Sheath Ultrasound

Point-of-care ultrasound (POCUS) is basically a portable ultrasound that doctors can use at a patient’s bedside. This has become increasingly popular over the past two decades. It helps doctors easily and quickly assess a patient’s condition, particularly in critical situations, like in the intensive care unit or during emergencies.

Emergency medicine doctors have been at the forefront of using and teaching others how to use POCUS. Back in 2001, the American College of Emergency Physicians (ACEP) even released the first guidelines on how to use ultrasound in emergencies. These guidelines covered everything from when to use the ultrasound, how to use it, and the importance of ongoing learning in this area.

As the use of ultrasound became more widespread, these guidelines developed further. Now, other medical specialists also use them, especially when it comes to using ultrasound to help perform certain procedures. This includes guiding the placement of needles or tubes into blood vessels, blocking nerves to control pain, and assessing patients before surgery.

One of the great things about POCUS is that it allows doctors to quickly answer specific medical questions. It also allows them to easily check how a patient’s condition is changing over time or to help carry out bedside procedures. Rather than delivering a detailed report, the main goal of POCUS is to quickly spot any problems. Furthermore, with the development of smaller, portable ultrasound machines, this type of imaging can now be used in remote locations or even on the battlefield.

Physicians, as well as advanced paramedics in these challenging settings, are being taught to use this technology. For example, they might use it to check a patient after an injury (eFAST), to look at the optic nerve in the eye when there might be increased pressure in the brain (ONSUS), to check whether a patient has enough fluid in their body by looking at a large vein in the abdomen (IVC collapsibility assessments), and many other uses.

This article will specifically discuss how doctors use POCUS to look at a patient’s optic nerve. This can be hugely helpful when doctors need to diagnose, monitor, or manage a condition where there is increased pressure inside the skull (elevated intracranial pressure or ICP).

Anatomy and Physiology of Optic Nerve Sheath Ultrasound

The eye is the perfect organ for ultrasound examination because it is located on the surface of the body and contains distinct fluid-filled areas which are great for capturing ultrasound images. To perform an ocular ultrasound, you need a basic understanding of the structure of the eye. The better you can identify the parts at the front of the eye, the more successful you will be in measuring the optic nerve sheath correctly.

The eyeball sits in a membrane sac called the Tenon capsule. It’s wrapped in a layer of fatty tissue, soft tissue and the walls of the eye socket. The eyeball is connected to the corneoscleral junction and the optic nerve. Extraocular muscle tendons penetrate the Tenon sac, attach to the sclera (the white part of the eye), and allow the eye to move in various directions. The anterior chamber (AC) and posterior chamber (PC) are two areas in the eye filled with fluid that you need to identify during the ultrasound. The lens of the eye separates these two chambers and they both contain a type of fluid that doesn’t echo sound waves.

Located behind the PC is the retina with a highly echoic (meaning it reflects sound waves really well) nervous membrane. Behind the retina and the optic disc, the optic nerve can be observed. The nerve looks like a pouch of brain tissue, with a sound wave-absorbing (anechoic) core, enclosed by a highly echoic sheath at the medial and lateral borders. The optic nerve is completely enclosed by three layers – dura, arachnoid, and pia matter – allowing it to carry cerebral spinal fluid and adjust its size based on the brain’s internal pressure changes. The covering of the optic nerve continues from the dural and subarachnoid space. The bulging part of the optic nerve, located roughly 3 mm behind the eyeball, is especially flexible and sensitive to pressure changes.

The regular optic nerve sheath diameter (ONSD) should not be larger than 4 mm in infants, 5 mm in children, and 5 mm in adults. Once the ONSD reaches 7.5 mm, it won’t expand much further.

Why do People Need Optic Nerve Sheath Ultrasound

An eye ultrasound, also known as ocular ultrasound, is a non-invasive test that uses sound waves to create images of the inside of your eye. It can help your doctor find or rule out problems that could be causing symptoms like vision loss, headaches, or issues related to a head injury. It’s particularly helpful for spotting issues like a detached retina, objects stuck in the eye, a dislocated lens, bleeding in the vitreous (the clear, jelly-like substance in the back of your eye), or other surface abnormalities that might be causing you to lose your sight.

In some cases, this type of ultrasound can also be used in the context of a traumatic brain injury, elevated pressure inside the skull, or inflammation of the optic nerve. The benefit of using an ocular ultrasound for these situations is that it’s a non-invasive procedure, which means it doesn’t come with risks of infection, bleeding, or radiation exposure that are associated with certain other methods of monitoring and testing, such as a CT scan.

Ocular ultrasounds can also help “rule out” high intracranial pressure (pressure within the skull) in non-emergency situations related to trauma. For example, if a device placed within the brain ventricles isn’t working properly, or to evaluate certain medical procedures involving the brain or spinal cord, or in people who previously had increased intracranial pressure. However, these uses haven’t been thoroughly studied or proven, so the results should always be evaluated in the broader context of your overall health.

The measurement of the optic nerve sheath diameter (ONSD), a part of your eye near your optic nerve, is currently thought to be the best non-invasive way to estimate intracranial pressure. In fact, there’s a specific formula used to estimate intracranial pressure based on the diameter of the optic nerve sheath. This can be especially useful for critically ill patients in whom other methods of monitoring intracranial pressure are not available or suitable. As a tool, this has been found useful in situations like intracranial hypertension, intracranial hypotension, brain damage due to lack of blood and oxygen, brain surgery, evaluating shunts (devices to relieve pressure in the brain), head injury, and strokes, among other things.

When a Person Should Avoid Optic Nerve Sheath Ultrasound

Surface ultrasound is generally a safe procedure that carries minimal to no risk for adult patients. If, however, there’s a chance that the patient has an injury where the eyeball is split open, it’s possible that the ultrasound could further hurt the eye. Under these circumstances, the procedure should not be carried out unless an eye doctor (ophthalmologist) has been consulted or is present to assist.

Equipment used for Optic Nerve Sheath Ultrasound

The Ocular Nerve Sheath Ultrasonography (ONSUS) is useful because it can be conducted with most ultrasound equipment that is abundantly available in emergency departments and intensive care units (ICUs) throughout the United States. There are special, high-frequency ocular ultrasound machines made specifically for eye studies, but most traditional ultrasound machines that have a high frequency (7 to 10 MHz or higher) linear array probes are enough to visualize the optic nerve with the closed eye technique.

“MHz” stands for megahertz, which is a unit of frequency used to measure how fast the waves in an ultrasound move. And “probes” refer to the handheld device that a doctor moves on the patient’s body to create an ultrasound image.

Some additional items that may be needed include a clear sterile intravenous catheter adhesive, which is used to create a barrier, and ultrasound gel, which helps improve the quality of the ultrasound image.

Who is needed to perform Optic Nerve Sheath Ultrasound?

Usually, a procedure known as ONSUS can be done by one person without needing help in positioning the patient. If the area is limited, it can be handy to have another person who knows how to use ultrasound there. This assistant can whelp in taking pictures, adjusting the picture quality, and fine-tuning other functions to get the best possible images.

Preparing for Optic Nerve Sheath Ultrasound

Patients who are awake and alert will be informed about the procedure. Patients who are dull or drowsy, intubated or heavily sedated should have a basic eye assessment to check for signs of eye injury like globe rupture or hyphema (a condition where blood collects in the front part of the eye). If there’s any evidence of globe rupture, like visible damage to the conjunctiva or the white part of the eye (known as sclera), severe swelling, bleeding covering the entire surface of the eye, low pressure inside the eye, or complete hyphema, the procedure should be stopped to avoid further damage to the eye, and an eye doctor should be consulted.

It could be helpful to tell awake patients to “look straight ahead” with a closed eye, or to check the eye alignment in less responsive patients before starting the procedure. This helps position the ultrasound tool properly and avoid incorrect images of the optic nerve. Some doctors may apply a type of gel or eye ointment to prevent air bubbles before using a clear sticky dressing. A liberal amount of ultrasound gel should be applied to the dressing before starting the scan.

The position in which the patient is placed should be based on their comfort and the ease of performing the examination, but usually, it should be on their back or slightly inclined. While the theory suggests that changes in the patient’s position and ventilator settings may affect the pressure inside the brain and the size of the optic nerve sheath, these changes are not seen in real life models. Several small studies have also shown that changes to patient position, or even the creation of air-filled belly during laparoscopic surgeries, do not significantly affect the optic nerve size.

How is Optic Nerve Sheath Ultrasound performed

An eye ultrasound uses high-frequency sound waves to make images of the inside of your eye. Before the exam starts, a sterile bandage is placed over your closed eye and a lot of ultrasound gel is applied. Then, a transducer which sends out sound waves and picks up the echoes is placed over the bandage, with light or no contact. The doctor performing the ultrasound will stabilize their hand on your nose, forehead, or midface to ensure they don’t put any pressure on the eye, which could cause discomfort and change the anatomy.

The transducer is moved across the eye in small steps, which help to get the best image of the optic nerve sheath. The optic nerve sheath is a tube that surrounds the optic nerve, which carries information from the eye to the brain. The image of the optic nerve sheath is then saved, and measurements are taken to help diagnose your condition. It’s important to remember that just one view isn’t enough – the same measurements should be done in a different plane to get the most accurate results.

If the doctor is having trouble getting a good image, they might try to aim the sound waves through the lens of the eye. Also, they may check the other eye to get an idea of where the optic nerve sheath might be in the eye being examined. Using more ultrasound gel or using a Doppler ultrasound to see the blood flow near the back of the eye are other techniques that can be used to get a better image.

The images and measurements are then evaluated in the context of your overall health. Based on the size of the optic nerve sheath, this can include information on the pressure inside your eye which can help diagnose conditions such as glaucoma.

There are a couple of different types of ultrasound that can be used – A-mode and B-mode. While A-mode is preferred due to the quality of the images it can produce, B-mode can still be used, especially if the patient is not able to stay still during the exam. Regardless of the type of ultrasound used, it is best to take the measurement of the optic nerve sheath about 3 mm behind the eye globe. This is where the pressure inside the eye has the biggest effect on the sheath.

The basic process of an eye ultrasound involves you lying flat, and the doctor placing the ultrasound transducer on your closed upper eyelid after applying some gel. The sound settings of the ultrasound are adjusted to as low as possible to prevent any damage to the eye structures. The image of the eye on the ultrasound screen is then adjusted to show the beginning of the optic nerve. After that, the measurement of one’s optic nerve sheath is taken about 3 mm behind the eye globe. Several measurements are taken for each eye and averaged to get the most accurate result.

Possible Complications of Optic Nerve Sheath Ultrasound

ONSUS, a type of eye test, is considered safe and not harmful if there is no eye injury. But if someone has an “open globe injury,” which means a severe eye injury where the eye is cut open, ONSUS might cause further damage to the eye. In this situation, the test should not be conducted unless an eye doctor, or ophthalmologist, gives their approval.

Even though some research studies show that ONSUS is quite accurate, there have been doubts about the method of this test. For instance, when it visualizes the covering of the optic nerve, and captures the images in two different views (transverse and sagittal), some researchers question its reliability.

Copetti et al., for example, argue that the common pictures of the optic nerve sheath seen in some of these studies may in fact be an unintentional result, or “artifact,” possibly due to the shadow from the lamina cribrosa – a part of the eye. They express a concern that these false positive outcomes might lead to unnecessary treatments, extra monitoring, or additional tests that are not needed. Therefore, they suggest using color doppler, a special type of ultrasound, to recognise the retinal artery and measuring the optic nerve from the side.

What Else Should I Know About Optic Nerve Sheath Ultrasound?

When patients in intensive care units (ICUs) are critically ill, it can be dangerous to move them to another part of the hospital for imaging scans like CT scans. The transfer from one part of the hospital to another can be life-threatening with a risk as high as 2%, and up to 68% have reported unwanted incidents during the transfer. Interestingly, about half the cases aren’t impacted by the results of CT scans, suggesting scans should be used cautiously and thoughtfully in ICU settings.

By using technology called ONSUS (Optic Nerve Sheath Ultrasound), doctors can make better decisions about whether a patient really needs a CT scan. This is particularly helpful when dealing with brain injuries, where the quick diagnosis and treatment are vital for the patient’s health. When a patient’s condition is worsening, ONSUS could be used to check for brain pressure build-up instead of moving them for a CT scan. Frequent ONSUS checks are recommended, as any change in the optic nerve can be a sign of increasing pressure in the brain.

This method has been used in various harsh environments like high altitudes and battlefields where no other imaging methods are available. Considering that traumatic brain injuries (TBIs) are common in such environments, ONSUS comes in handy for quick check-ups. Most of these injuries are classified as mild and don’t necessitate complex surgeries, but it’s often tough to evaluate them in such locations.

One of the benefits of using ONSUS is that it offers a fast and minimally invasive way to check for increased pressure in the brain. This is beneficial, especially when relocating a patient for neurosurgical care. The treatment of known or suspected increased brain pressure is a key part in treating severe brain injury. After providing standard care such as elevating the head, setting blood pressure and oxygen goals, and controlling blood sugar, treatment with a saline solution can be initiated, unless it’s contraindicated.

Since invasive methods to monitor pressure in the brain require specialized expertise and carry significant risks like infection and bleeding, are costly, and are often not readily available, using ONSUS could aid doctors in deciding when to start therapy and continuously check on the patient’s response. The procedure is simple, quick, inexpensive, can be done by the patient’s bedside, and doesn’t have serious risks like radiation exposure.

However, this method also comes with its own limits. It depends on the person operating the ultrasound, it’s influenced by the environment (like an emergency room or ICU), and the results can vary greatly. The shape of the optic nerve and factors like the motion of the eye, the length of the eyeball, the presence any eye diseases, and some physical characteristics can all affect the reading.

Frequently asked questions

1. How will an optic nerve sheath ultrasound help diagnose, monitor, or manage my condition? 2. Are there any risks or side effects associated with the procedure? 3. How accurate is the measurement of the optic nerve sheath diameter in estimating intracranial pressure? 4. Are there any limitations or factors that could affect the accuracy of the ultrasound results? 5. What other imaging or diagnostic tests may be necessary in conjunction with the optic nerve sheath ultrasound?

Optic Nerve Sheath Ultrasound can help measure the diameter of the optic nerve sheath, which is important for assessing intracranial pressure. By identifying any changes in the optic nerve sheath diameter, this ultrasound can provide valuable information about conditions such as increased intracranial pressure or optic nerve swelling. This can help in diagnosing and monitoring various neurological conditions.

There are several reasons why someone may need Optic Nerve Sheath Ultrasound. Some of these reasons include: 1. Evaluation of increased intracranial pressure: Optic Nerve Sheath Ultrasound can help in assessing the pressure inside the skull. Increased intracranial pressure can be caused by conditions such as brain tumors, hydrocephalus, or traumatic brain injury. 2. Diagnosis of papilledema: Papilledema is a condition characterized by swelling of the optic nerve due to increased intracranial pressure. Optic Nerve Sheath Ultrasound can help in confirming the presence of papilledema and determining its severity. 3. Monitoring of patients with head trauma: Optic Nerve Sheath Ultrasound can be used to monitor patients with head trauma for signs of increased intracranial pressure or development of complications. 4. Assessment of optic nerve abnormalities: Optic Nerve Sheath Ultrasound can be used to evaluate the optic nerve for abnormalities such as optic neuritis, optic nerve sheath tumors, or optic nerve head drusen. It is important to note that Optic Nerve Sheath Ultrasound should only be performed by a trained healthcare professional and in certain cases, consultation with an ophthalmologist may be necessary to ensure the safety and appropriateness of the procedure.

You should not get Optic Nerve Sheath Ultrasound if there is a chance that your eyeball is split open, as the procedure could further harm your eye. In such cases, it is important to consult or have an ophthalmologist present before proceeding with the ultrasound.

There is no mention of recovery time for Optic Nerve Sheath Ultrasound in the provided text.

To prepare for an Optic Nerve Sheath Ultrasound, the patient should be informed about the procedure. If the patient is awake and alert, they should be instructed to look straight ahead with a closed eye. For patients who are dull or drowsy, intubated, or heavily sedated, a basic eye assessment should be done to check for signs of eye injury. It may also be helpful for the patient to be in a comfortable position, such as lying flat or slightly inclined, and a sterile bandage should be placed over the closed eye with ultrasound gel applied before the exam starts.

The complications of Optic Nerve Sheath Ultrasound (ONSUS) include the potential for further damage to the eye if there is an "open globe injury." Additionally, there are doubts about the reliability of the test, with some researchers questioning the accuracy of the images captured in two different views. There is concern that false positive outcomes may lead to unnecessary treatments, extra monitoring, or additional tests that are not needed. Some researchers suggest using color doppler ultrasound as an alternative method.

Symptoms that would require Optic Nerve Sheath Ultrasound include vision loss, headaches, issues related to a head injury, elevated pressure inside the skull, inflammation of the optic nerve, and intracranial hypertension or hypotension.

Based on the provided text, there is no specific mention of the safety of Optic Nerve Sheath Ultrasound (ONSUS) in pregnancy. The text primarily focuses on the use of ONSUS in diagnosing and monitoring conditions related to increased pressure inside the skull. It does mention that surface ultrasound, which includes ONSUS, is generally a safe procedure for adult patients. However, it also states that if there is a chance of an injury where the eyeball is split open, the ultrasound could potentially harm the eye. Therefore, it is recommended to consult an eye doctor (ophthalmologist) before performing ONSUS in such cases. Since the text does not provide specific information about the safety of ONSUS in pregnancy, it is advisable to consult a healthcare professional or an ophthalmologist for a more accurate and detailed answer regarding the safety of ONSUS during pregnancy.

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