Definition & Overview
A subdural tap is a medical procedure that involves the removal of excessive fluid build-up in the subdural space, a condition caused by a disease or significant trauma to the head. In infants, the procedure can be performed by inserting a fine hollow needle through the fontanelle or the suture.
A fontanelle (fontanel) is a soft spot between the five major bones of an infant’s skull (2 frontal bones, 2 parietal bones, and an occipital bone). A soft membrane called a suture connects these bones. Sutures allow the bones to move to accommodate the growth of the skull.
In the past, a subdural tap was usually performed as a diagnostic procedure to determine the cause of an acute or chronic build up of subdural fluids that result in an increase in intracranial pressure. It could also be performed to remove an excessive amount of fluids, such as cerebral spinal fluid (CSF) and/or blood.
Today, computed tomography (CT) scan is widely available. As the procedure is non-invasive, it became the primary choice in diagnosing conditions that result in increased intracranial pressure. However, in situations wherein a CT scan is not available or the patient cannot be brought in for a CT scan, a subdural tap may have to be performed.
Who Should Undergo & Expected Results
A subdural tap is mostly performed in infants, but a similar procedure is also used in adults. Infants who are likely to require the procedure are those who display symptoms that suggest an increase in intracranial pressure, such as:
- Enlarged fontanel
- Mental status changes
- Respiratory difficulties
- Hypotonia (low muscle tone)
- Hemiparesis, which causes weakness or the inability to move one side of the body
The symptoms usually indicate the presence of a disease or trauma to the head.
A doctor trained in subdural tap can perform the procedure, but it is highly advisable that the case is consulted with a neurosurgeon. Imaging technologies such as an ultrasound, magnetic resonance imaging (MRI), or CT scan should always be utilised prior to performing the procedure if such technologies are available and the patient can be moved without causing further damage that can worsen his or her medical condition.
The parents or guardians of the patient should understand how the procedure works and why it needs to be performed. Additionally, they should also be aware that the procedure may need to be performed on a daily basis or on alternate days, depending on the severity of the condition.
Although invasive, a subdural tap is considered to be a safe procedure. However, there are risks of complications.
How is the Procedure Performed?
Unless a subdural tap is to be performed as an emergency procedure, the attending physician will notify the parent/guardian why the procedure must be performed, how it is going to be performed, what will happen during the procedure, and any risks or complications that may arise during and after the procedure.
In most cases, the infant will not require general or local anaesthesia. However, a sedative or pain medications may need to be administered to help calm the patient. A subdural tap is usually scheduled 2 to 3 hours after a meal. Doing so greatly reduces the possibility of nausea or vomiting during the procedure.
Prior to the procedure, the patient may need to undergo a CT scan, MRI, or ultrasound to help the doctor locate or identify the exact area where a subdural tap is required. A CT scan is an accurate method of locating excess fluid build up. However, if these technologies are not available or the patient’s current condition prevents him or her from being moved, then imaging tests may not be performed.
The doctor will first shave the area of the head where the procedure is to be performed. This will not only make it easier for the doctor to locate the exact area and perform the procedure but also minimise any chances of infections.
After shaving and cleaning the area with an antiseptic, the infant will be properly positioned and restrained comfortably.
The doctor will then insert a needle through the suture or fontanelle to take samples of the fluid from the area. The doctor will then remove the needle and may need to close the entry point with a single suture.
Possible Risks and Complications
A subdural tap may be safe but there are risks and possible complications. For instance, there’s a risk that the needle may be inserted too far causing damage to the underlying cortex.
Complications that may develop include bleeding caused by damage to small blood vessels, infection, and formation of fistula (abnormal passage) due to multiple subdural taps.
It is highly likely that the procedure will need to be performed repeatedly, which would either be on a daily basis or every other day. The risks and possibilities of complications remain the same each time the procedure is performed.
Yogesh Ghandhi, Don W. Penny;”Subdural Hematoma Aspiration in the Infant”; http://accessemergencymedicine.mhmedical.com/content.aspx?bookid=431§ionid=40216727
Lee Woods;”Atlas of Procedures in Neonatology 4th Edition”; http://doctorlib.info/pregnancy/procedures/18.html