head CT

What Do New Orleans and Canada Have in Common? Head CT Rules

virtualmedstudent

Background

CT scans have become the sine-qua-non of assessing traumatic brain injury. CT scans of the head are fast and relatively inexpensive. In addition, they are able to pick up injuries that require emergent intervention.

CT scanners are so ubiquitous, at least in the United States, that it is easy to order a scan regardless of whether or not it is clinically indicated. The “scan everybody with a head bonk” mentality is dangerous for several reason. First, it exposes the patient to unnecessary radiation. Second, the term “relatively” inexpensive is exactly that, “relative”. CT scans are still costly by comparison, and every unnecessary scan only adds to the economic health care crisis.

So who should we scan? The literature on who to scan is based heavily on the Glasgow Coma Scale (GCS). This scale divides patients with head trauma into three categories: mildly injured, moderately injured, or severely injured.

The Glasgow Coma Score is based on three behavioral components: eye opening, verbal performance, and motor responsiveness. Scores range from 3 to 15, with 15 being a "normal" score and 3 being completely comatose (or even dead!). Patients with a score of 8 or less are considered severely injured. Those with a score of 9 to 12 are considered moderately injured, and those with a score of 13 to 15 are considered mildly injured.

Patients with moderate to severe GCS scores should always be scanned. These people are at high risk for clinically important brain injury.

That brings us to the next question… What should we do with all the mild head bonks? The mildly injured patient usually looks good clinically (ie: a normal neurological examination), but may still harbor intracranial nastiness! So how do we determine which mild injuries to scan and which ones to send home?

The New Orleans Criteria

New Orleans Criteria:
Scan if GCS 15 and
any of the following
are present…

– Headache
– Vomiting
– Age 60 or older
– Short term memory
problems
– Seizure
– Intoxicated
– Visible injury above
clavicles
The answer lies in two commonly used guidelines. The first set of guidelines is known as the "New Orleans Criteria for Minor Head Injury". The New Orleans Criteria state that anyone with a normal GCS should be scanned if any of the following criteria are present: headache, vomiting, 60 years of age or older, short term memory problems, seizure, intoxication (ie: alcohol or drugs), or visible injuries above the clavicles.

The Canadian Head CT Rules

The second set of guidelines is known as the "Canadian Head CT Rules in Minor Head Injury". This set of guidelines states that any patient with a mild GCS score (ie: a 13, 14 or 15) are at high risk for neurosurgical intervention if the following factors are present: GCS score of less than 15 for longer than 2 hours after the injury, open or depressed skull fracture, signs of basilar skull fracture on physical examination, greater than two episodes of vomiting, and age greater than 65. In addition, patients at risk for brain injury (although not necessarily requiring surgical intervention) include those with amnesia longer than 30 minutes from the injury, or those involved in a dangerous mechanism of injury.

Comparing the Two Criteria

Canadian Head CT Rules:
Scan if GCS 13, 14, or 15 and any
of the following are present…

– GCS < 15 at 2 hours
– Open/depressed skull fracture
– Vomiting > 2 times
– Signs of basilar skull fracture
– Age 65 or older
– Dangerous mechanism
– Antegrade amnesia > 30 minutes
The two sets of criteria for scanning are surprisingly different. I would argue that the New Orleans Criteria are more "loose" compared to the Canadian Rules. For example, most people have a "headache" after traumatic injury, and based on the New Orleans criteria this alone would be enough to warrant a scan. In a head-to-head comparison both criteria were very sensitive at picking up clinically important head injuries, but the Canadian Rules were more specific.

Overview

Deciding who to scan after mild head injuries has been studied extensively. Currently, two common sets of criteria are used to decide who gets a CT scan. Both sets of criteria are sensitive in picking up clinically significant head injury, but the Canadian Head CT Rules are more specific than the New Orleans Criteria and may help further reduce unnecessary scanning.

References and Resources

  • Washington CW, Grubb RL Jr. Are routine repeat imaging and intensive care unit admission necessary in mild traumatic brain injury? J Neurosurg. 2012 Mar;116(3):549-57.
  • Stiell IG, Clement CM, Rowe BH, et al. Comparison of the Canadian CT Head Rule and the New Orleans Criteria in patients with minor head injury. JAMA. 2005 Sep 28;294(12):1511-8.
  • Teasdale G, Jennett B. Assessment of coma and impaired consciousness. A practical scale. Lancet. 1974 Jul 13;2(7872):81-4. This is the original GCS paper.
  • Bouida W, Marghli S, Souissi S, et al. Prediction value of the Canadian CT head rule and the New Orleans criteria for positive head CT scan and acute neurosurgical procedures in minor head trauma: a multicenter external validation study. Ann Emerg Med. 2013 May;61(5):521-7.

How To Systematically Interpret a Head CT: Blood Can Be Bad

virtualmedstudent

Head CTs are a common, inexpensive, and fast way of evaluating intracranial pathology. Although they do not give the anatomical detail of an MRI, they are still extremely important in diagnosing “gross” pathology that needs emergent intervention.

CT scans are based on the Hounsfield unit (HU), which is an indirect way to measure density. Interestingly, Sir Godfrey Newbold Hounsfield won a Nobel prize for his work on developing the CT scanner, but I digress…

The importance of the Hounsfield unit is that things that are hyper-dense (very dense) appear bright; those things that are hypo-dense (not very dense) appear dark. The different tissues and fluids within the confines of the skull have varying densities. The most dense materials, like bone, have very high Hounsfield units; less dense materials such as air and cerebrospinal fluid have very low Hounsfield units.

It is important to approach head CTs in a systematic fashion so that subtle (and not so subtle) pathology is not missed. The easiest way I have found to read a head CT is to remember the following mnemonic:

Blood Can Be Very Bad

The first “B” in the mnemonic stands for, you guessed it, blood. There are five different pathological locations that blood can be located: epidural, subdural, subarachnoid, intraventricular, and intraparenchymal. Depending on the age of the blood, it may be hyper-dense (acute/active bleeding), isodense (roughly 3 to 7 days old), or hypo-dense (older than 7 days).

CT scans of Intracerebral Hemorrhages
The "C" in the mnemonic stands for "cisterns". Cisterns are enlarged subarachnoid spaces where cerebrospinal fluid pools. The most important cisterns are around the brainstem. They include the interpeduncular, suprasellar, ambient, quadrigeminal and pre-pontine cisterns. A healthy amount of cerebrospinal fluid should “bathe” the brainstem; if there is increased intracranial pressure cerebrospinal fluid will get pushed out of these cisterns as brain tissue starts to herniate into them. And that as they say is “no bueno”.

The second "B" stands for "brain". Although blatant pathology such as blood clots are usually readily apparent, more subtle pathology can also be obtained from a CT. For example, blurring of the gray-white junction may indicate evolving stroke. Any areas of hypodensity (ie: dark areas) within the brain may indicate edema associated with a tumor.

The "V" represents the ventricular system. The ventricular system consists of a pair of lateral ventricles, a third ventricle, and a fourth ventricle (don’t ask me what happened to the first and second ventricle!). The ventricles are in communication with one another via holes known as foramen. The paired foramen of Monroe connect the lateral ventricles to the third ventricle; the cerebral aqueduct of Sylvius connects the third ventricle to the fourth ventricle. The fourth ventricle drains into the subarachnoid space surrounding the spinal cord via the foramen of Magendie and Lushka.

The ventricular system is quite symmetric. Any obvious asymmetries may indicate a pathologic process "pushing" on a ventricle causing it to become distorted. In addition, if the ventricles are larger than normal it may indicate the presence of hydrocephalus, a condition in which cerebrospinal fluid is not reabsorbed appropriately.

The final "B" in the mnemonic stands for "bone". The skull should be assessed for fractures, especially in trauma patients. A common place for fractures is at the skull base. Time should be spent assessing this area to rule out fractures that extend across the canals and foramen that house the carotid arteries, jugular veins, and cranial nerves.

Reading a head CT is the first step in determining what additional imaging studies are necessary, or what treatment should be given. By using the above mnemonic it allows the interpreter of the scan to quickly and effectively assess if there is underlying pathology that needs further evaluation.

Overview

The mnemonic – blood can be very bad – can be used to systematically interpret a head CT. The first "B" stands for blood. The "C" stands for cisterns. The second "B" stands for brain. The "V" represents the ventricular system. And the last "B" stands for bone. By looking at these five components it is possible to assess all the important pathology that may require further imaging and/or treatment.

References and Resources