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USMLE® Step 1 Question of the Day: Convex lesion

Osmosis Team
Published on Jan 17, 2024. Updated on Dec 15, 2023.

This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case focuses on a 25-year-old man who suffered head trauma in a bar altercation, resulting in loss of consciousness. Despite initial refusal for medical evaluation, he later exhibits alarming symptoms, including vomiting and sudden unconsciousness. Explore the diagnostic journey as a non-contrast CT reveals a convex lesion with inferolateral pupil dilation, shedding light on the distinct injury patterns and potential complications in this critical emergency department scenario. Test your diagnostic skills with this USMLE Step 1 question.


A 25-year-old man is brought to the emergency department following an altercation at a bar where he was hit over the left side of his head with a bottle. The patient subsequently lost consciousness and spontaneously awakened minutes later. He initially refused medical evaluation, but a few hours later, he began vomiting and agreed to be taken to the hospital. On arrival, the patient suddenly loses consciousness. Temperature is 36.0°C (96.8°F), pulse is 48/min, respirations are 8/min and irregular, and blood pressure is 144/40 mmHg. On physical examination, the patient does not respond to painful stimuli. The left pupil is dilated and inferolaterally abducted. A non-contrast CT is obtained and demonstrates a convex lesion with mass effect and downward displacement of the inferior temporal lobe. This patient's clinical presentation is best defined by which of the following injury patterns?

A. Intra-axial injury; supratentorial; primary brain injury

B. Extra-axial injury; supratentorial; primary brain injury

C. Extra-axial injury; supratentorial; secondary brain injury

D. Extra-axial injury; infratentorial; secondary brain injury

Scroll down for the correct answer!

The correct answer to today's USMLE® Step 1 Question is...

C. Extra-axial injury; supratentorial; secondary brain injury

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

A. Intra-axial injury; supratentorial; primary brain injury

Incorrect: Diffuse axonal injury (DAI) is an example of this injury pattern. Patients with DAI often present with deep coma without elevated intracranial pressure. DAI occurs as a result of traumatic shearing forces applied to the brain when the head is rapidly accelerated and/or decelerated affecting the gray-white junction. Another example of intra-axial injury is a focal cerebral contusion. This patient more likely has an extra-axial injury.

B. Extra-axial injury; supratentorial; primary brain injury 

Incorrect: Examples of this injury pattern include epidural, subdural, and subarachnoid hematomas without signs of brain herniation. This patient has imaging and exam findings indicative of herniation, a secondary brain injury.

D. Extra-axial injury; infratentorial; secondary brain injury

Incorrect: An example of this injury pattern includes downward cerebellar/tonsillar herniation or upward cerebellar/transtentorial herniation from a tumor or hematoma secondary to trauma. This patient’s presentation is consistent with supratentorial herniation.

Main Explanation

This patient presents with blunt head trauma and classic features of an epidural hematoma including a “lucid interval,” a lens shaped lesion, and signs of impending herniation. The physical examination demonstrates a blown, dilated pupil, indicating uncal herniation secondary to extrinsic compression of cranial nerve III. Lastly, the CT results confirm the diagnosis of an epidural hematoma, which is an extra-axial bleed, with secondary brain herniation. 

Traumatic brain injury (TBI) can be classified as primary (e.g. epidural hematoma) or secondary (e.g. brain herniation), the latter of which occurs due to mass effect and increased intracranial pressure. Another way to classify herniation is with respect to the tentorium: supratentorial herniation (cingulate, a.k.a. subfalcine), central transtentorial herniation, or infratentorial herniation (downward/cerebellar tonsillar herniation or upward cerebellar/transtentorial herniation). Finally, TBI that occurs within the brain parenchyma is classified as intra-axial (e.g. diffuse axonal injury) or extra-axial (e.g., epidural, subdural).


types of traumatic brain injuries

brain herniation

Major Takeaway

Pyruvate dehydrogenase complex (PDC) deficiency is an X-linked recessive disorder characterized by hypotonia, poor feeding, developmental and intellectual delay, and seizures. Laboratory testing will demonstrate elevated lactate and alanine levels. Treatment involves adhering to a ketogenic diet.  

References

Gupta, N., Rutledge, C. (2019) Pyruvate dehydrogenase complex deficiency: An unusual cause of recurrent lactic acidosis in a paediatric critical care unit. The Journal of Critical Care Medicine. 5(2), 71-75. Doi: 10.2478/jccm-2019-0012. 

Haddad, A., Mohiuddin, S.S. (2020) “Biochemistry, citric acid cycle”. StatPearls [Internet]. Web Address: https://www.ncbi.nlm.nih.gov/books/NBK541072/.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.