NBME 23 Pons (NBME Answers)

Retired NBME 23 Answers

nbme23/Block 1/Question#41 (reveal difficulty score)
A 62-year-old woman is brought to the ...
Pons 🔍 / 📺 / 🌳 / 📖
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submitted by sakbarh(5)

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She has many cardiovascular risk factors and likely suffered a stroke of the basilar artery causing locked in syndrome. According to FA this can cause a lesion at the pons, medullar, or lower midbrain -- however anatomically the basilar artery runs right on top of the pons so proximity most likely makes it the right answer.

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mousie The Boards and Beyond video of SC strokes was really helpful at explaining this if you are a video kind of person! +3

yotsubato What pushed me away from pons was "dysarthric speech" which implied she still could speak to some degree.... which made me pick medulla. +3

mimi21 I think FA may be misleading. Primarily it will effect the Pons because that is where the majority of the Basilar Artery is located. and I guess it could effect the other locations? but everywhere I have looked Locked-in syndrome is an issue with the Pons. But someone please continue to clarify, cause I was a bit tripped up at first with this question +

cbrodo Although FA says it can be pons, medulla, or lower midbrain, "locked-in" syndrome generally arises from BL pons lesions. Another way you can rule out medulla and midbrain in this question is the ocular movement findings. Since the patient has impaired horizontal gaze BL, you can conclude that the Abducens nuclei are involved on both sides. The abducens nuclei are located in the pons. +48

gh889 USMLE secrets also states that it is most commonly in the pons Bates states that locked-in syndrome preserves consciousness but these patients have limited speaking ability +1

skonys I approach all of these by the rule of 4 method with the 4 midline CN nuclei being multiples of 12. CN 3,4,6,12. Midbrain= PCA (3 & 4) Pons = AICA (5,7,8) & Basilar (6) Medulla = PICA (9,10,11) & ASA (12) Dysphagia, hoarseness, and absent gag reflex? Must be CN X therefore PICA stroke therefore Lateral Medulla (Wallenberg) Whole Face Paralysis? Must be CN VII therefore AICA thus Lateral Pontine. +2

fatboyslim @cbrodo Yes. Also, vertical eye movement is preserved because the vertical gaze center is in the midbrain, while the horizontal gaze center is in the pons. +1

sd22 Locked in syndrome also happens 2/2 to central pontine myelinolysis. Another reminder that, although she has a separate etiology, the location of the lesion is still in the pons. +

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submitted by ∗llamastep1(100)

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So did I just get lucky? I just did rule of 4s, bilateral motor meant medial and I interpreted horizontal eye movement as CN6 which is also medial and pons(CNs 5,6,7,8). Couldn't really place the disarthria but medial pons seemed like a good option.

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submitted by ∗nwinkelmann(366)

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The things that confused me/I forgot: horizontal gaze and dysarthric speech.

The horizontal gaze is a combination of signals between PPRF (paramedian pontine reticular formation), MLF, and other vision cranial nerves. PPRF is closely located to the MLF, which is in the dorsal pons, so a basillar artery infarct, i.e. ventral pons infarct, won't completely inhibit the PPRF, and thus why some horizontal gaze function is available.

Dysarthric speech = defect in articulation of speech mainly due to motor movements of speech, so because CST and CBT are impaired in basillar artery lesion, it makes sense that the motor function of speech would also be impaired.

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submitted by ∗sbryant6(205)

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Pontine arteries are small branches of the basilar artery that can rupture in the setting of poorly controlled hypertension.

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nor16 vertical gaze intact = mesencephalon intact horizontal gaze damaged = pons damaged (RPRF) Pons damaged = no access of corticobulbar tracts to motor nuclei in brain stem -> speech impaired +2

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