Uncus - Short Answer Questions

Q1. What is the uncus? The uncus is the anterior, hook-shaped (medially curved) part of the parahippocampal gyrus in the medial temporal lobe. It forms part of the limbic lobe and lies just above the parahippocampal gyrus, medial to the amygdala.

Q2. What structures does the uncus contain or relate to? The uncus overlies the amygdala anteriorly and part of the hippocampal head posteriorly. On imaging, the uncus lies medially, with the amygdala separated from the hippocampal head at this level (Grainger & Allison's Diagnostic Radiology, p. 1259-1260).

Q3. What is the functional significance of the uncus? It is part of the limbic system and olfactory cortex (primary olfactory/piriform cortex territory). Epileptogenic lesions in the region of the temporal uncus can produce "uncinate fits" - hallucinations of smell or taste, often accompanied by lip-smacking or chewing (mastication) movements - Localization in Clinical Neurology, p. 1396.

Q4. What is uncal (transtentorial) herniation? It is displacement of the medial temporal lobe, including the uncus, medially and downward through the tentorial notch (tentorial incisura), compressing the midbrain and adjacent structures. It is the most common pattern of herniation caused by an expanding supratentorial mass (Harrison's Principles of Internal Medicine, 22e; Schwartz's Principles of Surgery, 11e).

Q5. What are the key clinical signs of uncal herniation?

Ipsilateral pupillary dilation (blown pupil) from compression of the oculomotor nerve (CN III) as it runs along the edge of the tentorium
Progressive oculomotor palsy (ptosis, eye deviates down and out)
Contralateral hemiparesis from compression of the ipsilateral cerebral peduncle/corticospinal tract
Decreasing level of consciousness as the herniation progresses and compresses the midbrain reticular formation

Q6. Why can the hemiparesis in uncal herniation sometimes be on the same side as the herniation (a "false localizing" sign)? If the advancing uncus pushes the midbrain far enough that it impinges on the contralateral cerebral peduncle against the opposite tentorial edge (Kernohan's notch phenomenon), the resulting weakness will be ipsilateral to the herniating uncus rather than contralateral (Plum and Posner's Diagnosis and Treatment of Stupor and Coma, p. 2969-2974).

Q7. Which vessel is at risk during uncal herniation, and what is the consequence? The posterior cerebral artery (PCA) passes between the uncus and the midbrain; as the uncus herniates it can compress the PCA against the tentorial edge, producing an occipital lobe (calcarine cortex) infarct, sometimes with hemorrhagic conversion (Schwartz's Principles of Surgery, p. 1332-1340; Neuroanatomy through Clinical Cases, 3rd ed.).

Q8. What is the sequence of pupillary findings as uncal herniation progresses? Early: ipsilateral pupil becomes sluggish then dilated and fixed (CN III compression). If untreated, progression to midbrain compression produces bilateral fixed, midposition pupils as the herniation becomes central/bilateral.

Q9. How is uncal herniation typically managed emergently? Immediate measures include head elevation, hyperventilation to lower PCO2, osmotic therapy (mannitol or hypertonic saline), and urgent neurosurgical decompression (e.g., evacuation of the mass lesion or decompressive craniectomy) to relieve the compressive mass effect before irreversible brainstem injury occurs.

Q10. What is an "uncinate fit"? A temporal lobe seizure arising from or involving the uncus, classically presenting with an olfactory or gustatory hallucination (often an unpleasant, indescribable smell) accompanied by automatisms such as lip-smacking or chewing movements.

If you'd like, I can expand any of these into a longer explanation, add a labeled image of the uncus/parahippocampal region, or turn this into a printable revision sheet.

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