The hummingbird sign refers to the appearance of the midbrain on a sagittal MRI or CT scan. It indicates compression of the midbrain by the surrounding structures, giving it a characteristic tapered, beak-like appearance reminiscent of a hummingbird’s beak.
What causes the hummingbird sign?
The most common causes of the hummingbird sign are:
- Brain herniation – The brain tissue, cerebellum, and brainstem can herniate through the tentorium or foramen magnum, compressing the midbrain.
- Hydrocephalus – Buildup of cerebrospinal fluid in the brain’s ventricles puts pressure on the midbrain.
- Posterior fossa mass lesions – Tumors or other mass lesions within the posterior cranial fossa, such as cerebellar tumors, can compress the midbrain posteriorly.
- Tentorial notch mass lesions – Lesions near the tentorial notch, such as pineal region tumors, can compress the midbrain from above.
Of these, transtentorial brain herniation is the most common cause of the hummingbird sign. The rigid tentorium separates the supratentorial and infratentorial compartments. As brain swelling occurs, the brain can herniate downward through the tentorial opening, compressing the midbrain.
How does transtentorial herniation cause the hummingbird sign?
In transtentorial herniation, pressure from the descending temporal lobes against the midbrain causes it to take on a tapered, beak-like appearance. This is due to compression of the midbrain at two points:
- At the tentorial notch anteriorly, compressing the cerebral peduncles against the edge of the tentorium
- Posteriorly, compressing the midbrain where it is anchored to the clivus by the tectal plate
This gives the midbrain a distinctive peaked, V-shaped appearance reminiscent of a hummingbird’s beak in sagittal views. The cerebral peduncles appear straightened and compressed. Increased pressure can lead to ischemia and medial necrosis of the midbrain.
What are the signs and symptoms of transtentorial herniation?
Transtentorial herniation causes both general and focal neurological signs and symptoms:
- Declining level of consciousness progressing to coma
- Pupillary changes like pupillary dilation or unreactive pupils
- Abnormal posturing like decorticate or decerebrate rigidity
- Cushing’s triad of hypertension, bradycardia, and irregular respirations due to compression of the brainstem
- Parinaud’s syndrome:
- Vertical gaze palsy – inability to look up or down due to compression of the superior colliculi in the midbrain tectum
- Dilated pupils – compression of the pretectal area
- Eyelid retraction – loss of input to the third cranial nerve (CN III)
- Convergence-retraction nystagmus – eyes pulled inward due to unopposed action of the medial recti muscles
How is the hummingbird sign diagnosed?
The following neuroimaging studies can reveal the hummingbird sign:
- CT scan – Plain CT may show obliteration of the quadrigeminal plate cistern posterior to the midbrain.
- MRI – T1-weighted saggital MRI best demonstrates the hummingbird beak appearance of the midbrain.
The hummingbird sign is best visualized on midline saggital MRI or CT images. The peaked contour of the compressed midbrain is seen, along with flattening of the pons. MRI may also show associated findings like hydrocephalus, mass lesions, or hemorrhage causing the herniation.
Table summarizing the key features of the hummingbird sign
| Imaging Feature | Description |
|---|---|
| Peaked midbrain contour | The midbrain has a tapered, V-shaped appearance like a hummingbird’s beak |
| Compressed cerebral peduncles | The cerebral peduncles appear straightened and compressed against the tentorium |
| Obliterated quadrigeminal plate cistern | Loss of cerebrospinal fluid space posterior to the midbrain |
| Flattened pons | The pons loses its usual convexity and appears flattened |
How is transtentorial herniation treated?
Emergency medical treatment is needed to reduce intracranial pressure and relieve herniation. This may include:
- Elevating the head of the bed to 30 degrees
- Hyperventilation to induce hypocapnia and cerebral vasoconstriction
- IV mannitol to reduce brain volume through osmosis
- Sedation and paralytics to prevent voluntary or reflex Valsalva maneuvers
- Surgery to remove mass lesions or drain CSF via ventriculostomy
- Decompressive craniectomy to allow the brain to swell outward without compression
With early treatment, the structural deformity caused by transtentorial herniation may reverse. However, ischemic brain injury can still occur. Delay in treatment can lead to death or permanent neurological deficits.
What is the prognosis of transtentorial herniation?
The prognosis depends on the duration and degree of brainstem compression. Mild cases with early treatment have a good prognosis. Severe, untreated herniation can lead to death or permanent coma.
Key prognostic factors include:
- Age and pre-existing health status
- Cause and rate of progression of increased intracranial pressure
- Presence of associated mass lesions or hemorrhage
- Timeliness of diagnosis and treatment
- Degree of brainstem ischemic injury before treatment
Even with recovery, some permanent sequelae may remain, like cranial nerve palsies, movement disorders, or cognitive deficits due to midbrain injury.
Conclusion
The hummingbird sign is a characteristic imaging finding of midbrain compression seen in transtentorial herniation. It appears as a peaked contour of the midbrain on saggital MRI/CT due to downward herniation of the temporal lobes through the tentorial opening. This causes neurological deterioration, requiring prompt treatment to halt progression and relieve compression. While early recognition and treatment are key for a good outcome, permanent neurological damage can still occur depending on the degree of brainstem injury.
In summary:
- The hummingbird sign indicates compression of the midbrain, usually due to transtentorial herniation
- It shows a characteristic tapered, beak-like appearance of the midbrain on saggital imaging
- It is caused by temporal lobe herniation compressing the midbrain at two points
- Associated findings include quadrigeminal plate cistern obliteration and flattening of the pons
- It causes both generalized and focal neurological findings
- Urgent measures are needed to reduce intracranial pressure and relieve compression
- The prognosis depends on prompt diagnosis and treatment before irreversible brainstem injury occurs
