The bird sign refers to a specific finding on brain imaging, particularly MRI, indicating infarction or ischemia in the pons region of the brainstem. The pons is a crucial part of the brainstem that contains nerve fibers carrying signals between the brain and the rest of the body. When the blood supply to the pons is disrupted, it can result in a characteristic appearance on imaging that has been termed the “bird sign” or “penguin sign” due to its resemblance to a bird or penguin shape. Detecting the bird sign is important, as it indicates a stroke or other serious neurological event requiring prompt medical attention.
Anatomy of the Pons
The pons is located in the upper part of the brainstem, between the midbrain and the medulla oblongata. It acts as a bridge connecting the cerebellum and cerebrum. The pons contains important nuclei and tracts that relay signals between the two parts of the brain, as well as between the brain and spinal cord. Some key structures found in the pons include:
- Corticospinal tracts – Carry motor signals from the brain to the spinal cord
- Sensory tracts – Carry sensory signals from the spinal cord to the brain
- Pontine nuclei – Relay signals from the cerebrum to the cerebellum
- Cranial nerve nuclei – Control functions like eye/face movements and chewing
- Reticular formation – Regulates arousal and sleep/wake cycles
The pons receives blood supply from two main arteries:
- Anterior inferior cerebellar artery (AICA)
- Paramedian basilar arteries (PMBA)
A disruption in blood flow through either of these vessels can lead to ischemic injury in the pons.
Causes of the Bird Sign
The bird sign indicates that there is infarction (tissue death from inadequate blood/oxygen) or ischemia (inadequate blood flow) in the pons. Some potential causes include:
- Stroke – Blockage or rupture of a blood vessel supplying the pons
- Vertebrobasilar insufficiency – Reduced blood flow through the vertebral or basilar arteries
- Pontine hemorrhage – Bleeding into the pons
- Central pontine myelinolysis – Damage to the myelin sheath around nerves
- Tumors – Mass lesions compressing the pons
- Inflammation – Demyelinating disorders like multiple sclerosis
- Trauma – Injury to the brainstem
Strokes, either ischemic or hemorrhagic, are the most common cause of the bird sign. However, any condition affecting the blood supply to the pons or directly damaging the pons tissue can potentially result in the imaging findings.
Imaging Features of Bird Sign
The bird sign is best visualized through MRI of the brain, though it may also be seen on CT scans. The main features include:
- Area of abnormal signal intensity in the pons – Depending on the imaging sequence, this could appear darker or brighter than normal pons tissue
- Triangular shape – The abnormality has a wedge-shaped or teardrop configuration
- Pointed beak – One end comes to a point resembling a beak
- Lesion centered on midline – Located centrally within the pons
- Discrete borders – Stands out in contrast to rest of pons tissue
The lesion tends to span across the transverse width of the pons, disrupting corticospinal and sensory tracts. On T2-weighted and FLAIR MRI sequences, it will typically appear hyperintense or bright compared to normal pons tissue. Gradient echo sequences may show blooming artifact. Diffusion weighted imaging usually demonstrates restricted diffusion in the acute setting.
Significance of Bird Sign
Identification of the bird sign has important diagnostic and prognostic implications:
- Confirms acute ischemic or hemorrhagic event in the pons
- Can localize lesion to aid in identifying etiology
- Helps distinguish stroke from other causes of vertigo/dizziness
- Indicates risk for associated neurological deficits
- Predicts likely poor functional recovery and higher mortality if sign is sizable
- Warrants urgent management of blood pressure and intracranial pressure
Patients with a bird sign often develop severe neurological symptoms including vertigo, diplopia, dysarthria, facial weakness, hemiparesis, and ataxia. Those with larger lesions may progress to coma or even herniation. Therefore, promptly identifying the bird sign can guide rapid treatment to stabilize the patient and try to preserve neurological function.
Differential Diagnosis
There are some other findings that may mimic a bird sign on neuroimaging. The main entities in the differential diagnosis include:
- Bat wing sign – Bilateral pontine lesions in central pontine myelinolysis
- Heart sign – Midbrain ischemia with bilateral medial occipital lesions
- Mickey Mouse sign – Midbrain atrophy with pontine sparing
- Morning glory sign – Midbrain tumor causing aqueductal obstruction
- Opercula sign – Bilateral paramedian thalamic infarcts
Carefully evaluating the exact location, shape, and signal characteristics will help distinguish between these findings. The bird sign’s characteristic triangular pontine distribution helps confirm the diagnosis. Review of clinical history and other imaging also aids in narrowing the differential.
Treatment
There are several key aspects to managing patients with a bird sign:
- Admit to stroke unit or neurological ICU for close monitoring
- Control blood pressure to limit hemorrhage expansion or ischemia
- Maintain adequate oxygenation and cerebral perfusion
- Consider thrombolytics if within window from symptom onset
- Surgical decompression may be needed for large hemorrhages
- Treat underlying conditions like blood clots, atherosclerosis, hypertension
- Rehabilitation to maximize recovery of neurological function
Preventing secondary brain injury and stabilizing the patient are the priorities in the acute setting. Long term, managing risk factors like hypertension and stroke prophylaxis help reduce recurrence. Physical, occupational and speech therapy are key to regaining abilities like movement, speech and swallowing.
Prognosis
Prognosis depends heavily on the size and nature of the lesion:
- Small ischemic strokes generally have better outcomes
- Hemorrhages carry higher mortality, up to 50% with large bleeds
- Basilar artery occlusions have poor prognosis due to extensive ischemia
- Risk of recurrence without proper secondary prevention
- Many survivors left with residual neurological deficits
Complete recovery is unusual with the bird sign given the strategic location and functions of the pons. Patients may be left with permanent problems like slurred speech, imbalance, double vision and weakness. Overall, the bird sign indicates significant neurological injury and risk of long term disability or death. However, outcomes can vary substantially based on etiology and rapidity of treatment.
Conclusion
The bird sign, also known as the penguin sign, refers to a characteristic finding on brain MRI or CT suggestive of an acute ischemic or hemorrhagic stroke affecting the pons region of the brainstem. Identification of this triangular-shaped lesion centered on the pons should prompt urgent neurological evaluation and treatment to try to minimize permanent damage and disability. While the bird sign often portends a poor prognosis, early intervention can improve outcomes in some cases of stroke or other reversible etiologies. Careful follow up imaging is important to look for evolution of the lesion over time. The bird sign remains an important diagnostic clue to elucidate causes of acute onset vertigo, imbalance or other brainstem signs. Increased awareness of this distinctive imaging pattern can help direct appropriate therapy when this neurological emergency arises.
