The hummingbird sign refers to a specific finding that can be seen on magnetic resonance imaging (MRI) scans of the brain. It describes the appearance of the midbrain on MRI due to tissue loss in certain neurodegenerative conditions. The midbrain is the portion of the brain that connects the brainstem to the forebrain and is involved in functions like eye movement, hearing, motor control, alertness, and sleep. On MRI, the midbrain normally has a rounded, oval shape but in certain conditions, it can take on an elongated, pointed appearance that has been likened to the silhouette of a hummingbird in flight. This is referred to as the hummingbird sign or the hummingbird’s beak sign.
What causes the hummingbird sign?
The hummingbird sign is most commonly associated with progressive supranuclear palsy (PSP), which is a rare brain disorder that causes difficulty with walking, balance, eye movements, speech, swallowing and thinking. It is part of a category of neurodegenerative diseases known as tauopathies, which are characterized by abnormal accumulation of tau protein in the brain.
In PSP, tau protein builds up in nerve cells of the midbrain, causing them to deteriorate and die. This atrophy or tissue loss in the midbrain is what leads to the characteristic MRI finding of midbrain atrophy, creating the elongated, pointed appearance that resembles a hummingbird.
While PSP is the most common cause, the hummingbird sign can also be seen in some other neurodegenerative conditions like:
- Corticobasal degeneration
- Multiple system atrophy
- Lewy body dementia
- frontotemporal lobar degeneration
These conditions can all involve midbrain atrophy due to cell loss in that region. The specific pattern of tissue loss results in the MRI finding of midbrain deformation producing the hummingbird silhouette.
How is the hummingbird sign identified on MRI?
The hummingbird sign is best visualized using T1-weighted MRI sequences. On these scans, the midbrain takes on a characteristic elongated and tapered appearance, resembling the body and beak of a hummingbird in flight.
Some key features that radiologists look for are:
- Pointed elongation of the midbrain
- Narrowing of the midbrain, particularly at the junction of the midbrain and pons which forms the “beak” of the hummingbird
- Atrophy or loss of volume in the midbrain compared to normal
- A linear, wedge-shaped hyperintensity within the midbrain in some cases
These features reflect the atrophy and tissue loss in the midbrain structures like the substantia nigra and tegmentum that give rise to the hummingbird silhouette.
How specific is the hummingbird sign for PSP?
The hummingbird sign was initially described in relation to PSP, and its presence on MRI is still considered one of the characteristic imaging markers of the disease. Research indicates that it has high specificity for PSP, meaning that when the hummingbird sign is seen on MRI, it is highly likely that the patient has underlying PSP pathology.
In one study looking at PSP patients, the hummingbird sign was seen in 90-100% of cases. Other studies have reported a specificity ranging from 82-96% for PSP.
However, it is important to note that while it is highly suggestive of PSP, the hummingbird sign is not entirely specific for this condition. As mentioned above, midbrain atrophy with this appearance can also occur in other tauopathies and neurodegenerative disorders. In corticobasal degeneration particularly, the hummingbird sign may be seen in up to 75% of patients.
So while an important diagnostic clue, the hummingbird sign must be interpreted in the full clinical context, with correlation to the patient’s symptoms and other MRI findings, rather than as an isolated imaging sign.
What is the significance of identifying the hummingbird sign?
Identifying the presence of the hummingbird sign on MRI has important implications:
- It provides supportive evidence for the diagnosis of PSP or other tauopathies where midbrain involvement occurs.
- It helps distinguish PSP from conditions like Parkinson’s disease which would not have this midbrain deformation on imaging.
- It indicates more advanced or severe disease, since the hummingbird sign reflects progression of neurodegeneration to involve the midbrain.
- It provides prognostic information, as the hummingbird sign is associated with rapid disease progression in PSP.
- It guides treatment, as certain medications may be initiated to help manage symptoms related to midbrain pathology like abnormalities of eye movement in PSP.
- In research, it allows early identification of patient cohorts with evidence of tau pathology for clinical trials evaluating new therapeutic agents for PSP and similar disorders.
So in the proper clinical scenario, recognizing the hummingbird appearance of midbrain atrophy can be a meaningful diagnostic and prognostic MRI finding that impacts patient management.
Differential Diagnosis
While a highly specific marker for PSP in most cases, there are some other conditions which can produce MRI changes mimicking the hummingbird sign. These include:
- Parkinson’s disease – While Parkinson’s disease typically spares the midbrain, in advanced stages there can be midbrain volume loss adjacent to the substantia nigra. However, true hummingbird deformity is uncommon.
- Corticobasal degeneration – As mentioned, CBD commonly causes the hummingbird sign, seen in up to 75% of patients. Clinically CBD tends to have asymmetrical symptoms which help distinguish it from PSP.
- Multiple system atrophy – MSA can have midbrain atrophy but usually spares the tegmentum and tends to involve the pons, producing a less tapered, less pointed silhouette than in PSP.
- Lewy body dementia – Rarely LBD can have midbrain changes resembling the hummingbird sign, but will also have other characteristic MRI findings like reduced dopamine transporter uptake.
- Stroke – Midbrain infarcts, though rare, can deform the midbrain creating a silhouette potentially mistaken for the hummingbird sign.
- Toxic-metabolic disorders – Rare causes like methanol toxicity have been reported to produce MRI changes similar to the hummingbird sign.
Correlating the imaging with the clinical scenario, laboratory findings and other MRI features can help distinguish these mimics from true PSP in most cases. The finding of a classic hummingbird appearance of the midbrain should prompt consideration of PSP as the leading diagnosis.
Illustration of the Hummingbird Sign
Below is an illustrated comparison of a normal midbrain appearance versus the hummingbird sign:
Normal Midbrain | Hummingbird Sign in PSP |
Key differences on the PSP MRI:
– Elongation and tapering of midbrain
– Narrow “beak” appearance
– Loss of normal oval contour
These reflect the midbrain atrophy that produces the characteristic hummingbird silhouette.
Conclusion
The hummingbird sign refers to a specific MRI finding of midbrain atrophy that produces a tapered, elongated appearance resembling a hummingbird in flight. It is most closely associated with progressive supranuclear palsy, where midbrain tau pathology results in tissue loss and deformity. While highly characteristic of PSP, the sign can occur in other neurodegenerative disorders involving the midbrain. Recognition of the hummingbird sign has important diagnostic and prognostic implications, providing evidence of an underlying tauopathy like PSP and suggesting more rapidly progressive disease. When correlated properly with clinical findings, this striking midbrain deformity on MRI serves as a useful supportive marker for the diagnosis of PSP and similar disorders.