The hummingbird sign is a radiological finding seen on MRI scans that is indicative of midbrain atrophy. Midbrain atrophy refers to shrinkage or tissue loss in the midbrain region of the brain stem. The midbrain is located between the pons and thalami and contains important structures like the substantia nigra and cerebral peduncles. Atrophy or damage to this region can result in various movement disorders and cognitive deficits. On MRI scans, midbrain atrophy shows up as thinning of the midbrain and enlargement of the third ventricle, giving it a characteristic hummingbird-like appearance. This article will discuss the hummingbird sign in more detail – what it indicates, its causes, associated symptoms, diagnosis, and treatment options.
What Does the Hummingbird Sign Indicate?
The hummingbird sign refers specifically to the appearance of the midbrain on MRI scans when it has undergone atrophy or tissue loss. Some key features of the hummingbird sign are:
– Thinning of the midbrain region so it appears narrowed or pencil-like
– Enlargement of the third ventricle which appears like a dark fluid-filled cavity
– Reduction in size of the cerebral peduncles which appear thin and spindly
This gives the midbrain a slender, tapered look reminiscent of a hummingbird’s body and beak – the third ventricle representing the body and the peduncles the beak. The name “hummingbird sign” was coined in radiology literature to describe this distinct appearance.
The hummingbird sign indicates there is some pathological process causing shrinkage and damage to the midbrain. It is associated with neurodegenerative conditions that progressively damage the brain stem. While other brain structures may also be affected, the midbrain shows one of the earliest signs of atrophy. The hummingbird sign is therefore an important radiological marker of midbrain degeneration.
Causes of the Hummingbird Sign
Some key causes of midbrain atrophy that produce the hummingbird sign include:
Progressive Supranuclear Palsy
Progressive supranuclear palsy (PSP) is the most common cause of the hummingbird sign. PSP is a rare neurodegenerative disorder that involves accumulation of tau protein in the brain, leading to damage and cell death. Key features are problems with eye movements, balance, walking, speech, swallowing, mood and behavior changes. As PSP progresses, the midbrain is one of the first areas affected. Midbrain atrophy shows up in over 90% of PSP cases.
Parkinson’s Disease
While not as frequent as in PSP, midbrain atrophy can sometimes occur in Parkinson’s disease. Loss of dopamine-producing cells in the substantia nigra of the midbrain causes the motor symptoms of Parkinson’s. Around 25% of Parkinson’s patients eventually develop midbrain atrophy.
Corticobasal Degeneration
This rare neurodegenerative disorder characterized by problems with movement, cognition and language can also sometimes involve midbrain atrophy and the hummingbird sign on MRI scans.
Multiple System Atrophy
Multiple system atrophy is a rapidly progressive disorder that affects movement and autonomic function. When Parkinsonian features are predominant, midbrain atrophy may be seen.
Other Causes
Other less common causes include Huntington’s disease, Creutzfeldt-Jakob disease, multi-infarct dementia, stroke affecting the midbrain region, and hydrocephalus causing compression. In a small percentage of cases, no specific cause can be found for midbrain atrophy.
Symptoms Associated with the Hummingbird Sign
Hummingbird sign indicates that the midbrain has undergone pathological changes. This manifests with various neurological symptoms depending on the structures affected:
Movement Problems
Damage to the substantia nigra and cerebral peduncles causes problems with coordination, balance and movement control. Symptoms may include:
– Muscle rigidity or stiffness
– Shuffling gait
– Freezing of gait while walking
– Difficulty initiating movements
– Slow movements and decreased arm swing when walking
– Tremors
– Falls
– Impaired balance and coordination
Eye Movement Abnormalities
Since cranial nerve nuclei controlling eye movements are located in the midbrain, their dysfunction leads to ocular abnormalities like:
– Impaired vertical gaze, especially downgaze
– Blurred or double vision
– Light sensitivity
– Reduced blinking
– Involuntary eye movements called saccades
Cognitive Difficulties
Shrinkage of midbrain nuclei like the substantia nigra can cause cognitive problems like:
– Poor attention and concentration
– Reduced motivation
– Impaired memory
– Personality changes
– Speech difficulties like dysarthria
– Mood disorders like depression
Other Symptoms
Additional symptoms may include:
– Swallowing difficulties called dysphagia due to dysfunction of swallowing coordination centers in the midbrain
– Sleep disturbances like vivid dreaming, acting out while dreaming and disrupted sleep
– Abnormalities in blood pressure, heart rate and body temperature regulation due to involvement of midbrain autonomic centers
– Hearing loss or tinnitus from damage to midbrain auditory pathways
How is the Hummingbird Sign Diagnosed?
The following methods help in diagnosing midbrain atrophy seen as the hummingbird sign:
Brain MRI
MRI is the standard imaging test to reveal midbrain atrophy producing the hummingbird sign. MRI provides clear pictures of the brain’s anatomy and is very sensitive to any structural changes occurring in disease. It can detect subtle midbrain tissue loss early even before clinical symptoms appear.
Medical History
The doctor will elicit a detailed history about symptoms like movement problems, cognitive difficulties and visual changes to determine if they are indicative of a midbrain disorder. History of risk factors and illness progression provides clues.
Neurological Examination
Comprehensive neurological testing will assess functions like reflexes, eye movements, motor function, balance and walking, speech, swallowing, vision, hearing and mental status. This can help localize the affected area to the midbrain.
Other Tests
Additional tests may sometimes be done to supplement MRI and neurological examination. This includes tests to assess cognition, visual processing, movement analysis, and autonomic function. Blood tests may help rule out other causes.
Treatment for the Hummingbird Sign
There are no treatments currently available that can reverse or halt the midbrain atrophy behind the hummingbird sign. Therapy aims to manage symptoms and improve quality of life as much as possible.
Medications
Medications may provide symptomatic relief of some problems associated with midbrain atrophy:
– Levodopa/carbidopa for rigidity, bradykinesia
– Amantadine for movement impairment
– Trihexyphenidyl for tremor
– SSRIs for depression
– Cholinesterase inhibitors for memory and cognition issues
Physical, Occupational and Speech Therapy
These help manage disabilities related to movement, balance, coordination, daily activities, swallowing difficulties and speech. Exercises may improve mobility, strength and flexibility.
Visual Aids
Prisms, special lenses and other devices can help correct vision problems like impaired gaze and double vision.
Prevention of Falls and Injuries
Safety precautions including installing grab bars, railings, adequate lighting and removing tripping hazards can help prevent dangerous falls. Walking aids like canes or walkers provide support.
Modifying Activities
Adapting daily tasks to manage disabilities improves ability to perform routine activities independently.
With supportive care, individuals with the hummingbird sign can have enhanced functioning and improved quality of life. Research is ongoing to find drugs that may slow progression of midbrain atrophy.
Conclusion
The hummingbird sign refers to the characteristic MRI picture of midbrain atrophy, with pencil-thin peduncles and dilated ventricles giving a hummingbird silhouette. It indicates pathological damage to important midbrain structures. Progressive supranuclear palsy is the most common cause, but other neurodegenerative disorders like Parkinson’s disease, corticobasal degeneration and multiple system atrophy may also sometimes produce this imaging finding.
Symptoms of midbrain involvement include movement abnormalities like rigidity, tremor and postural instability, visual deficits like impaired downgaze and eye movement abnormalities, and cognitive changes. MRI is the most sensitive diagnostic test, while clinical history and neurological exam can help determine if midbrain dysfunction exists. There are currently no good treatments to reverse midbrain atrophy. Management aims to alleviate symptoms, retain functionality, prevent injury and maintain quality of life with a multidisciplinary approach using medications, therapy and lifestyle adaptation. Research to develop neuroprotective drugs is essential to change the course of disabling midbrain disorders associated with the hummingbird sign.
