Why Brainstem Lesions Are Not as Complex as You Were Taught
How to Localise a Brainstem Lesion Using Clinical Signs: A Dorsal Midbrain Syndrome Case Study
By Dr Will Bierrum, Neurology Registrar and Founder of MedXStart
Brainstem localisation is an area many non-neurologists and resident doctors find challenging. When a patient presents with complex eye movement abnormalities, pupillary changes, and cranial nerve signs, the examination findings themselves can tell you precisely where the lesion is before any imaging is requested. The key is applying a systematic pattern recognition framework that moves from symptoms to syndrome to aetiology. This case demonstrates exactly how that works.
The Case
A 34 year old man is referred urgently to neurology with a three week history of double vision and headaches. His partner has noticed his eyes look unusual and describes a constant staring appearance. He has been feeling generally unwell and reports blurred vision when reading. There is no significant past medical history.
On examination you find bilateral upper eyelid retraction giving a wide staring appearance, an inability to look upward voluntarily with completely preserved downward gaze, and eyes that roll upward reflexively when the head is tilted forward. The pupils are mid-dilated and fail to constrict to direct light but constrict normally when the patient focuses on a near object. On attempted upgaze, the eyes jerk inward and retract simultaneously.
Where is the lesion?
The Framework: Symptoms to Syndrome to Aetiology
In neurology, the clinical approach follows the same sequence every time. Identify the symptoms. Build the syndrome from the pattern of findings. Then consider the aetiology.
Before reaching for imaging, ask: what does this pattern of findings tell me about the anatomy? In brainstem disease, the examination findings are exquisitely localising if you know what to look for. The key principle is that different levels of the brainstem produce different patterns of cranial nerve involvement and recognising those patterns is the foundation of brainstem localisation.
How to Localise Brainstem Lesions Clinically
The brainstem can be divided into three levels: midbrain, pons, and medulla. Each level has anatomical neighbours that produce characteristic additional signs when involved.
Midbrain lesions affect the structures of the dorsal midbrain — the vertical gaze centres, the pupillary light reflex pathways, and the lid retractors. The key signs are upgaze palsy, light-near dissociation, Collier's sign, and convergence-retraction nystagmus. The facial nerve nucleus is remote from the midbrain, so facial weakness is not a typical feature.
Pontine lesions sit adjacent to the facial nerve nucleus. This anatomical proximity means that a pontine lesion often has ipsilateral facial weakness alongside its other features. The pons is also the headquarters for horizontal conjugate gaze, so horizontal gaze palsy rather than vertical gaze palsy is the expected finding. Pontine lesions classically produce pinpoint pupils from disruption of descending sympathetic fibres.
Medullary lesions involve the lower brainstem and its neighbouring cranial nerve nuclei. The medulla sits adjacent to the nuclei of the lower bulbar cranial nerves — IX, X, and XII — so medullary lesions typically produce additional features of vertigo, nausea and vomiting, dysarthria, dysphagia, and hoarseness. The lateral medullary syndrome, also known as Wallenberg syndrome, is the classic example. Damage to the lateral medulla leads to ipsilateral facial numbness, contralateral limb sensory loss, ipsilateral Horner's syndrome, and bulbar involvement.
This three level framework means that when you see a brainstem syndrome, you ask two questions. Is there vertical or horizontal gaze involvement? And which cranial nerve nuclei are affected alongside the eye signs? The answers help guide you to the level.
Why the Answer in This Case is the Dorsal Midbrain
The correct answer is the dorsal midbrain. This is Parinaud syndrome, also known as dorsal midbrain syndrome.
The fibres controlling voluntary upgaze run superficially across the posterior commissure at the back of the midbrain. They are the first structures to be compressed when a lesion pushes down from above in this case a pineal region tumour. The light reflex fibres run through the adjacent pretectal area and are similarly compressed, abolishing the pupillary light response. The accommodation fibres take a slightly deeper course and are spared, producing light-near dissociation. Collier's sign — bilateral lid retraction — results from disruption of the fibres controlling lid depression. Convergence-retraction nystagmus on attempted upgaze is pathognomonic of this syndrome.
The doll's head manoeuvre is the key examination finding. When the head is tilted forward and the eyes roll up reflexively, this proves that the ocular motor nerves and eye muscles are intact. The problem is supranuclear. The voluntary command pathway has been disrupted, not the final common pathway. There is no facial weakness because the facial nucleus is not in the midbrain. There is no vertigo or dysphagia because the medulla is not involved.
Why the Other Options Are Wrong
Bilateral frontal lobes control horizontal saccadic eye movements to the contralateral side via the frontal eye fields. They have no role in vertical gaze and no involvement in pupillary reflexes. Bilateral frontal damage produces personality change, disinhibition, and primitive reflexes. None of which are present here.
The pons is the headquarters for horizontal conjugate gaze. A pontine lesion produces horizontal gaze palsy, pinpoint pupils, and ipsilateral facial weakness from involvement of the adjacent facial nucleus. This is the anatomical opposite of what we see in this case. The presence of facial weakness in any brainstem syndrome should always raise pontine involvement as the first consideration.
The cerebellum fine tunes the execution of eye movements but cannot generate or paralyse them. Cerebellar pathology produces incoordination, gaze-evoked nystagmus, broken pursuit, ataxia, intention tremor, and scanning dysarthria. None of those features are present here.
The Underlying Cause
Applying the symptoms to syndrome to aetiology framework, the next step is to consider what lesion could compress the dorsal midbrain in a 34 year old man. The pineal gland sits directly adjacent to this region, making pineal region tumours — pinealoma, germinoma, or metastasis — a possibility. Obstructive hydrocephalus from aqueduct compression is another critical consideration. In older patients with vascular risk factors, midbrain ischaemic stroke becomes more likely. The syndrome is the same regardless of cause. It is the aetiology that determines urgency and management direction. MRI is a go-to investigation as it has a much higher sensitivity for brainstem structures compared with a CT scan.
Frequently Asked Questions
What is dorsal midbrain syndrome? Dorsal midbrain syndrome, also known as Parinaud syndrome, is a cluster of eye movement and pupillary abnormalities caused by damage or compression to the dorsal midbrain. The classic features are upgaze palsy, Collier's sign, light-near dissociation, and convergence-retraction nystagmus.
How do you differentiate a midbrain lesion from a pontine lesion clinically? Midbrain lesions cause vertical gaze palsy without facial weakness. Pontine lesions cause horizontal gaze palsy with ipsilateral facial weakness because the facial nucleus is adjacent to the pontine gaze centres.
How do you differentiate a pontine lesion from a medullary lesion? Pontine lesions produce facial weakness and horizontal gaze palsy. Medullary lesions produce lower bulbar cranial nerve signs — vertigo, nausea, vomiting, dysarthria, dysphagia, and hoarseness — because the nuclei of cranial nerves IX, X, and XII sit in the medulla.
What causes dorsal midbrain syndrome in a young patient? In young patients without vascular risk factors, the most important cause to exclude is a structural cause such as pineal region tumour compressing the dorsal midbrain from above. Obstructive hydrocephalus should also be considered. The timing of onset and development of symptoms will help differentiate the possible aetiologies. For example, regardless of the patient’s age, if the syndrome happens within seconds you would think vascular e.g. stroke rather than a progressive neoplastic cause. MRI is the first line investigation.
What is light-near dissociation? Light-near dissociation describes pupils that fail to constrict to direct light but constrict normally to accommodation. It is a cardinal feature of dorsal midbrain syndrome because the light reflex fibres are compressed while the accommodation fibres are spared.
What is the doll's head manoeuvre and why is it important in dorsal midbrain syndrome? The doll's head or oculocephalic manoeuvre involves tilting the patient's head forward to observe whether the eyes roll upward reflexively. A positive response in a patient with upgaze palsy proves that the eye muscles and cranial nerves are intact and that the lesion is supranuclear — confirming the dorsal midbrain as the site of pathology.
The Clinical Takeaway
This case illustrates the central principle of neurological pattern recognition. A systematic examination, interpreted through the symptoms to syndrome to aetiology framework and combined with an understanding of brainstem anatomy, allows precise localisation. The brainstem level can be identified clinically by asking which gaze direction is affected and which cranial nerve nuclei are involved alongside the eye signs. Midbrain means vertical gaze palsy without facial involvement. Pons means horizontal gaze palsy with facial involvement. Medulla means bulbar involvement with vertigo and dysphagia.
Recognising the patterns help you make sense of the anatomy and localisation.
That is the approach taught in the Neurology Pattern Recognition Guide, available now at medxstart.co.uk. Whether you are a junior doctor building clinical confidence, a medical registrar preparing for MRCP PACES, or a non-neurologist wanting a reliable framework for the neurological patients on your ward, the guide gives you the systematic approach that turns complex neurology into logical, learnable patterns.
Dr Will Bierrum is a neurology registrar and founder of MedXStart, the success platform for modern doctors.
Follow @willbierrum on Instagram for weekly clinical neurology teaching.
Educational purposes only. Not medical or clinical advice.