Weber Syndrome

Etiology

A lesion in the ventromedial portion of midbrain leads to Weber syndrome. The midbrain receives its blood supply from the paramedian mesencephalic branches (basilar), peduncular perforating branches (posterior cerebral artery) as well as from the superior cerebellar artery, and the choroidal arteries.[3]

Isolated infarctions of the paramedian mesencephalic and the peduncular perforating branches are the most common cause of Weber syndrome.[4] Less common etiologies include hemorrhage, aneurysms, tumors, and demyelinating diseases. [4]

Antecedent history of hypertension, diabetes, hypercholesterolemia is often associated with Weber syndrome, like other strokes.

In one case series, isolated midbrain infarction was seen in only 7.6% of the cases and most were associated with the concurrent infraction in other vascular territories of the vertebrobasilar system.[3]The causes for the midbrain infarction were categorized in the decreasing order of frequency into[3]

1. Cardioembolism
2. Insitu thrombosis
3. Large artery to artery embolism and
4. Intrinsic perforator branch disease (lacunar infarct)

Epidemiology

The incidence of isolated midbrain infarction causing Weber syndrome is not known mostly it occurs in conjunction with infarction in other areas of the vertebrobasilar system. One study found an isolated midbrain infarction occurring in only 0.7% of patients with posterior circulation strokes.[3]

History and Physical

Patients with weber syndrome present with drooping of the eyelid, diplopia associated with contralateral partial or complete weakness of upper limb and lower limb. The presence of crossed deficits, either sensory or motor, along with occasional association with ataxia (involvement of the red nucleus) and parkinsonian rigidity (involvement of the substansia nigra) are the characteristic neurological findings in this syndrome.[4]

On neurological examination, higher mental functions are usually normal and not commonly affected in an isolated Weber syndrome. Ptosis may be present, and on opening the affected eye, the eye is mostly shifted downwards and outwards, due to the unopposed action of superior oblique and lateral rectus muscles.

Dilated unresponsive pupils may occur if both upper and middle midbrain are affected, pupils may be spared if only lower midbrain is affected by the inciting insult. Contralateral ataxia and sometimes parkinsonian type of rigidity may be seen associated with the weakness.

Khalil et al. reported the association of Weber syndrome with vertical gaze palsy owing to the involvement of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the midbrain, dorsal to the red nucleus.[5]

The fascicular arrangement of the midbrain oculomotor nerve is speculated to be a pupillary component, extraocular movement, and eyelid elevation in that rostrocaudal order.[6]

Evaluation

As with each case of suspected stroke, the patient should first have a thorough neurologic examination and a measure of the National Institute of Health Stroke Scale (NIHSS) score. [7] The emergent diagnosis is made by an urgent Computerised Tomography (CT) of the brain, since it is fast and more readily available in most centers. However, since there can be the propagation of the stroke to other vascular territories within the vertebrobasilar system, other additional radiological modalities such as Magnetic Resonance Imaging (MRI) along with stroke sequences, angiographic and the perfusion studies may also need to be done.

Treatment / Management

The immediate management is similar to other patients with stroke with an emphasis on the airway, breathing, and circulation along with quick and complete neurological assessment.

The patients with an isolated Weber syndrome are mostly neurologically stable. However, scenarios wherein the large infarction and there is a risk of the propagation of the disease process, thrombolytic therapy may be justified.[3] Neurosurgical management is only warranted in

1. Cases with massive posterior circulation infarction, wherein there is need of emergent posterior fossa decompression 
2. External ventricular drain placement in cases of evolving hydrocephalus
3. As an adjunct to surgical management for tumors and aneurysm causing Weber syndrome.

Apart from these, supportive care is most important, including early mobilization, bladder bowel care, and nutritional support.

Differential Diagnosis

Other midbrain syndromes are Benedikt, Claude's and Nothnagel syndrome.

Benedikt syndrome, also called Paramedian midbrain syndrome, is also caused by an insult in the tegmentum of the midbrain and cerebellum. It is characterized by the presence of oculomotor nerve palsy and cerebellar ataxia including tremor and involuntary choreoathetotic movements.

Claude's syndrome is similar to Weber syndrome in that it presents with ipsilateral oculomotor nerve palsy with contralateral hemiparesis but also includes contralateral ataxia. It occurs due to the involvement of oculomotor nerve, red nucleus and brachium conjunctivum.

Another famously described Weber syndrome is the Sturge-Weber syndrome (SWS), also called encephalotrigeminal angiomatosis, which is a neurocutaneous syndrome with angiomas involving the leptomeninges and skin of the face, typically ophthalmic (V1) and maxillary (V2) distribution areas of the trigeminal nerve. The hallmark of SWS is a facial cutaneous venous dilation, also referred to as a nevus flammeus or port-wine stain (PWS).

Yet another Weber syndrome is the Klippel-Trenaunay-Weber syndrome (KTWS), characterized by a triad of port-wine stain, varicose veins, and bony and soft tissue hypertrophy involving an extremity.

Prognosis

Prognosis depends on the etiology of Weber syndrome. Early detection, timely management, effective supportive therapy to prevent complications and precautionary measures to prevent recurrence of similar stroke are the most important factor determining the neurological outcome in these patients.

Complications

Weber syndrome is ten times more likely to occur in conjunction with pontine infarction. It has been to be involved as a middle territory syndrome of the posterior circulation in as many as 50% of patients.[3] There is the risk of massive posterior circulation infarction with tonsillar herniation, and eventual death. There may be evolving hydrocephalus as well.

After recovery from the acute event, early mobilization is advised strictly to avoid secondary complications like contractures, bedsores, aspiration pneumonia, deep vein thrombosis, pulmonary embolism and secondary infections such as pneumonia and urinary tract infections. Early dysphagia screening and if any swallowing difficulty is present, Nasogastric feeding needs to be initiated. Early physiotherapy to recover motor power, prevent spasticity, and thereby improving the quality of life is of paramount importance.[8]