Foster Kennedy syndrome (FKS) is a clinical syndrome characterized by compressive optic atrophy in one eye and papilledema due to increased intracranial pressure in the contralateral eye. It was reported by Robert Foster Kennedy in 1911.
FKS is classified into three types.
Type 1: Optic atrophy on one side and papilledema on the contralateral side. This is the most typical form and is secondary to an intracranial tumor (often a meningioma).
Type 2: Bilateral papilledema and unilateral optic atrophy.
Type 3: Progression from bilateral papilledema to bilateral optic atrophy.
Pseudo-Foster Kennedy syndrome (PFKS) presents with fundus findings similar to type 1 FKS but is caused by conditions other than intracranial tumors. The most common etiology of PFKS is sequential anterior ischemic optic neuropathy (AION) in both eyes, especially non-arteritic anterior ischemic optic neuropathy. Clinically, PFKS is encountered more frequently than true FKS.
QWhich is more common: true Foster Kennedy syndrome or pseudo-Foster Kennedy syndrome?
A
Clinically, PFKS is encountered more frequently. True FKS is relatively rare because it is caused by intracranial tumors. When unilateral optic atrophy and contralateral papilledema are observed, PFKS (especially non-arteritic anterior ischemic optic neuropathy) is often considered first. However, imaging studies are essential to rule out a tumor.
Sudden vision loss: Acute visual impairment in one eye in the setting of pre-existing optic neuropathy in the fellow eye. In non-arteritic anterior ischemic optic neuropathy, it is often noticed upon waking.
In the early stages of papilledema, there may be only transient visual obscurations lasting a few seconds, with little subjective awareness of vision loss.
Clinical Findings (Findings Confirmed by Physician Examination)
Ipsilateral optic atrophy: The optic disc on the tumor side appears pale. It shows compressive simple atrophy (well-defined margins, flat, narrowed blood vessels).
Contralateral papilledema: Papilledema due to increased intracranial pressure is observed. The atrophied optic nerve can no longer develop edema.
Positive RAPD: RAPD is present on the side of optic atrophy.
Visual field defects: The atrophic side shows various optic neuropathy patterns, while the edematous side shows enlarged blind spots and nerve fiber layer defect patterns.
Pseudo-Foster Kennedy Syndrome (PFKS)
Optic atrophy in one eye: Secondary atrophy due to a previous ischemic event such as nonarteritic anterior ischemic optic neuropathy. It may appear inflammatory (grayish, with slightly blurred margins).
Acute papilledema in the contralateral eye: In new nonarteritic anterior ischemic optic neuropathy, segmental or diffuse swelling is observed. Peripapillary hemorrhages are often present.
Visual impairment: Both eyes show varying degrees of decreased visual acuity and visual field defects.
A small optic disc in the contralateral eye, known as a “disc at risk,” is a characteristic finding of nonarteritic anterior ischemic optic neuropathy.
Non-arteritic anterior ischemic optic neuropathy: Occurs bilaterally with a time lag. This is the most common cause. Onset in the fellow eye occurs in 15–30% within 5 years, with a median of 7–12 months 1).
Idiopathic intracranial hypertension (IIH): Mimics PFKS when more severe in one eye.
Neurosyphilis
Risk factors for non-arteritic anterior ischemic optic neuropathy are as follows.
Hypertension
Diabetes
Hypercholesterolemia
Smoking history
Sleep apnea syndrome
In non-arteritic anterior ischemic optic neuropathy, circulatory insufficiency of the short posterior ciliary arteries is suspected as the cause, and a drop in blood pressure during daily fluctuations may be a trigger.
QWhat is the probability of non-arteritic anterior ischemic optic neuropathy occurring in both eyes?
A
Non-arteritic anterior ischemic optic neuropathy occurs in the fellow eye within 5 years in 15–30% of cases 1). The median interval between episodes is 7–12 months. This sequential bilateral involvement is the most common cause of PFKS.
The following examinations are important for differentiating FKS and PFKS.
Neuroimaging (CT/MRI): Contrast-enhanced CT or MRI of the head and orbit is essential when FKS is suspected. It confirms the presence and extent of the tumor. Even in atypical PFKS cases, it is performed to rule out intracranial tumors.
Fundus examination: Check the pattern of optic disc atrophy and edema.
OCT (Optical Coherence Tomography): Measurement of peripapillary retinal nerve fiber layer (cpRNFL) thickness allows quantitative assessment of optic atrophy. Since cpRNFL increases during acute papilledema, macular ganglion cell complex (GCC) analysis is useful for early detection of axonal damage.
Visual field testing: Evaluate visual field patterns on the atrophic and edematous sides. In non-arteritic anterior ischemic optic neuropathy, horizontal hemianopia (especially inferior) is common.
Fluorescein angiography: In non-arteritic anterior ischemic optic neuropathy, delayed filling of the optic disc is observed. In arteritic cases, delayed filling or defects in the peripapillary choroid are characteristic.
Blood tests: In elderly patients, measure ESR and CRP to rule out giant cell arteritis (GCA).
Cerebrospinal fluid pressure measurement: Necessary for definitive diagnosis of papilledema. Exclude space-occupying lesions by CT/MRI before performing.
The distinguishing points between FKS and PFKS are shown below.
Distinguishing point
True FKS
PFKS
Course of visual impairment
Progressive (slow)
Sudden onset (acute)
Intracranial pressure
Elevated
Usually normal
Loss of smell
Often present
Usually absent
QWhat tests can distinguish FKS from PFKS?
A
The most important test is CT/MRI to check for intracranial tumors. FKS shows intracranial tumors, while PFKS does not. Additionally, medical history (progressive vs. sudden onset), presence of anosmia, and cerebrospinal fluid pressure are useful for differentiation.
Treatment of FKS focuses on managing the causative tumor.
Surgical resection: If surgery is indicated, resection is the best option to reduce tumor compression and relieve intracranial pressure.
Preoperative embolization: For vascular-rich tumors, preoperative embolization can reduce intraoperative bleeding and suppress postoperative complications.
Corticosteroids: For symptomatic brain tumors, they are administered to reduce peritumoral edema and intracranial pressure.
Stereotactic radiotherapy: When non-surgical intervention is preferred or in elderly patients (70 years or older), stereotactic radiotherapy is an option considering surgical risks. For optic nerve sheath meningiomas, good outcomes have been reported with three-dimensional fractionated radiotherapy and intensity-modulated radiotherapy.
Treatment of FKS
Surgical resection: First choice for reducing compression effects and relieving intracranial pressure.
Stereotactic radiotherapy: Alternative for elderly patients or those at high surgical risk.
Steroids: Used to reduce peritumoral edema during the perioperative period.
Treatment of PFKS
AAION: High-dose systemic steroid therapy should be initiated immediately.
Non-arteritic anterior ischemic optic neuropathy: No established effective treatment exists. Management of vascular risk factors is central.
Idiopathic intracranial hypertension: Weight management, acetazolamide administration. In severe cases, shunt surgery or optic nerve sheath fenestration.
Treatment of PFKS depends on the underlying disease.
AAION: If caused by giant cell arteritis, high-dose prednisone therapy should be started immediately. The highest priority is to prevent spread to the fellow eye.
Non-arteritic anterior ischemic optic neuropathy: No established effective treatment exists. Anticoagulation therapy, vasodilators, oral steroids, and optic nerve sheath decompression have been attempted, but no significant improvement in visual prognosis has been proven.
Optic neuritis: Intravenous methylprednisolone followed by oral prednisone is considered.
For compressive optic neuropathy, steroid pulse or half-pulse therapy is first-line. After 1–3 courses, switch to oral steroids. Avoid rapid tapering as it may cause recurrence of optic neuropathy.
QIs there an effective treatment for non-arteritic anterior ischemic optic neuropathy?
A
Currently, there is no proven effective treatment for non-arteritic anterior ischemic optic neuropathy. Management focuses on controlling vascular risk factors (hypertension, diabetes, dyslipidemia). For details, see the section on “Standard Treatment.”
6. Pathophysiology and Detailed Mechanism of Onset
In non-arteritic anterior ischemic optic neuropathy, the most common cause of pseudo-Foster Kennedy syndrome, the condition develops through the following mechanisms.
Microinfarction occurs in the optic nerve head due to circulatory insufficiency of the short posterior ciliary arteries.
When the optic disc is small (disc at risk), axons are compressed at the lamina cribrosa, exacerbating circulatory disturbance.
Non-arteritic anterior ischemic optic neuropathy develops in one eye, progressing from acute optic disc edema to optic atrophy within 1–2 months.
When new non-arteritic anterior ischemic optic neuropathy develops in the contralateral eye, asymmetric findings of atrophy and swelling occur.
There are two pathways leading to optic atrophy. One pathway progresses through papilledema (such as optic neuritis, anterior ischemic optic neuropathy, and congestive optic disc) to atrophy, and the other pathway progresses directly from a normal optic disc to atrophy, as seen in retrobulbar optic neuritis and compressive optic neuropathy.
The mechanism of congestive optic disc is that increased intracranial pressure elevates the subarachnoid space pressure around the optic nerve, compressing the optic nerve and causing stagnation of axonal flow, leading to papilledema.
