Foster Kennedy Disease (Spinal and Bulbar Muscular Atrophy)

Overview

Foster Kennedy disease (FKD), also known as spinal and bulbar muscular atrophy (SBMA), is a rare, X‑linked recessive neuro‑degenerative disorder that primarily affects the lower motor neurons in the spinal cord and brainstem (bulbar region). The disease leads to progressive muscle weakness, atrophy, and certain endocrine abnormalities.

Who it affects

• Almost exclusively males, because the responsible gene is on the X chromosome.
• Typical onset is between the ages of 30 and 50, though some cases have been reported as early as the teens or as late as the 70s.

Prevalence

• Estimated prevalence in the United States is about 1–2 per 100,000 men.¹
• Higher carrier frequencies have been found in East Asian populations (roughly 1 in 500 men) compared with European ancestry (≈1 in 1,000).²

Symptoms

The clinical picture of FKD evolves slowly over years. Symptoms may be subtle at first and often overlap with other motor neuron diseases, making early recognition challenging.

Motor symptoms

• Proximal and distal muscle weakness: Weakness usually starts in the lower limbs (especially thigh and calf muscles) and later involves the forearms and hand muscles.
• Muscle atrophy (wasting): Visible thinning of the biceps, forearms, thighs, and calf muscles.
• Fasciculations: Small, involuntary muscle twitches, most noticeable in the tongue and limbs.
• Bulbar weakness: Difficulty chewing, swallowing (dysphagia), and speaking (dysarthria). Voice may become nasal or hoarse.
• Reduced reflexes (hyporeflexia) or absent deep tendon reflexes.
• Foot deformities: Pes cavus (high arch) or hammer toes due to chronic weakness.

Non‑motor symptoms

• Androgen insensitivity: Reduced libido, erectile dysfunction, gynecomastia, and infertility—linked to the mutation’s effect on the androgen receptor.
• Metabolic changes: Mild insulin resistance, dyslipidemia, and occasional type 2 diabetes.
• Fatigue and generalized weakness.
• Emotional impact: Anxiety or depression related to progressive loss of function.

Causes and Risk Factors

Genetic cause

FKD is caused by an expansion of a CAG trinucleotide repeat in the AR (androgen receptor) gene on the X chromosome (Xq11‑12). Normal alleles contain 9–35 repeats; affected individuals have ≥38 repeats, with longer repeats generally correlating with earlier onset and more severe disease.³

How the mutation leads to disease

1. The expanded polyglutamine tract creates a misfolded androgen receptor protein.
2. Misfolded proteins accumulate in motor neurons, causing cellular stress, impaired transcription, and eventual neuronal death.
3. Because the receptor also mediates androgen signaling, the mutation interferes with normal hormone function, giving rise to endocrine abnormalities.

Risk factors

• Family history: A mother who is a carrier or an affected male relative dramatically increases risk.
• Carrier status in women: Female carriers are usually asymptomatic but can pass the mutated gene to 50% of their children (sons will be affected, daughters become carriers).
• Ethnicity: Slightly higher carrier rates reported in East Asian populations.

Diagnosis

Diagnosing FKD involves a combination of clinical evaluation, genetic testing, and exclusion of other neuromuscular disorders.

Clinical evaluation

• Detailed history focusing on gradual onset of limb weakness, bulbar symptoms, and any endocrine changes.
• Neurological exam to assess muscle strength, tone, reflexes, and fasciculations.

Electrodiagnostic studies

• Electromyography (EMG): Shows chronic denervation with motor unit loss, especially in bulbar and limb muscles.
• Nerve conduction studies (NCS): Typically normal sensory studies, helping differentiate from peripheral neuropathies.

Imaging

• MRI of the brain and spinal cord is usually normal but helps rule out structural lesions.

Genetic testing

The definitive test is a PCR‑based assay that measures the number of CAG repeats in the AR gene. A repeat length ≥38 confirms the diagnosis. Testing is recommended for the patient, at‑risk male relatives, and potentially carrier testing for female relatives.

Laboratory tests

• Serum testosterone, LH, and FSH levels (often normal or slightly altered).
• Creatine kinase (CK) may be mildly elevated due to muscle breakdown.
• Metabolic panel to screen for insulin resistance or dyslipidemia.

Treatment Options

There is currently no cure for FKD, but several strategies can slow progression, relieve symptoms, and improve quality of life.

Pharmacologic therapies

• Androgen deprivation therapy (e.g., leuprorelin, dutasteride): Small trials suggest that lowering circulating testosterone may reduce mutant receptor toxicity. Evidence is mixed, and therapy is not universally recommended.⁴
• Anticholinergic agents (e.g., amitriptyline) for dysphagia: Can reduce spasticity of pharyngeal muscles.
• Physical symptom drugs: Baclofen or tizanidine for muscle cramps; gabapentin for neuropathic pain.
• Management of endocrine issues: Testosterone replacement is generally avoided; estrogen antagonists may help gynecomastia.

Procedural interventions

• Speech and swallow therapy: Regular sessions with a speech‑language pathologist to maintain safe swallowing and communication.
• Percutaneous endoscopic gastrostomy (PEG): Considered when dysphagia leads to weight loss or aspiration risk.
• Assistive devices: Custom orthotics, walkers, or power wheelchairs as mobility declines.

Lifestyle and supportive measures

• Exercise: Low‑impact aerobic activity (e.g., swimming, stationary cycling) 3–4 times per week helps preserve muscle strength and cardiovascular health.
• Strength training: Light resistance with physiotherapist supervision; avoid high‑intensity overload that may cause muscle injury.
• Nutrition:: High‑protein diet with adequate calories; monitor weight to prevent malnutrition.
• Stress management: Mind‑body techniques (yoga, meditation) can mitigate anxiety and fatigue.