Understanding Your ALSP Diagnosis

The diagnosis of ALSP (Adult-onset leukoencephalopathy with axonal spheroids and pigmented glia) marks the end of what many families describe as a “diagnostic odyssey”—a long, often exhausting search for answers ^[1]. Because ALSP is ultra-rare and its symptoms overlap with more common conditions, published cohort data suggests only about 25% of patients receive the correct diagnosis initially ^[1]. Most families have spent years navigating misdiagnoses, with Multiple Sclerosis (MS) and Frontotemporal Dementia (FTD) being the most common incorrect labels ^[1][2].

Understanding this diagnosis requires looking at how the naming of the disease has evolved and how a single genetic change disrupts the brain’s internal maintenance system.

One Disease, Many Names

In the past, doctors used several different names to describe what we now know is the same condition spectrum. This was because different researchers focused on different features seen under a microscope. You may see these historical terms in older medical records or research papers:

• HDLS (Hereditary Diffuse Leukoencephalopathy with Spheroids): Focused on the “spheroids” or swollen nerve fibers ^[3].
• POLD (Pigmentary Orthochromatic Leukoencephalopathy): Focused on the “pigmented” immune cells ^[2].
• CSF1R-related Leukoencephalopathy: The modern genetic name, identifying the specific gene involved ^[3].

Today, ALSP is the accepted “umbrella” term that brings all these historical descriptions together into one diagnosis ^[3][4].

The Biological Mechanism: Microglia and Myelin

ALSP is primarily a disease of the microglia—the brain’s resident immune cells ^[5]. Think of microglia as the “janitors” and “security guards” of the brain; they are responsible for cleaning up debris, defending against threats, and supporting the health of other brain cells.

In most cases of ALSP, a pathogenic variant (often called a mutation) in the CSF1R gene prevents these cells from working correctly. This gene provides instructions for a receptor (a “docking station”) on the surface of microglia that they need to survive and stay healthy ^[5][6]. When this receptor function is altered, it is believed to cause a cascade of effects:

1. Microglial Dysfunction: The number of healthy, functional microglia may decrease, or they may become improperly activated or unbalanced (dyshomeostasis), though the exact balance of this is an active area of research ^[7][8].
2. White Matter Damage: Without healthy microglia to support them, the cells that create myelin (the protective insulation around nerve fibers) begin to fail ^[9].
3. Axonal Spheroids: The nerve fibers themselves (axons) become damaged and develop large, balloon-like swellings called spheroids ^[9][10].

This progressive loss of white matter is why ALSP is classified as a leukodystrophy (a disease that affects the white matter of the brain).

Incidence and Age of Onset

ALSP is considered an ultra-rare disorder. While the exact population prevalence is unknown—largely because so many cases are misdiagnosed—recent studies have identified hundreds of families worldwide ^[11][12].

In certain published referral cohorts, the typical age when symptoms first appear is in the early-to-mid 40s (the mean age is approximately 43) ^[1][13]. However, there is a very wide range; symptoms have been documented in people ranging from their late teens to their 70s ^[12]. Interestingly, some limited research has suggested that women may develop symptoms slightly earlier than men, often in their late 30s or early 40s, though this difference requires further study ^[12].

Genetics and Inheritance

Most cases of ALSP are inherited in an autosomal dominant pattern. This means if you carry a pathogenic variant in the CSF1R gene, there is a 50% chance of passing it to each of your children. However, inheriting the gene does not provide a precise timeline for the disease. ALSP exhibits age-dependent penetrance (meaning the likelihood of showing symptoms increases with age) and variable expressivity (meaning the severity and type of symptoms vary widely, even among relatives with the exact same genetic variant). Predictive testing for adult family members who do not yet have symptoms should always be voluntary and accompanied by professional genetic counseling.

Why the Diagnosis is Difficult

The “odyssey” to an ALSP diagnosis happens because the symptoms—such as changes in personality, memory loss, or difficulty walking—look very similar to other neurological diseases ^[1]. To support a clinical diagnosis of ALSP, doctors look for specific clues:

• Genetic Testing: Finding a pathogenic variant in the CSF1R gene is the “gold standard” for diagnosis ^[6].
• Imaging Clues: On an MRI, doctors look for confluent (connected) white matter lesions, especially in the front of the brain ^[14]. A CT scan may show unique, “stepping-stone” calcifications (small calcium deposits) in the white matter that can support an ALSP diagnosis, although their absence does not rule it out ^[15].
• Clinical Patterns: The combination of cognitive decline, psychiatric changes (like depression or anxiety), and motor issues (like stiffness or tremors) helps distinguish it from other forms of dementia, though not everyone has all these symptoms initially ^[1][13].

Validating your experience is important: if it took years and multiple specialists to reach this point, that is a reflection of the rarity and complexity of the disease, not a failure of your or your family’s care ^[1]. Knowing the definitive genetic cause now allows you and your family to shift from the “search for a name” to building a personalized, supportive care plan.