How Are Frontotemporal Dementia and ALS Connected?

Yes, it is true that behavioral variant frontotemporal dementia (bvFTD) and amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease, are closely related. Doctors now understand that these two conditions are often part of the same disease spectrum ^[1]. While bvFTD primarily affects behavior, personality, and cognition, ALS affects the motor neurons (the nerve cells that control movement), causing physical weakness and muscle decline ^[2]. It is important to know that having bvFTD does not mean you will inevitably develop ALS. However, some individuals develop only FTD, some develop only ALS, and a subset of patients develop a combination of both, known as FTD-ALS overlap ^[1][3].

The Genetic Link: C9orf72

One of the most significant discoveries connecting these two diseases is a shared genetic link. A specific type of genetic change called a repeat expansion in the C9orf72 gene is the most common known genetic cause for both familial (inherited) FTD and familial ALS ^[4].

This genetic expansion is highly unpredictable. It has variable “penetrance,” meaning that inheriting the altered gene does not guarantee a person will develop the disease, nor does it predict the age of onset ^[4]. Furthermore, this expansion can express itself differently even within the same family ^[4]. One family member might develop behavioral symptoms of bvFTD, another might develop the physical symptoms of ALS, and a third might experience both ^[3]. It can also be found in some “sporadic” cases where there is no known family history ^[4]. Because genetic testing has profound implications for relatives and family planning, it is strongly recommended to work with a genetic counselor before deciding whether to pursue testing.

The presence of this genetic link also suggests that the diseases share similar underlying cellular changes. Researchers have found that a buildup of a protein called TDP-43 is a common feature in the vast majority of ALS cases and about half of FTD cases ^[5]. This protein buildup disrupts normal cell function ^[6]. However, TDP-43 is a pathology finding usually established after death through research, not a routine clinical test used to diagnose you in a doctor’s office.

Adding ALS Symptoms to FTD

When ALS overlaps with bvFTD, the patient experiences physical motor issues alongside cognitive and behavioral changes ^[1]. While bvFTD changes behavior, the addition of ALS introduces progressive physical symptoms, including:

• Focal weakness: Weakness that starts in a specific area, like a hand, arm, or leg, and spreads ^[2].
• Muscle atrophy: Wasting away or shrinking of muscles ^[7].
• Stiffness and spasticity: Tightness in the muscles that makes movement difficult ^[7].
• Bulbar symptoms: Difficulty with swallowing, chewing, and the physical act of speaking (dysarthria) ^[8].
• Respiratory issues: Gradual shortness of breath as the muscles controlling breathing weaken ^[2].

Do not self-diagnose these symptoms; weakness or swallowing problems can have many other causes and should be evaluated by a neurologist.

Interestingly, cognitive and behavioral symptoms can sometimes differ slightly in an overlap case. Studies suggest a group-level tendency where people with FTD-ALS may experience more profound apathy (lack of motivation) and language difficulties compared to people with bvFTD alone, who often show more social disinhibition ^[9][10]. However, these are general tendencies, not absolute diagnostic rules. Cognitive and physical symptoms can develop simultaneously, or one can precede the other by months or years ^[1].

Impact on Survival and Disease Course

A dual diagnosis of FTD and ALS is understandably overwhelming. Because ALS causes progressive failure of the muscles that control swallowing and breathing, the physical decline tends to drive the disease’s timeline, and the presence of ALS unfortunately shortens the overall survival time compared to having bvFTD alone ^[1][7].

For example, a major review of patients with the C9orf72 mutation found that the median survival after symptoms began was 9.0 years for those with only FTD, but dropped to 3.0 years for those with combined FTD-ALS ^[11]. It is vital to understand that a “median” is simply the midpoint of a specific study group—it is not an average, and it is not a personal countdown or an individual prediction ^[11]. Factors such as the age of onset and whether symptoms start with swallowing difficulties (bulbar onset) heavily influence a person’s unique timeline ^[11]. Your neurologist is the best source for individualized prognosis.

Building a Multidisciplinary Care Team

Because of the physical complications introduced by ALS, care needs to become multidisciplinary very quickly. A care team will help manage both the behavioral strategies necessary for bvFTD and the physical supports required for ALS. This team should include:

• Neurologists from an ALS/FTD clinic
• Speech-language pathologists (for communication aids and swallowing tests)
• Respiratory therapists
• Dietitians and nutritionists
• Physical and occupational therapists
• Social workers and palliative care specialists (to assist with comfort, quality of life, and advance care planning)

Warning Signs to Monitor

It is critical to monitor breathing and swallowing safely.

• Seek Emergency Medical Help if: You or your family member experiences severe breathing difficulty, choking with an inability to clear the airway or secretions, bluish discoloration of the lips/face, or a complete inability to swallow saliva.
• Contact Your Care Team Promptly if: You notice recurrent coughing or choking during meals, a wet or gurgly voice after eating, unexplained weight loss, a weak cough, difficulty breathing when lying flat, morning headaches, or unusual daytime sleepiness. These can be subtle signs of respiratory or swallowing weakness.