Symptoms and Warning Signs of HDL Syndromes

When a person experiences symptoms that look exactly like Huntington’s Disease (HD) but tests negative for the HTT gene mutation, doctors look toward a group of rare conditions called Huntington Disease-Like (HDL) syndromes ^[1][2]. These syndromes are often referred to as phenocopies—conditions that “mimic” the appearance of another disease ^[2].

While these syndromes share a common “triad” of symptoms with classic HD, they often have unique “red flag” signs that can help your medical team find the right diagnosis.

The Shared Triad: Movement, Thinking, and Mood

Like classic HD, most HDL syndromes involve three main categories of symptoms ^[2][1]:

1. Movement Disorders: The most common shared symptom is chorea, which comes from the Greek word for “dance.” These are involuntary, jerky, or fidgety movements that flow from one muscle group to another ^[3].
2. Cognitive Decline: This involves changes in executive function (mental skills that help you get things done), such as trouble planning, organizing, or multi-tasking ^[4].
3. Psychiatric Symptoms: Shared symptoms include depression, anxiety, irritability, or apathy (a loss of motivation) ^[4].

“Red Flag” Symptoms: Clues to an HDL Diagnosis

If you or a loved one is experiencing any of the following symptoms, it may point toward a specific HDL syndrome rather than classic HD.

1. Seizures and Self-Mutilation (Chorea-Acanthocytosis)

In classic adult-onset HD, seizures are very rare. However, they are a major red flag for a condition called Chorea-Acanthocytosis (ChAc) ^[5][6].

• Feeding Dystonia: This is a very specific symptom where the act of eating triggers involuntary tongue movements that push food out of the mouth ^[7][8].
• Tongue and Lip Biting: Patients with ChAc may forcefully and involuntarily bite their own tongue, lips, or the inside of their cheeks ^[7][9].
• Drop Attacks: Sudden, brief loss of muscle tone that causes the knees to buckle and the person to fall without warning ^[6].

2. Severe Ataxia (SCA17 / HDL4)

While HD patients may become unsteady as the disease progresses, ataxia (a specific type of clumsiness and lack of coordination) is the hallmark of Spinocerebellar Ataxia Type 17 (SCA17) ^[10][11].

• The “Drunken” Gait: Unlike the restless, fidgety walk of HD, SCA17 often presents with a wide-based, stumbling gait early in the disease ^[10].
• Eye Movement Changes: Doctors may look for specific oculomotor abnormalities (how your eyes track objects) that differ from the slow eye movements typical of HD ^[12].

3. Rapid Progression and Personality Shifts (HDL1)

HDL1 is an extraordinarily rare prion disease (a disorder caused by abnormally folded proteins) linked to the PRNP gene ^[13]. It is important not to panic over this possibility, as it has only been documented in a very small number of families globally.

• Unpredictable Speed: When it does occur, HDL1 can progress much faster than classic HD ^[13]. In some cases, it can lead to severe disability much more rapidly, though in others it may last up to 20 years ^[13].
• Early Dementia: Significant memory loss and personality changes often appear much earlier in HDL1 than in classic HD ^[13].

4. Psychosis and Frontal-Lobe Symptoms (C9orf72)

Mutations in the C9orf72 gene are most commonly linked to ALS, but they can also look like HD ^[14].

• Early Behavioral Changes: This syndrome may present with “frontal-lobe” symptoms like extreme impulsivity, social inappropriateness, or a total loss of empathy ^[4].
• Psychosis: Hallucinations or delusions are more common in C9orf72 phenocopies than in early-stage classic HD ^[4][15].

Summary of Symptom Differences

Note: HDL3 is a historical term used for a specific family in 2001; it is no longer a standard clinical diagnosis ^[13].