The Diagnostic Roadmap: Essential Tests for LKS

Diagnosing Landau-Kleffner Syndrome (LKS) is a process of “connecting the dots” between your child’s behavior and their brain’s electrical activity during sleep. Because the symptoms can mimic other conditions—such as autism or hearing loss—doctors use an individualized set of tests to evaluate the cause of the language loss ^[1][2].

The Important Role of the Sleep EEG

A critical evaluation for LKS is an Electroencephalogram (EEG) that records the brain while your child is in non-rapid eye movement (NREM) sleep ^[3]. This is often called a “sleep EEG” or “overnight video-EEG.”

During the day, a child with LKS might have a perfectly normal EEG. However, when they fall into deep, slow-wave sleep, the brain’s electrical activity can change dramatically ^[3][4]. Capturing an adequate amount of sleep is important, as this is when the characteristic electrical spikes appear ^[5].

Understanding the Spike-Wave Index (SWI)

When your neurologist reviews the sleep EEG, they look for a pattern often described under the umbrella of Electrical Status Epilepticus during Sleep (ESES) or Continuous Spike-Waves during Sleep (CSWS) ^[6][7]. To measure the severity of these spikes, they may calculate the Spike-Wave Index (SWI).

• What it measures: The percentage of NREM sleep time that contains abnormal electrical spikes ^[8].
• The thresholds: Definitions and counting methods vary by center. Historically, an SWI of 50% to 85% of NREM sleep was used to classify ESES/CSWS ^[9][10]. However, an SWI below 50% does not rule out LKS, and the index is just one piece of context, not a universal diagnostic cutoff.
• Why it matters: A higher index indicates a significant epileptiform burden, but the “burden” of the spikes doesn’t always perfectly match the severity of the language impairment ^[11][12]. The neurologist will relate it to language, cognition, and behavior.

Common Diagnostic Steps

Beyond the EEG, several other tests are typically considered to ensure the diagnosis is comprehensive and to create a roadmap for support.

1. Audiology (Hearing Test)

Before diagnosing a brain-based language disorder, doctors must evaluate physical hearing. In LKS, a child often fails to respond to their name, making them appear deaf. A formal audiology assessment helps evaluate for physical hearing loss ^[1]. Most children with LKS will have normal peripheral hearing, meaning their ears hear the sound, but their brain cannot decode it ^[13]. Audiology helps characterize exactly what the child can and cannot process.

2. Speech-Language Assessment

A specialist must evaluate the “gap” in your child’s skills. They will measure:

• Receptive Language: How much of what is said to them they can actually understand ^[14].
• Expressive Language: Their ability to produce words and sentences ^[14].
• Visual Communication: Whether they can still follow instructions using pictures or gestures, which is often a strength in children with LKS ^[15].

3. Neuropsychological Evaluation

This is a baseline “snapshot” of your child’s overall cognitive function, including memory, attention, and behavior ^[14]. Because LKS can sometimes cause children to act impulsively or seem “autistic,” this evaluation provides context for their development and behavior ^[2][11]. Note that a neuropsychological evaluation can be difficult to interpret while a child is severely aphasic.

Additional Evaluations to Consider

Depending on your child’s presentation, your medical team may consider:

• [ ] Overnight or Sleep-Deprived EEG: Did it capture an adequate amount of NREM sleep?
• [ ] SWI Calculation: Has the neurologist explained the severity of the spikes?
• [ ] Audiogram: Has peripheral hearing been formally tested?
• [ ] Baseline Speech Evaluation: Do you have a report detailing their current receptive and expressive language levels?
• [ ] Neurological/Medical Workup: Has a pediatric neurologist evaluated for other conditions, ordering an MRI or blood work to evaluate for structural, inflammatory, or autoimmune causes (like autoimmune encephalitis) if indicated? ^[16]
• [ ] Genetic Testing: Have you discussed testing, such as an epilepsy gene panel that might include the GRIN2A gene, which is found in a subset of cases on the epilepsy-aphasia spectrum? ^[11] A negative result does not exclude LKS, but it can provide more context.