Introduction

Cerebral folate deficiency (CFD) is often overlooked and difficult to diagnose. This is due to its specificity to the brain and central nervous system, the inability to test it through standard blood tests, and its similarities or even overlap with common developmental disorders, such as autism. 

In this piece, we explain what CFD is and why it’s often overlooked, what 5-MTHF does, and how CSF 5-MTHF provides a specific, actionable marker to confirm deficiency early and guide personalised treatment (typically folinic acid). We’ll also cover when to test, how results are interpreted with age-stratified ranges, what a spinal tap involves, and practical next steps after a positive outcome.

What is cerebral folate deficiency?

Cerebral Folate Deficiency (CFD) is a rare neurometabolic disorder in which the fluid around the brain (cerebrospinal fluid, CSF) has abnormally low 5-MTHF levels, even when the same folate in the blood is normal. In short, the body is unable to transport enough necessary 5-MTHF to the central nervous system (CNS).^1,2

CFD typically emerges in early childhood and can begin as early as foetal development. The disorder is often overlooked, as its early signs are easy to miss. Sleep disturbances, irritability, and delays in learning new motor and language skills are examples of CFD symptoms, which are commonly mistaken for conditions such as autism or general developmental delay, or in some cases occur side by side with the disorder.³

As early signs are easily misattributed, recognising the month-to-year trajectory of CFD can help in earlier diagnosis and treatment.

Timeline of symptomatic progression of CFD

• 4-6 months: The more subtle symptoms of sleep disturbance and irritability surface
• 6-24 months: Psychomotor delays and decelerating head growth, meaning the circumference of the child's head increases at a slower rate than average. These can be accompanied by seizures and a lack of muscle coordination (ataxia)
• 3 years: Decline in vision
• 6 years: Hearing loss (sensorineural hearing loss) with a persistent regression and disability in motor skills⁴

What is 5-methyltetrahydrofolate (5-MTHF)?

5-MTHF is part of the family of essential molecules, known as folates or vitamin B9. It is mainly found in plasma and is the biologically active form of folate.

Folates are found in foods or in food supplements, in the form of folic acid. They can then be converted to 5-MTHF using an enzyme called MTHFR^.(5)Good sources of these vitamins include leafy greens, legumes and fortified grains.⁶

5-MTHF function

5-MTHF is involved in DNA and RNA repair and production. It does so by acting as a coenzyme that recycles homocysteine back into methionine, which in turn triggers the process of producing a crucial methyl donor called S-adenosylmethionine (SAM). This last step is essential, as the chemical tags which SAM donates, called methyl groups, are heavily used in:

• Gene regulation: turning genes "on" and "off"
• Regulating membranes, helping in maintaining the main lipids of membranes, called phospholipids
• Production of energy compounds, such as creatine

After its methyl groups are donated for its appropriate function, SAM is then turned back into homocysteine, which your body either recycles to be used once again in this 5-MTHF ó SAM cycle, or is transformed into cysteine, an essential amino acid which can be used for building proteins.⁷

Why does 5-MTHF deficiency occur in the CNS?

The active form of folate can be standard in blood yet low in the brain and CSF. The mechanism by which folate enters the CNS depends on a transport system at the choroid plexus, known as folate receptor-α (FRα). When this system is disrupted, the brain becomes deficient in essential folates even if the rest of the body is not, leading to CFD.

There are a few different ways in which this happens, such as loss of genetic variants encoding FRalpha, autoimmune blockades, and dysregulations in transport physiologies. It can even occur as a secondary disorder to already present mitochondrial disorders, such as Kearns-Sayre or a consequence of off-target effects of therapeutic drugs.⁸

Genetic

Genetic causes of CFD are linked to loss-of-function variants of the FOLR1 gene, which encodes for FRα. This causes a reduced expression, ligand binding, and trafficking of Fralpha, which blocks brain folate transport. 

When Fralpha is absent or dysfunctional as a result of these gene variations, folates such as 5-MTHF cannot be delivered into CSF or the brain. This ultimately results in the textbook symptoms of CFD that are identified.⁹

Autoimmune

Autoantibodies to FRα can interfere with the choroid-plexus transport ‘gate’, lowering CSF 5-MTHF and producing a CFD phenotype.¹⁰ Two patterns of this autoimmune action are blocking and binding, which are similarly seen in the autism spectrum.

Blocking: Autoantibodies block the binding site of folates.

Binding: Autoantibodies bind to FRα outside of the binding location.

Think of the latter as "jamming the door mechanism", rather than "blocking the keyhole". While blocking works by directly competing with the folates, binding is more closely related to immune-mediated receptor dysfunctions.¹¹

Consequences of disease or drugs

Mitochondrial diseases such as Kearns-Sayre Syndrome (KSS):

• 5-MTHF deficiency can arise in patients with mitochondrial diseases. One example of this would be KSS, a disease that is characterised by large-scale deletions in mitochondrial DNA. This leads to neuromuscular problems affecting the eyes, heart, and muscles all over the body. Monitoring folates is essential, as known 5-MTHF deficiencies in this disease are a possible occurrence, highlighting the importance of biological sample techniques.^12,9

Anti-epileptic drugs (AEDs):

• Another possible contributor to CFD may be the adverse effects of therapeutic drugs. Anti-epileptic drugs have the possibility of impairing the conversion and transport of folates into CSF. Treatments such as carbamazepine or valproate have been shown to decrease folate levels in serum and can affect levels of oxidative stress.^13,14

Why should CSF 5-MTHF levels be used as a diagnostic marker for CFD?

There are many difficulties faced with CFD. The disorder's similarities to other early learning disabilities, such as autism, both in symptomatic and in antibody presentation, make CFD incredibly difficult to catch and treat early. The risk of CFD occurring as a secondary disorder to other primary factors also highlights the importance of finding a biological marker to not only diagnose, but also to monitor throughout treatment.

The goal for health providers and researchers is to find biomarkers that are minimally invasive yet specific; this is where CSF levels of 5-MTHF come in.  

Unlike blood serum, the use of CSF reflects folate transport across the choroid plexus and into the central nervous system and brain, a direct window into brain folate status. This makes folate CSF levels an extremely valuable and quantifiable marker. This can allow clinicians to detect these deficiencies before any lasting damage can occur, and address them with personalised therapies, such as folinic acid supplementation.

How do spinal taps work?

Spinal taps, or lumbar punctures, can be a scary concept. Luckily, the procedure rarely ever comes with complications, and the information clinicians can gain from it is highly beneficial.

The procedure involves inserting a needle between the lumbar bones, typically L3-4 or L4-5, to reach the CSF. The fluid is collected into a few small sterile tubes, where, in this case, the sample is protected from light and frozen for 5-MTHF testing.^15,16 The result is interpreted against age-stratified reference ranges to judge whether levels are low for the patient’s developmental stage.

When to test?

It is recommended to consider testing CSF 5-MTHF in infants at around 4- 6 months, at the onset of symptoms of sleep disturbances, irritability, followed by psychomotor delays or seizures. This should especially be considered if the child's blood serum levels of 5-MTHF are normal. 

Other imaging tools, such as MRI scans, may also be used to indicate if any delayed myelination or hypomyelination is occurring. Additionally, if the child has any known familial cases of FLOR1 variations paired with these symptoms, CSF tests could be crucial in treating the disease as early as possible. 

As mentioned previously, if patients and providers are aware of correlated diseases such as KSS, or if the patient is prescribed AEDs (which are considered to increase the risk of CFD alongside its symptoms), monitoring these levels in blood and CSF would be highly beneficial.

How are results interpreted?

Results of folate levels are interpreted against age-stratified reference intervals, which are what clinicians consider normal ranges tailored to the individual's age. Due to the body changing as they grow, folate levels in a newborn differ from those of adults and teenagers.

For 5-MTHF, infants naturally have higher levels than teens and adults, with levels of 40-240 nmol/L between 0 and 6 months, and 40-240 nmol/L from 6 to 24 months.¹⁸ So an infant with levels of 5-MTHF equivalent to 25nmol/L but with normal serum 5-MTHF, would be considered as having cerebral folate deficiency.

The test does not diagnose on its own but rather is an essential step in the diagnosis process. Other key components, such as symptomatic analysis, imaging analysis through MRI and other metabolic values, will be considered before the doctor makes a diagnosis and guides the next steps of treatment.

What are the next steps after diagnosis?

Typically, treatment involves replenishing folate levels in the CNS. This could be done through folinic acid (leucovorin), which can bypass the folate transport system at the choroid plexus.¹⁷ Further managing the disease would involve getting to the bottom of the cause in the specific patient, whether it be genetic, autoimmune or other, and personalising treatment to the individual. 

Summary

Cerebral folate deficiency (CFD) is a rare neurological disorder in which 5-methyltetrahydrofolate (5-MTHF) levels in cerebrospinal fluid (CSF) are abnormally low, despite normal blood folate levels. This occurs due to impaired transport of 5-MTHF across the choroid plexus into the brain, often caused by mutations in the FOLR1 gene, autoantibodies against folate receptor-α (FRα), mitochondrial disorders (e.g., Kearns-Sayre syndrome), or the effects of anti-epileptic drugs.

Symptoms typically begin in infancy (4–6 months) with irritability and sleep disturbances, progressing to psychomotor delays, seizures, visual decline, and hearing loss by childhood. Because symptoms often resemble autism or developmental delay, CFD is frequently overlooked.

Measuring CSF 5-MTHF provides a specific biomarker of brain folate status—unlike blood tests, which cannot detect central deficiency. A spinal tap (lumbar puncture) is used to collect CSF, and results are compared against age-specific reference ranges. Low CSF 5-MTHF in the presence of normal serum levels confirms CFD.

Early diagnosis is crucial, as treatment with folinic acid (leucovorin) can bypass defective transport and restore folate in the CNS, improving symptoms. Identifying the underlying cause (genetic, autoimmune, or secondary) allows for personalised therapy and ongoing monitoring.

In essence: CSF 5-MTHF testing is a critical, specific diagnostic tool for detecting and managing cerebral folate deficiency—enabling early intervention and better neurological outcomes.