The study began with three unrelated children who shared an unusual constellation of symptoms: abnormally large heads beginning in infancy, recurrent episodes of elevated pressure inside the skull, difficulties with movement and coordination, autistic features, and seizures. Magnetic resonance imaging revealed a distinctive pattern of edema, with swelling initially confined to the outermost layers of cerebral cortex before spreading into the underlying white matter. Unusual signal changes in the medulla, the lower portion of the brainstem, further distinguished these cases from better-known causes of genetic brain edema.

Genetic sequencing uncovered the culprit: a heterozygous missense variant in SLC4A4, the gene encoding the electrogenic sodium bicarbonate cotransporter NBCe1. This variant, absent from population databases, substitutes a threonine for a highly conserved isoleucine residue in the eighth transmembrane domain of the protein. The residue sits immediately below the ion coordination site, a position critical for the transporter’s ability to shuttle bicarbonate across cell membranes while maintaining proper electrochemical gradients.

Importantly, the clinical picture differed dramatically from proximal renal tubular acidosis, the condition classically associated with biallelic loss-of-function mutations in SLC4A4. Children with that disorder suffer from systemic acidosis, short stature, and eye and tooth abnormalities, but they do not develop chronic brain edema. The patients in this study showed only mild laboratory evidence of acidosis and no overt renal symptoms, indicating that the new variant causes disease through a mechanism distinct from simple protein inactivation.

To understand how the mutation wreaks havoc at the molecular level, researchers used confocal imaging, patch-clamp electrophysiology, and voltage-clamp studies in frog oocytes. They found that the mutant protein reaches the cell membrane far less efficiently than its normal counterpart. More strikingly, even the fraction that does reach the surface behaves aberrantly: instead of cleanly transporting sodium and bicarbonate together, the mutant transporter admits a continuous, weakly selective depolarizing current that leaks positive ions into cells, disrupting membrane potential.

This depolarizing leak proved consequential for astrocytes, the star-shaped glial cells that express NBCe1 more abundantly than any other brain cell type. Immunohistochemistry on human brain tissue confirmed dense NBCe1 staining in astrocytic processes throughout gray and white matter, particularly at blood-brain interfaces. Astrocytes normally rely on NBCe1 to buffer extracellular acidity generated by neuronal firing, and the transporter can move bicarbonate in either direction depending on the cell’s membrane voltage. The leak shifts astrocytes toward inward bicarbonate transport even at rest, promoting cellular swelling and, ultimately, tissue-wide edema.

Structural modeling using AlphaFold 3 did not reveal an obvious pore or channel forming in the mutant protein, suggesting instead that the leak arises from a subtle destabilization of the conformational switching mechanism that transporters use to ferry substrates across membranes. Similar mutations in related SLC4 family members, such as the red cell anion exchanger AE1, have been shown to convert disciplined transporters into indiscriminate cation leaks, hinting at a broader principle of transporter dysfunction that this study now extends to the brain.

The translational payoff emerged when researchers tested oral bicarbonate supplementation in two of the affected children. Over nearly two years of treatment, both showed marked clinical improvement, including better motor control, enhanced social engagement, and normalization of head growth rate. Sequential brain MRIs demonstrated reduced white matter edema, resolution of brainstem abnormalities, and improved quantitative diffusion measures, confirming that restoring extracellular bicarbonate can dampen the pathological leak and relieve astrocyte swelling.

The work establishes SLC4A4-related encephalopathy as a new member of a small family of astrocyte-driven brain edema disorders, alongside megalencephalic leukoencephalopathy with subcortical cysts. By connecting acid-base regulation directly to brain volume control, the findings underscore that pH homeostasis is not merely a supporting player in brain physiology but a central determinant of tissue integrity. They also raise the prospect that other unexplained cases of childhood macrocephaly and edema may respond to the same simple metabolic intervention.

The connection between pH regulation and intracranial pressure has long been appreciated in clinical practice, most visibly in the use of controlled hyperventilation to lower arterial carbon dioxide. When CO2 falls, extracellular fluid becomes alkaline, cerebral vessels constrict, and pressure within the skull drops. What the new study adds to this familiar picture is a genetic demonstration that the converse relationship also holds: when bicarbonate handling at the single-cell level is disrupted, brain tissue volume can spiral upward chronically rather than fluctuating with ventilation. In this sense, the SLC4A4 variant provides a natural experiment that isolates acid-base transport as a direct determinant of brain water distribution, something that pharmacological manipulation alone could never cleanly establish.

The quantitative imaging methods used to track the patients deserve particular attention. Beyond conventional T2 and FLAIR sequences, the investigators extracted diffusion-weighted metrics from defined regions of interest, including mean diffusivity, fractional anisotropy, and radial diffusivity. These measures probe the microscopic architecture of white matter: mean diffusivity rises when extracellular water accumulates and tissue microstructure breaks down, while radial diffusivity is sensitive to the integrity of the myelin sheaths surrounding axons. Serial measurements of this kind allowed the team to document not just visible swelling but subtle changes in tissue microstructure over years, and then to watch those same metrics improve during bicarbonate therapy. Such longitudinal quantitative imaging is still uncommon in rare-disease research and offers an objective endpoint for future trials.

The inheritance patterns observed across the four affected individuals carry practical implications for genetic counseling. In two of the children, the variant was absent from both parents, consistent with a de novo event, meaning that recurrence risk for subsequent siblings is low though not zero due to the possibility of germline mosaicism. The third child, however, inherited the variant from his mother, who herself only became symptomatic in mid-adulthood after minor head trauma. This markedly later and milder presentation in the transmitting parent illustrates how the same molecular defect can produce dramatically different clinical trajectories, likely reflecting a combination of modifier genes, environmental insults, and the cumulative burden of edema episodes over decades. It also serves as a caution that heterozygous SLC4A4 carriers should not be assumed to be unaffected simply because they lack the severe infantile phenotype.

The episodic nature of the patients’ deterioration is itself informative. Recurrent crises of raised intracranial pressure were repeatedly triggered by minor head trauma or intercurrent infections, and responded to corticosteroids and, in one child, to acetazolamide, a carbonic anhydrase inhibitor that reduces cerebrospinal fluid production and shifts acid-base balance. These triggers plausibly act by imposing additional osmotic or inflammatory stress on astrocytes whose buffering capacity is already compromised by the leaky transporter. The observation that systemic infections can precipitate neurological decompensation echoes patterns seen in other channelopathies and suggests that fever, altered ventilation, and metabolic acidosis during illness may transiently worsen the already deranged bicarbonate equilibrium in these patients’ brains.

The radiating pattern of enlarged perivascular spaces visible on the patients’ scans offers a further clue to the underlying pathology. Perivascular spaces are fluid-filled compartments that follow vessels deep into the brain parenchyma, and their enlargement is typically interpreted as impaired clearance of interstitial fluid. In this disorder, their prominence in the swollen white matter suggests that the edema fluid is not uniformly distributed but accumulates along perivascular drainage routes, consistent with astrocytic endfeet at the blood-brain interface being a primary site of pathological swelling. The dense NBCe1 staining observed at these same interfaces in human control tissue strengthens the argument that the blood-brain boundary is where the mutant transporter exerts its greatest damage.

Comparison with megalencephalic leukoencephalopathy with subcortical cysts, the prototypical monogenic brain edema disorder, highlights both similarities and differences. MLC arises from defects in MLC1, GlialCAM, GPRC5B, or aquaporin-4, proteins that govern astrocyte volume regulation and cell-cell adhesion, yet none of these directly implicate acid-base transport. The shared imaging signature, subcortical white matter swelling with cyst formation and temporal lobe involvement, suggests convergent downstream pathways, but the response to bicarbonate in the SLC4A4 patients raises the question of whether subtle pH disturbances might also contribute to edema in MLC, or whether bicarbonate therapy might have adjunctive value there. Testing such hypotheses will require careful metabolic monitoring, since the patients here showed only mild serum bicarbonate reduction despite dramatic neurological benefit from supplementation.

The mechanistic finding that a transporter can become a cation leak without forming an obvious pore has implications well beyond this one gene. SLC4 family members and related transporter superfamilies are increasingly recognized as capable of adopting leak modes when their conformational gating is destabilized, and the parallel with the anion exchanger AE1 in red blood cells suggests a shared biophysical principle. For clinicians, the message is that missense variants in transporter genes cannot be assumed to be simple loss-of-function; electrophysiological characterization may reveal gain-of-toxic-function behavior with entirely distinct tissue consequences and, as demonstrated here, distinct treatment opportunities. Screening unexplained childhood macrocephaly cases for SLC4A4 variants, and considering early bicarbonate supplementation when such variants are found, now stands as a concrete clinical action supported by this work.

Subject of Research: NBCe1 sodium bicarbonate cotransporter dysfunction causing genetic brain edema in children

Article Title: A Depolarizing Leak in Sodium Bicarbonate Cotransporter NBCe1 Causes Brain Edema

Article References: Bisseling, Q., Parker, M. D., Kerst, S., Pasternack, R. A., Tondreau, J., Breur, M., van Rooijen‐van Leeuwen, G. M., Tonduti, D., Salsano, E., Darling, A., van Wijk, J. A. E., Törnroth‐Horsefield, S., Bugiani, M., Pouwels, P. J. W., Waisfisz, Q., van der Knaap, M. S., & Min, R. (2026). A Depolarizing Leak in Sodium Bicarbonate Cotransporter NBCe1 Causes Brain Edema. Annals of Clinical and Translational Neurology, 13(9), 1817-1830. https://doi.org/10.1002/acn3.70363

Image Credits: AI Generated

DOI: 10.1002/acn3.70363

Keywords: NBCe1, SLC4A4, brain edema, astrocytes, bicarbonate transporter, macrocphaly, intracranial pressure, acid-base homeostasis, ion leak, genetic encephalopathy, bicarbonate treatment, MRI

The study addresses one of the most urgent problems in neurocritical care: elevated intracranial pressure, or ICP. The skull is a rigid compartment containing the brain, blood and cerebrospinal fluid. When trauma, bleeding, stroke or another disease causes brain tissue to swell, pressure can rise within this confined space. As ICP increases, it can reduce cerebral perfusion—the pressure gradient that drives blood through brain tissue—and compromise the delivery of oxygen and glucose. In severe cases, high pressure can trigger ischemic injury, distort brain structures and cause herniation, in which brain tissue is forced through openings within the skull. Osmotic therapy is intended to interrupt this process. By increasing the concentration of osmotically active particles in the bloodstream, hypertonic saline draws water out of swollen brain cells and into the circulation, where it can eventually be removed by the kidneys. The treatment can also expand intravascular volume, potentially supporting blood pressure and cerebral perfusion in patients who are simultaneously suffering from blood loss or shock.

The investigation was led by Hye Won Choi and Jeong-Am Ryu at Samsung Medical Center in Seoul. The researchers reviewed the records of 993 adults who had received 7.5% hypertonic saline in a neurosurgical intensive-care unit between January 2015 and December 2024. All participants had received the concentrated saline bolus, allowing the central comparison to focus on whether adding continuous saline infusion was associated with better outcomes. Of the total cohort, 851 patients received bolus therapy alone, while 142 received the boluses along with a continuous intravenous infusion of 3% saline. The primary analysis concentrated on 212 patients classified as having severe brain injury, defined by a Glasgow Coma Scale score from 3 to 9 on admission. The Glasgow Coma Scale measures eye opening, verbal responses and motor responses, producing a score from 3 to 15. Lower scores indicate deeper impairment of consciousness. When speech could not be assessed because a patient was intubated or had a tracheostomy, the investigators estimated the verbal component using a previously described regression method.

The two saline strategies have different pharmacological roles. At the Seoul hospital, a standard 7.5% saline bolus consisted of 100 milliliters infused over 20 to 30 minutes. Clinicians used it when patients experienced sudden neurological deterioration, when monitored ICP remained above 20 millimeters of mercury or when brain scans showed signs of impending herniation. The bolus produces a rapid increase in plasma osmolality, creating an immediate gradient that can shift water away from swollen tissue. Its effect, however, may fade over several hours, and repeated doses can cause abrupt changes in serum sodium. The continuous treatment was delivered as 3% saline at 30 milliliters per hour through a central venous catheter, with clinicians targeting a serum sodium concentration of 145 to 155 milliequivalents per liter. Rather than waiting for pressure to surge and then reacting with another large dose, the infusion was intended to sustain the osmotic gradient between acute interventions. The concept resembles maintaining a stable pressure differential instead of repeatedly applying a sequence of short, powerful pushes.

Brain swelling itself is not a single process. In the early phase of traumatic or other severe brain injury, cytotoxic edema can develop when depleted cellular energy stores disrupt ion pumps. Sodium and other ions accumulate inside cells, and water follows, causing neurons and glial cells to swell. Later, damage to the blood–brain barrier can produce vasogenic edema. This allows plasma proteins and fluid to move into the extracellular space, increasing tissue volume. Hypertonic saline may influence both forms of edema, although its ability to draw water depends on the integrity and permeability of the barrier between blood and brain. The study’s results support the possibility that the duration and steadiness of osmotic exposure may matter as much as the highest sodium value achieved. In the severe-injury subgroup, the combined-therapy group had better ICP control despite lower peak serum sodium. The observation is important because clinical strategies have often emphasized reaching a predetermined sodium target, potentially overlooking how quickly sodium rises, how long the osmotic effect persists and how sharply concentrations fall afterward.

After statistical adjustment for differences between patients, the association with survival remained large. The investigators used inverse probability of treatment weighting, a method that attempts to balance measured characteristics between treatment groups by giving different statistical weights to individual patients. In the severe brain injury analysis, combined therapy was associated with an adjusted odds ratio for ICU death of 0.23, with a 95% confidence interval of 0.08 to 0.65 and a p value of 0.006. For 28-day mortality, the adjusted odds ratio was 0.20, with a 95% confidence interval of 0.07 to 0.55 and a p value of 0.002. The researchers also performed five sensitivity analyses, including analyses using multiple imputation for missing information and an E-value assessment of the potential influence of unmeasured confounding. Each analysis reportedly supported the direction of the main findings. In the overall cohort, however, the mortality differences did not reach conventional statistical significance: the odds ratio was 0.58 for ICU mortality and 0.59 for 28-day mortality. A time-to-event analysis across the full cohort did show a significant survival difference, measured by a Kaplan–Meier log-rank p value of 0.006.

The apparent benefit does not prove that continuous saline caused patients to survive. This was a single-center retrospective cohort study, meaning the researchers examined treatment decisions and outcomes that had already occurred rather than assigning patients to treatment randomly. Physicians chose to add continuous 3% saline when they believed sustained osmotic support was needed. That clinical judgment may have been based on factors not fully captured in the records, such as the trajectory of ICP, subtle neurological changes, the severity of swelling on imaging or the anticipated need for surgery. It is also possible that patients selected for combined therapy received other differences in care, including more intensive monitoring or earlier neurosurgical intervention. Statistical weighting can adjust for recorded variables, but it cannot eliminate confounding by indication when important differences remain unmeasured. The authors note that even the study’s robustness analyses and high E-values cannot fully exclude this problem. The lower mortality could therefore reflect the combined regimen, associated aspects of care, patient selection or some mixture of all three.

The findings also highlight why hypertonic saline has attracted particular interest in critically ill patients. Mannitol, another osmotic agent, can lower ICP but promotes diuresis, increasing urine output and potentially reducing circulating blood volume. That effect may be disadvantageous in patients with traumatic injuries, bleeding or unstable blood pressure, where maintaining cerebral perfusion is already difficult. Hypertonic saline can increase intravascular volume while creating the osmotic gradient needed to move water from brain tissue. Its concentrated formulations also deliver a large osmotic load in a relatively small volume, a useful property when clinicians need a rapid response without administering substantial fluid. In the hospital involved in the study, 7.5% saline was prepared by diluting 60 milliliters of 11.7% sodium chloride with sterile water. Local regulatory guidance discourages direct use of undiluted 11.7% saline, so the compounded 7.5% formulation served as the institution’s standard bolus treatment. These details matter because saline concentrations, dosing practices, monitoring protocols and thresholds for treatment may differ substantially among hospitals.

The work arrives amid uncertainty about whether continuous hypertonic saline improves long-term outcomes. The large COBI randomized trial examined continuous infusion of 20% hypertonic saline in patients with moderate-to-severe traumatic brain injury and did not find a significant improvement in neurological outcomes six months later, although a post hoc analysis suggested a possible mortality benefit. That study and earlier research have helped establish that hypertonic saline can reduce ICP, but lowering pressure is not automatically equivalent to improving recovery. A therapy could normalize a physiological measurement while failing to prevent neuronal death, disability or complications elsewhere in the body. The new study focuses on a different regimen—7.5% boluses combined with 3% continuous saline—and on short-term outcomes in a broader neurocritical population treated at one center. Its most provocative result is not simply the survival association, but the relationship between sodium kinetics and pressure control. If sustained gradients are more important than sodium peaks, future protocols may be designed around the shape and stability of the concentration curve rather than a single target value. That hypothesis will require carefully controlled testing.

Prospective, multicenter trials are now needed to determine whether the strategy can safely improve outcomes and which patients are most likely to benefit. Such studies would need to define when the continuous infusion begins, how long it continues, how sodium and osmolality are monitored, and how clinicians respond to kidney dysfunction, fluid overload or electrolyte disturbances. They would also need to measure neurological outcomes beyond survival, because avoiding death does not necessarily mean preserving independence or cognitive function. Until those data are available, the findings should be viewed as a compelling signal rather than a new standard of care. Still, the study offers a clinically intuitive possibility: in a brain under extreme pressure, repeatedly delivering a rescue dose may not be as effective as combining rapid responses with a carefully controlled background treatment. The result has already sharpened a central question for neurocritical care—whether the brain benefits more from the highest osmotic force clinicians can safely produce, or from a steadier gradient that prevents pressure from rebounding in the first place.