Hair-straightening treatments have become increasingly popular worldwide over the past decade. Following concerns regarding the toxicity of formaldehyde-containing formulations, many manufacturers introduced products marketed as formaldehyde-free alternatives. However, accumulating evidence indicates that acute kidney injury (AKI) may also occur following exposure to these products. Recent clinical and experimental studies have identified glyoxylic acid, rather than formaldehyde itself, as the most likely nephrotoxic compound responsible for acute oxalate nephropathy, whereas glycolic acid appears to have a much weaker effect on oxalate production [1, 2].

In 2023, Bnaya et al reported the first multicenter case series describing 26 women who developed AKI shortly after hair-straightening procedures [3]. The patients had been exposed to different commercially available hair-straightening products rather than a single formulation. The authors noted that most keratin-based hair-straightening products available in Israel contained glycolic acid derivatives and hypothesized that systemic absorption of these compounds, followed by hepatic metabolism to oxalate, could explain the observed acute oxalate nephropathy [3]. Several patients experienced recurrent episodes, and some required temporary renal replacement therapy. Kidney biopsies performed in selected patients demonstrated acute tubular injury associated with calcium oxalate deposition, supporting acute oxalate nephropathy as the predominant pathogenic mechanism [3].

Since the first case series, additional cases have been reported in Europe and North Africa. Robert et al reported recurrent episodes of AKI following exposure to hair-straightening products containing glyoxylic acid [4]. Experimental studies further demonstrated calcium oxalate crystal deposition following topical exposure to glyoxylic acid, supporting its potential role in kidney injury [2]. More recently, similar observations have been reported in Switzerland and North Africa, suggesting that this emerging cause of AKI may be underrecognized [5, 6].

Although oxalate nephropathy appears to be the predominant pathological finding, urinary crystals and hyperoxaluria are not consistently identified, and some patients may recover completely with supportive treatment alone [3, 5]. Increased awareness of this potential complication is therefore essential.

Despite the increasing number of published cases, this entity remains largely underrecognized in routine clinical practice. We report the case of a previously healthy 37-year-old woman who developed severe AKI shortly after exposure to a commercially available hair-straightening product, with complete recovery of kidney function following conservative management.

A 37-year-old previously healthy woman with no history of hypertension, diabetes mellitus, or chronic kidney disease was admitted for severe AKI. Her medical history was notable only for chronic lumbosciatica related to a lumbar disc herniation diagnosed in 2023, for which she occasionally used non-steroidal anti-inflammatory drugs (NSAIDs). However, she reported no NSAID use before symptom onset and denied recent exposure to herbal medications or other known nephrotoxic agents.

According to the patient’s report, the hair-straightening procedure was performed in a professional hair salon using a commercially available hair-straightening product identified as Black Diamond^®. According to publicly available product information from Morocco, Black Diamond^® is marketed as a formaldehyde-free hair-straightening product. However, the complete ingredient list for the formulation used by our patient was not available. Therefore, we could not independently verify whether it contained glyoxylic acid or other potentially nephrotoxic compounds. The patient reported previous exposure to other commercially available hair-straightening products but could not recall whether this was her first exposure to the Black Diamond^® product. The procedure was performed in a ventilated environment by a hairdresser wearing protective gloves. No ocular irritation or respiratory symptoms were reported during or after the procedure. She had no previous history of adverse reactions following cosmetic hair treatments.

One day after the hair-straightening procedure, the patient developed abdominal pain, repeated vomiting, constipation, and asthenia. Because of persistent symptoms, she was initially admitted to the intensive care unit and subsequently transferred to the nephrology department. The patient’s clinical course is summarized in Table 1.

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Table 1. Clinical Timeline

On admission, the patient was hemodynamically stable, with a blood pressure of 125/75 mm Hg, an oxygen saturation of 98% on room air, and preserved urine output. Physical examination revealed no peripheral edema, signs of fluid overload, or other significant abnormalities.

Initial laboratory investigations performed at hospital admission confirmed severe AKI. Serum creatinine was 492 µmol/L (5.56 mg/dL), and the C-reactive protein level was elevated at 80 mg/L. The urine protein-to-creatinine ratio was 0.19 g/g. Urine culture was sterile. Urinary sediment examination revealed 1 × 10³ leukocytes/mL and 3 × 10³ erythrocytes/mL, with no urinary casts or urinary crystals. Serum lipase levels were within the normal range. Serological testing for hepatitis B virus and hepatitis C virus was negative. The calculated anion gap at presentation was 15 mmol/L. Serum osmolality was not measured; therefore, the osmolar gap could not be calculated. The principal laboratory findings are summarized in Table 2.

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Table 2. Laboratory Findings at Admission

Renal ultrasonography demonstrated normal-sized kidneys with preserved corticomedullary differentiation. The right kidney measured 130 × 68 × 56 mm, whereas the left kidney measured 120 × 66 × 60 mm. No hydronephrosis or nephrolithiasis was detected. The urinary bladder was normal, and no intraperitoneal effusion was observed. Overall, no alternative explanation for the patient’s AKI was identified based on the clinical presentation, laboratory investigations, and renal ultrasonography.

The patient was admitted 4 days after the hair-straightening procedure, at which time the serum creatinine concentration was 492 µmol/L (5.56 mg/dL). She received supportive treatment consisting of intravenous isotonic saline hydration, which resulted in progressive improvement in kidney function. Renal replacement therapy was not required. Serum creatinine decreased to 146 µmol/L (1.65 mg/dL) at hospital discharge (11 days after the procedure), 117 µmol/L (1.32 mg/dL) at the first outpatient follow-up (19 days after the procedure), and normalized to 88 µmol/L (0.99 mg/dL) at the final follow-up (26 days after the procedure). Renal biopsy and urinary oxalate measurements were not performed because of the favorable clinical course. The evolution of renal function is presented in Table 3.

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Table 3. Recovery of Kidney Function

At the last follow-up visit, the patient remained asymptomatic, with complete recovery of kidney function.

AKI associated with hair-straightening products has recently emerged as a distinct and probably underrecognized clinical entity. In the present case, the close temporal relationship between cosmetic exposure, the onset of gastrointestinal symptoms, and the subsequent development of severe AKI supports a probable causal relationship. The absence of an alternative explanation following comprehensive clinical, biological, microbiological, and radiological investigations further strengthens this diagnosis.

To date, the largest published study is the multicenter case series by Bnaya et al, which reported 26 women who developed AKI following hair-straightening procedures performed in different salons [3]. Most patients presented with abdominal pain, nausea, vomiting, or flank pain within hours to a few days after exposure. Several patients experienced recurrent episodes after repeated exposure, and temporary renal replacement therapy was required in some cases [3]. Our patient exhibited a remarkably similar clinical course, developing abdominal pain, repeated vomiting, constipation, and asthenia within 24 h after the hair-straightening procedure, followed by severe but completely reversible AKI. More recently, a similar case was reported in North Africa, suggesting that this condition is not restricted to the populations initially described and is likely underrecognized in other regions [6].

Although the precise pathophysiological mechanism has not been fully elucidated, glyoxylic acid is currently considered the most likely nephrotoxic component of many formaldehyde-free hair-straightening products [1, 4]. Following cutaneous absorption, glyoxylic acid is metabolized into oxalate, which combines with calcium to form calcium oxalate crystals within the renal tubules, leading to tubular obstruction, epithelial injury, and acute oxalate nephropathy [2, 4]. Experimental studies by Robert et al demonstrated calcium oxalate crystal deposition and acute tubular injury after topical exposure to glyoxylic acid-containing formulations, providing strong biological evidence supporting this mechanism [2, 4]. Although repeated vomiting may have contributed to volume depletion, the severity of AKI, the close temporal relationship with cosmetic exposure, and the similarity to previously reported cases suggest that vomiting alone is unlikely to account for the observed kidney injury. Instead, gastrointestinal symptoms may represent an early manifestation of systemic toxicity preceding AKI.

Kidney biopsy findings reported by Bnaya et al consistently demonstrated acute tubular injury associated with calcium oxalate crystal deposition, confirming acute oxalate nephropathy as the predominant histopathological lesion [3]. Nevertheless, kidney biopsy is not mandatory in every patient. In our case, the rapid and sustained improvement in kidney function following supportive treatment made an invasive diagnostic procedure clinically unnecessary. Urinary and plasma oxalate levels were not systematically reported in the case series by Bnaya et al [3]. Therefore, the diagnosis was primarily supported by the temporal relationship with exposure and, in selected patients, by histopathological evidence of calcium oxalate crystal deposition.

An important finding in our patient was the absence of urinary crystals despite severe AKI. Similar observations have been reported in previously published cases, indicating that crystalluria is not consistently detected, even in patients with biopsy-proven acute oxalate nephropathy [4, 5]. Likewise, urinary oxalate excretion was not measured because of the favorable clinical course. Therefore, neither the absence of crystalluria nor the lack of urinary oxalate measurements should discourage clinicians from considering this diagnosis when the temporal relationship between cosmetic exposure and AKI is highly suggestive. The complete recovery of kidney function observed in our patient is also consistent with the favorable outcomes reported in several patients managed conservatively [3, 5].

An elevated C-reactive protein concentration was also observed at presentation and gradually normalized during follow-up. Although nonspecific, this finding may reflect the inflammatory response associated with acute tubular injury and should be interpreted within the overall clinical context.

Our case presents several noteworthy features. First, the commercial hair-straightening product was identified according to the patient’s report. Second, complete recovery of kidney function was documented through serial biochemical measurements. Third, no alternative cause of AKI was identified despite an extensive diagnostic workup. Fourth, despite the absence of crystalluria and histopathological confirmation, the clinical presentation was highly consistent with previously reported cases. Finally, this report further supports the growing body of evidence that hair-straightening products represent an emerging and probably underrecognized cause of AKI. A recent systematic review further highlighted the increasing number of published cases worldwide, suggesting that this complication may be more common than currently recognized [7].

This report has several limitations. Kidney biopsy, urinary oxalate measurements, and chemical analysis of the cosmetic product were not performed. Consequently, histopathological confirmation of acute oxalate nephropathy and direct confirmation of glyoxylic acid exposure were not available. In addition, the commercial product was identified based on the patient’s report and could not be independently verified. Nevertheless, the close temporal relationship between exposure and symptom onset, the exclusion of alternative causes of AKI, and the complete recovery following supportive treatment strongly support a probable causal relationship.

In conclusion, recent exposure to hair-straightening products should be systematically investigated in patients presenting with otherwise unexplained AKI, particularly when gastrointestinal symptoms precede kidney injury. Greater awareness of this emerging complication may facilitate earlier diagnosis, prevent unnecessary investigations, and improve patient outcomes.

This case highlights hair-straightening products as an emerging and probably underrecognized cause of AKI. The close temporal relationship between cosmetic exposure and symptom onset, the exclusion of alternative causes, and the complete recovery of kidney function support a probable causal relationship. Clinicians should systematically inquire about recent hair-straightening procedures in patients presenting with otherwise unexplained AKI, particularly when gastrointestinal symptoms precede renal dysfunction. Increased awareness of this emerging entity may facilitate earlier diagnosis, prevent unnecessary investigations, and improve patient outcomes. Further studies are warranted to better characterize the nephrotoxic mechanisms associated with hair-straightening products, particularly those containing glyoxylic acid.

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