Published: August 19, 2020 | 6 mins read

Calcium Oxalate Monohydrate Kidney Stones [Subtypes + Causes]

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Calcium oxalate stones are the most prevalent type of kidney stone formed in North America. There are two different types based on their predominant crystalline form (internal crystal structure). The two variations are either Calcium Oxalate Monohydrate (Type 1) or Calcium Oxalate Dihydrate (Type 2).

Monohydrate kidney stones come in 5 distinct morphological types (Type 1a to 1e). Dihydrate kidney stones come in 3 different morphological types (Type 2a to 2c). There are also Calcium Oxalate kidney stones that are a mixture of both Monohydrate and Dihydrate crystals. We will discuss Dihydrate and mixed stones in a future blog.

Calcium Oxalate Monohydrate Kidney Stones (Type 1)

Monohydrate kidney stones form due to an elevated concentration of oxalate ions in the urine without an associated elevation in urinary calcium. This condition is also known as Hyperoxaluria. The degree and duration of Hyperoxaluria determine the morphological subtype (Type 1a to 1e). Additionally, it is essential to note that Hyperoxaluria results from both genetic factors (inherited) and dietary factors (idiopathic).

Monohydrate Type 1a Kidney Stones

Monohydrate Type 1a

Type 1a kidney stones are the most common kidney stone type. On the exterior, Type 1a kidney stones are spherical, with a smooth, dark brown surface. Their interior is composed of compact concentric layers with a radiating organization starting from a nucleus. This interior structure represents an intermittent (on again and off again) formation of crystals related to peaks of Hyperoxaluria resulting from dietary habits (idiopathic).

Three conditions may be involved in the intermittent and moderate excessive concentration of oxalate in urine. The most common cause is a low water intake resulting in low dieresis. The other factors are:

  1. High consumption of oxalate-rich foods such as dark chocolate, beetroots, spinach, rhubarb, sorrel, star fruit, and many more (click HERE for a comprehensive list of oxalate containing foods).
  2. High consumption of hydroxyproline-rich foods that induce a metabolic synthesis of oxalate by the liver
  3. Low intake of calcium, resulting in increased absorption of oxalate ions by the gut.

Interestingly, most Type 1a kidney stones are dark brown because their growth is intermittent, allowing urinary pigments to attach to the crystals. However, in some cases, the surface of Type 1a stones exhibit a very thin (and often incomplete) white layer made of tiny calcium oxalate Monohydrate crystals. Because the underlying layers of crystals are dark brown, the thin superficial deposit appears as a grayish film. This grayish film is significant because it illustrates a very recent peak of Hyperoxaluria (shown below). Thus, dark brown Type 1a kidney stones (shown above) are considered dormant or metabolically inactive. By contrast, grayish Type 1a kidney stones are in an active stage.

                    Monohydrate Type 1a-1Monohydrate Type 1a-2Monohydrate Type 1a-3

As an aside, several Type 1a kidney stones show a umbilication (a small depression at the stone surface). This depression is the footprint of the tip of the papilla (internal structure of kidney- Figure 4), where the calcium oxalate kidney stone formed and was initially attached.

Monohydrate Type 1b Kidney Stones

Monohydrate Type 1b

Type 1b & Type 1d kidney stones are much less common than Type 1a kidney stones. On the surface, Type 1b kidney stones have a brown to dark brown color, are mammillary (oval-shaped rather than spherical/round), and have a rough surface. The interior of Type 1b stones is unorganized and never exhibits umbilication. This disorganized internal structure is due to their formation being primarily driven by moderate Hyperoxaluria, stasis (bladder not able to completely empty), and/or a progressive crystallin conversion from Dihydrate to Monohydrate.

Monohydrate Type 1c Kidney Stones

Monohydrate Type 1c

Type 1c kidney stones are (in the majority of instances) a result of Primary Hyperoxaluria Type 1 (PH1), the most severe of all forms of nephrolithiasis. Primary Hyperoxaluria is a rare inherited genetic condition present at birth. The liver of individuals with this condition does not create enough of a particular enzyme that regulates the production of oxalate. As such, this kidney stone type is very rare.

This kidney stone type consists of pure Calcium Oxalate Monohydrate crystals. However, Type 1c kidney stones demonstrate a very different and peculiar morphology represented by a pale-yellow budding surface without any umbilication. Internally, the structure is loose and unorganized.

When analyzed with an electron microscope, the unique crystalline structure of common idiopathic Calcium Oxalate Monohydrate kidney stones is distinctly identifiable compared to those structures associate with kidney stones formed by Primary Hyperoxaluria Type 1 (PH1). This structure variation is significant because when a kidney stone that appears clear is found, it should immediately alert to the probability of Primary Hyperoxaluria Type 1 (PH1). Or, at least, of a condition associated with heavy Hyperoxaluria.

Monohydrate Type 1d Kidney Stones

Monohydrate Type 1d

Type 1d kidney stones are quite different in terms of their appearance and morphology. These kidney stones have a very smooth surface and are beige or pale brown. Internally, they are composed of thin concentric layers without radiations. Type 1d kidney stones often form in numbers, as they develop in conditions of stasis (cannot empty bladder) or hydronephrosis (backup of urine in kidneys) in confined cavities. The smooth and plan surface is a result of the many stones rubbing against each other.

Monohydrate Type 1e Kidney Stones

Monohydrate Type 1e

Type 1e kidney stones represent a specific condition associated with Hyperoxaluria resulting from an intestinal dysfunction of diverse origins. A few examples of the dysfunctions leading to this condition are:

  • Inflammatory bowel disease
  • Ileal resection for Crohn’s Disease (removal of diseased tissue at end of small intestine)
  • Children with a severe pancreatic deficit
  • Post-bariatric surgery (such as jejunoileal bypass or Roux-en-Y gastrojejunal bypass)

This type of kidney stone is locally budding with an oval shape and a rough brown-yellow pale surface. Internally, it is poorly organized with brown-yellow areas with locally concentric dark brown layers with radial organization.

Another condition associated with the formation of Type 1e kidney stones is Type 2 Diabetes. Roughly 6% of kidney stones formed in diabetic patients show this type of morphology. This correlation suggests that Hyperoxaluria is at least in a subgroup for these individuals, a metabolic disorder associated with Type 2 Diabetes. A 2010 paper on the subject pointed out Hyperoxaluria as a risk factor for kidney stones in diabetic patients. Various pathways involving carbohydrates and polls may be involved in the formation of metabolic precursors of oxalate such as glyoxal or glyoxylate.

Stay tuned for our next blog on Calcium Oxalate Dihydrate kidney stones. 

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