Development of onychodystrophies

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Introduction
Nail dystrophies are a fairly common clinical problem in dermatology. The development of a certain type of onychodystrophy is caused by a variety of exogenous and endogenous factors, which, among other things, contributes to the difficulty of diagnosing this group of disorders. Dystrophic nail lesions can develop as an independent disease (most often due to mechanical trauma), as part of a symptom complex associated with dermatological pathology (psoriasis, eczema, lichen planus, etc.), or as a manifestation of internal organ pathology, including malignant neoplasms (Hippocratic nails, koilonychia, etc.). Despite well-described patterns and associations of nail lesions in dermatological and non-dermatological diseases, their detection can sometimes pose challenges for the clinician.
Being a primarily non-infectious process, nail dystrophies can persist for a considerable period of time, evolving and changing, worsening the pathological process and leading to irreversible changes. Moreover, patients with dystrophic nail changes seek medical attention much later than their onset, which further negatively impacts the prognosis. Understanding the patterns of onychodystrophic development over time can be very useful for assessing the severity of the clinical manifestations of these changes and diagnosing the underlying disease. This can apply not only to isolated nail lesions as an independent pathology but also to identifying the underlying disease. A striking example in this case is isolated nail lesions in psoriasis, which are observed in 5-10% of patients [1] and are difficult to diagnose without skin manifestations and joint involvement. This article examines several examples of onychodystrophic development and presents data on the etiopathogenesis and clinical presentation of these disorders. This article also suggests the need to differentiate onychodystrophic types into primary and secondary, based on the morphological elements of the skin. Beau's Lines and Onychomadesis
A Beau's line (transverse groove of the nail, Beau-Reilly line, or Beau's lines) is a groove-like depression running from one lateral edge of the nail to the other (Fig. 1).
This symptom was first described by the French physician Joseph Honore Simon Beau (1806-1865) in 1846 [2]. To date, the etiopathogenesis of the disease remains poorly understood. It is believed that the appearance of Beau's lines is caused by a temporary cessation of nail growth (up to 2-3 weeks) due to severe systemic stress [3]. A study by Kim et al. [4] noted that the most common causes of Beau's lines are medications (primarily chemotherapy drugs), non-autoimmune systemic diseases, mechanical trauma, and infections. Beau's lines can also be observed in patients after myocardial infarction, diabetic microangiopathy, deep-sea diving, fever of any origin, exposure to low temperatures, psychoemotional stress, starvation, or malnutrition [5, 6, 7]. Beau's lines are also characteristic of certain inflammatory dermatoses, both in severe cases with pronounced general symptoms, such as psoriasis [8], and when lesions are localized in the distal phalanges of the fingers, as in hand eczema [9]. The variety of causes for Beau's lines results in their low diagnostic specificity.
Clinically, this type of dystrophy presents as one or more transverse grooves that replicate the shape of the lunula and always emerge from the nail matrix. Beau's lines most often affect the nail plates of the first through third fingers and are easily identified both visually and by palpation. The presence of Beau's lines on all 20 nails indicates severe systemic diseases such as mumps, pneumonia, coronary vein thrombosis, Kawasaki disease, syphilis, and hypoparathyroidism [10, 11, 12].
With resolution of the pathological process and/or cessation of the triggering factor within 2-3 weeks, Beau's lines disappear as the nail grows and its surface is restored. If the pathological process and/or triggering factor persists for more than 2-3 weeks, Beau's lines transform into another type of dystrophy – onychomadesis. Onychomadesis is the process of separation of the nail from the nail bed in its proximal part due to prolonged dysfunction of the nail matrix (Fig. 2).

Unlike Beau's lines, the most common causes of onychomadesis are infections, particularly enterovirus vesicular stomatitis, medication intake (primarily antiepileptic drugs), and non-autoimmune systemic diseases [4]. According to Hardin and Haber [13], the main causes of onychomadesis are autoimmune diseases, other severe illnesses, medication intake, neonatal diseases, infections, and idiopathic disease. It is also assumed that not only inhibition of nail growth but also thinning during a long-term pathological process are responsible for the development of onychomadesis [14]. With the resolution of the pathological process and/or cessation of the trigger factor, the affected nail in onychomadesis grows back and is replaced by a healthy nail plate. In cases of prolonged pathological process, atrophy of the nail bed and the development of anonychia may occur (Diagram 1). It should also be noted that in severe, long-term pathological processes, Beau's lines and onychomadesis may coexist on the same or different nails (Fig. 3).
Koilonychia
Koilonychia (koilonychia, spoon nails) is a centrally located saucer-shaped depression of the nail plate such that when a drop of water is applied to the nail bed, it does not flow out (water-drop test) [15]. The causes of koilonychia are quite varied, but one of the most characteristic diseases is iron deficiency anemia and associated conditions [16, 17] (Fig. 4).
Among the main hypotheses for the development of this condition is a primary change in the angle of the nail matrix in relation to the surrounding connective tissue. Also among the causes of koilonychia is the low position of the distal part of the nail matrix relative to the proximal part [18]. It should be noted that the development of koilonychia is a normal variant for children in the first years of life and also occurs in elderly and senile patients as a result of acral ischemia [19]. The development of koilonychia is characteristic of psoriasis and nail mycosis, apparently due to a violation of the consistency (thinning) of the nail plate [20, 21].
The initial form of koilonychia is a petaloid (leaf-shaped, leaf-shaped) nail, characterized by flattening of the nail plate [22]. Among the types of onychodystrophies, the term platonychia is also described, also characterizing flattening of the nail plate. It can be assumed that we are talking about the same pathological change, which ultimately is the initial form of koilonychia. This assumption is also supported by the fact that the literature presents data on a possible association of platonychia with iron deficiency states [23]. Hypertrophic osteoarthropathy
Hippocratic nails (unguis hyppocratici, drumstick fingers, digital clubbing, watch-glass nails) are a specific type of onychodystrophies characterized by hypertrophy of all tissues of the distal phalanges of the fingers and toes. The exact mechanism of this disorder is unknown. It is believed that the disease is underlain by a number of pathophysiological events, including:
- vascular dilation due to vascular incompetence;
- hypersecretion of growth factors (platelet-derived growth factor, hepatocyte growth factor) by lung tissue;
- hyperproduction of prostaglandin E2 by other tissues;
- increased concentration of megakaryocytes in the systemic circulation [24].
Originating from the bone marrow, megakaryocytes are normally retained and destroyed in the pulmonary capillary bed, preventing their entry into the systemic circulation. It is believed that in certain respiratory disorders (malignant lung tumors), megakaryocytes can bypass the pulmonary capillary bed, entering directly into the systemic circulation. Settling in the capillary network of the distal extremities, megakaryocytes release platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF). PDGF and VEGF cause connective tissue hypertrophy and increase capillary permeability [25]. The mechanisms described above promote fibroblast growth and the formation of interstitial edema in the distal phalanges of the fingers, which ultimately leads to the development of clinical manifestations of this type of onychodystrophies.
According to A. Seaton et al. (1989) [26], non-cardiothoracic and pulmonary causes of Hippocratic nail development are distinguished, as well as pathological processes in the mediastinum and heart disease (Table 1).

Clinically, the nails and terminal phalanges of the fingers increase in size, becoming wide, convex, and domed—like a watch glass. The nails retain a shiny surface and normal color, but rarely become softer and more elastic. Typically, all 20 fingers are affected. The normal curvature of the nail plate is increased in the transverse, longitudinal, and anteroposterior directions; sometimes the free edge of the nail is slightly curved downward. The proximal and lateral folds have a bluish, stagnant color. The fingers take on a clubbed appearance (Fig. 5).
When clinically assessing Hippocratic nails, 3 geometric measurements are taken into account [11]:
- Lovibond angle: the angle between the base of the nail bed and the adjacent skin of the end of the finger (cuticle). When forming Hippocratic nails, this angle exceeds 165 degrees (the norm is less than 160 degrees);
- Curth angle: the angle of the distal interphalangeal joint. During the formation of Hippocratic nails, it is 160 degrees or less (the norm is 180 degrees);

- Schamroth symptom (window): disappearance of the diamond-shaped gap between the nails of opposite hands when placed together.

As the pathological process progresses, this type of dystrophy goes through several stages of development: Five stages of clinical manifestations of Hippocratic nails can also be distinguished [27]:

- no visible changes;
- mild changes: change in the Lovibond angle; Schamroth symptom is positive. Pathology may not be immediately apparent visually;

- moderate changes: pronounced convexity of the nail fold. Pathology is apparent visually;

- severe changes: pronounced thickening of the entire distal phalanx of the finger;

- hypertrophic osteoarthropathy.

The presence of nail changes may be the only sign of somatic pathology. In this case, patients often do not experience significant complaints from internal organ diseases and do not seek medical attention. Early detection of hypertrophic osteoarthropathy can be crucial for subsequent patient management and prompt medical attention.
It should also be noted that the presence of this type of onychodystrophy is not always a marker of systemic pathology, but rather a hereditary trait in certain individuals (Hippocratic pseudo-nails).
Longitudinal Ridges and Onychorrhexis
Longitudinal ridges (striations) are faint superficial lines (grooves) located along the nail plate. This type of onychodystrophies can be caused by a number of exogenous and endogenous factors, but can also be a normal variant, especially in elderly and senile patients [28] (Fig. 6).
The main causes of longitudinal nail grooves include trauma to the nail plate, impaired peripheral circulation, and impaired innervation in the distal phalanges.
The prolonged existence, growth, and widening of longitudinal grooves can lead to the development of onychorrhexis—a splitting of the nail at the free edge (Fig. 7).
This type of onychodystrophy can be considered a severe form of longitudinal grooves. Onychorrhexis is one component of the symptom complex of brittle nails, along with onychoschisis and superficial keratin granulation [29]. Onychorrhexis is caused by dehydration of the nail plate due to contact with aggressive environments (acids, alkalis). Onychorrhexis also occurs in a number of skin diseases, being the leading symptom of nail damage in lichen planus and often occurring in fungal nail infections [30].

Trachyonychia
Trachyonichia (trachionychia, twenty-nail dystrophy, or sandpaper nails) is a nail disorder characterized by the appearance of multiple, pronounced longitudinal grooves (opaque trachyonychia) or uniform, multiple, pinpoint depressions (shiny trachyonychia). Nails with trachyonychia are sometimes compared to having been rubbed against sandpaper. Trachyonychia can affect all 20 nail plates, or just a few [31]. Trachyonychia can occur in patients of any age, but is more common in children. This type of dystrophy can be idiopathic or result from a wide range of dermatological and non-dermatological disorders [31].
Today, trachyonychia is commonly divided into two subtypes: opaque and shiny [32, 33] (Fig. 8).
This classification was first described and proposed by Baran [34] in 1981. This classification involved classifying trachyonychia subtypes based on their clinical manifestations and severity. The opaque subtype of trachyonychia is considered more severe and less treatable; it is characterized by the presence of coarse, thickened, longitudinally striated nail plates. The shiny subtype of trachyonychia is considered less severe and has a more favorable prognosis for recovery; it is characterized by multiple punctate depressions on the surface of the nail plate and its shiny appearance.
Although the histological differences between the two subtypes of trachyonychia were well described by Tosti et al. back in 1995 [35], it can be assumed that the subtypes of trachyonychia may not be two different variants of the disease, but stages of a single pathological process with worsening of the condition as the disease persists. This hypothesis can be supported by well-defined punctate depressions merging into pronounced longitudinal grooves in patients with certain dermatological diseases (Fig. 9).
Onychauxis and Onychogryphosis
Onychauxis is a thickening of the nail plate with a characteristic darkening, sometimes even blackening (Fig. 10). The clinical picture of the disease is quite varied and depends on the etiology. The most common causes of onychauxis are arthrosis of the first metatarsophalangeal joint (halux rigidus), old age, and wearing improperly fitted shoes. This condition is always associated with subungual hyperkeratosis, fungal infection, and pain. Generally, onychauxis is the result of chronic trauma to the nail [36]. Onychogryphosis (also known as Ram's horn nails) is a severe thickening of the nail plate, with a dark yellow color and a change in growth direction. In onychogryphosis, the nail plate initially grows upward and then toward the other toes (Fig. 11). This type of onychodystrophy is also largely the result of prolonged mechanical trauma and essentially represents a severe form of onychauxis. The main difference between onychogryphosis and onychauxis is the change in nail growth direction, with the nail extending beyond the nail bed and growing toward the other toes [36].

Practical Significance
Understanding the patterns and developmental characteristics of various types of onychodystrophies can play an important role in the diagnosis and differential diagnosis of nail lesions. Considering the rather wide variety of onychodystrophies and their possible combinations, it is advisable to use the division of nail dystrophies into primary and secondary by analogy with the morphological elements of skin rashes (Table 2).

This distinction can be crucial and should be considered both when collecting and analyzing a patient's medical history and during long-term monitoring of the pathological process affecting the nail plates. This distinction only considers the types of onychodystrophies described in this article and does not include the specific course and outcomes of nail lesions in certain diseases, such as congenital anonychia, nail psoriasis, nail lesions associated with lichen planus and alopecia areata, etc.

Conclusion
Assessing the severity and progression of onychodystrophies over time can play a crucial role in diagnosing a wide range of dermatological and systemic disorders. Given that treatment of nail diseases is also time-consuming and may not yield a favorable therapeutic response, an initial, adequate assessment of the clinical manifestations of nail lesions can significantly influence patient management and avoid early errors. This article presents key examples of the evolution of onychodystrophies. An attempt has also been made to systematize this information in the form of a diagnostic table. This article does not cover specific aspects of the development of onychodystrophies in certain dermatoses and hereditary diseases, which requires further study of this issue and updating of the information provided.

About the authors

Roman Vladimirovich Saranyuk

Author for correspondence.
Email: roman.saranuk@gmail.com
ORCID iD: 0000-0001-9676-1581
Russian Federation, Kursk; Kursk

Tatyana A. Gosteva

Email: ya-lisenok-@mail.ru
ORCID iD: 0000-0003-0059-9159
Russian Federation, Kursk; Kurchatov, Kursk Region

References

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