BN may be a consequence of factors that alter nail plate production and/or factors that damage the NP.
Under physiologic conditions the NP is a fully keratinized structure made up of about 25 layers of tightly packed keratinocytes, with a thickness of 0.5–1 mm and a smooth surface. The majority of the NP derives from the proximal nail matrix (NM) and around 1/3 by the distal matrix and bed. The NM is localized under the proximal nail fold. Epithelial growth and keratinization of NM keratinocytes follow an oblique axis; during this process of maturation and differentiation cells move upward and distally. Normal nail growth is highly dependent on vascularization and inflammation [4, 5]. Impairment of this process results in NP abnormalities.
The clinical aspects of NB due to NM damage can vary depending on the site and the extent of the injury. NP thinning due to proximal NM damage always involves the whole nail length and is often associated with abnormalities in the superficial NP. Damage to the distal matrix, on the other hand, may produce alterations in the shape of the NP free edge, but not of the superficial NP.
BN may be caused by several factors: first of all, a physiological increased nail fragility is typical of aging. Other local or systemic causes include inflammatory nail diseases, onychomycosis, systemic conditions, medication ingestion, and trauma (occupational and professional). When a secondary cause has not been identified, the disease is defined as idiopathic.
Clinically, three main clinical aspects of NF have been described:
Onychorrhexis: particularly frequent among middle-age women and characterized by longitudinal splitting and shallow parallel furrows running on the superficial layer of the nail (Fig. 1). The severity of the clinical presentation depends on the degree of the causative factor on the NM. Longitudinal ridges may vary from a few superficial ridges up to deep furrows, affecting from a small part of the nail surface to at least 70% of the total nail surface. Likewise, longitudinal splitting may vary from a few superficial splits up to multiple deep fissures. A single longitudinal split of the entire NP is sometimes observed (split nail) (Fig. 2). The pain induced by the deep splits may be the cause for medical advice being sought.
Lamellar onychoschizia: caused by the impairment of intercellular adhesive factors of the NP. As a result, the NP exfoliates into fine horizontal layers (Fig. 3). Nail hardness is mainly secondary to high sulphur amino acids, particularly cystine, and tight keratin cross-links. The keratin protein in nails is hardest at a slightly acid pH. Adhesion between cells is facilitated by membrane-coating granules with linkage by lipids, such as acyl ceramides. Triangular pieces of NP may easily be torn off from the free margin (crenellated splitting). In other cases, breaking of the lateral edges could cause transverse splitting. This type of nail fragility is almost exclusive to fingernails and it is typical of patients who wash their hands too frequently (e.g., homemakers, but also doctors and nurses). Proximal lamellar splitting may occasionally be observed in lichen planus or during etretinate or acitretin therapy.
Superficial granulation of keratin is a brittleness confined to the dorsal NP, not to be confused with superficial white onychomycosis. Superficial granulation of keratin is often reported in patients wearing nail polish for months, sometimes applying it over the previous coating. In this way NP keratins undergo to a gradual ‘granulation’ or exfoliation with the formation of small white-yellow patches and striations (Fig. 4). Keratin degranulation is a type of pseudoleukonychia, where the white discoloration of the nail is due to white particles (the damaged onychocytes) on the NP surface.
According to causative factors, we can distinguish 2 forms of NF: a primary “idiopathic” form and NF secondary to different causes.
Idiopathic Nail Brittleness (Brittle Nail Syndrome)
This is the most common cause of NB, almost exclusively seen in the fingernails. In women, the intercellular keratinocyte bridges are constitutionally weaker than in males and this can be considered a cause of the higher frequency of this complaint among females. Moreover, normal nails contain 5% lipids and there is a decrease in cholesterol sulphate of the NP with age, especially in post-menopausal women, suggesting an important role of lipids in the development of NB . Some authors have identified reduced water content in the nail plate (< 16%) as a possible cause of BN . In contrast, other studies found no significant difference in water content of brittle compared with normal nails [7, 8].
Secondary Nail Brittleness
Inflammatory Nail Disorders
Several dermatoses such as psoriasis, lichen planus, lichen striatus, alopecia areata, Darier’s disease and eczema may involve the nail apparatus [9, 10]. Usually these disorders are diagnosed as separate entities although features of nail fragility are common in these patients.
In fact, up to 50% of patients affected by psoriasis present nail abnormalities that are frequently associated with NB. When the proximal part of the NM is involved, the NP presents irregular and deep pits (Fig. 5), while a more extensive psoriatic involvement of the NM induces a friable and brittle NP .
Lichen planus in its various forms can also affect the nails. About 10% of patients with lichen planus have specific nail involvement . Depending on the degree of inflammation, the nail changes consist of thinning, longitudinal ridging and splitting of the NP (Fig. 6). Rarely, erosive lichen planus may involve the nails . In lichen striatus, nail involvement is similar to that of lichen planus, but typically only half of the NP is affected .
Approximately 2/3 of patients with alopecia areata have nail changes, such as pits secondary to proximal NM involvement, onychorrhexis, thinning and trachyonychia (Fig. 7). Changes may be seen in one, several or all the nails.
Periungual eczema, seen in contact dermatitis or atopic dermatitis, is frequently associated with NP fragility (Fig. 8).
Superficial white onychomycosis is an example of a cause of NF secondary to NP damage. In this type of onychomycosis, the fungal hyphae colonize the most superficial layers of the NP, and it becomes white, opaque, and friable in multiple small spots secondary to keratin digestion by fungi  (Fig. 9).
In distal subungual onychomycosis, fungi invade the nail bed from the distal free margin of the NP, inducing NF in the distal portion (Fig. 10).
Several infectious diseases such as syphilis and pulmonary tuberculosis can give nonspecific alterations to the nails such as nail thinning and fissuring of the free margin and an overall fragility [16, 17].
Systemic Diseases and General Conditions
BN have also been linked with systemic diseases, nutritional deficiencies and medication ingestion, but they are often only a non-specific sign that accompanies these conditions [6, 7].
The impairment of peripheral circulation secondary to arteriopathy, neurological disorders, chronic anemia and arteriosclerosis may lead to a reduced NM vascularization with production of a thin NP .
Patients with endocrine disorders may present with BN, slow nail growth and longitudinal ridging and fissuring. Nail changes, characterized by brittleness and softness, are present in about 5% of cases of hyperthyroidism and are often reversible following successful therapy. The nails are affected in 90% of patients with hypothyroidism and typically appear thin, brittle, slow-growing and with longitudinal or transverse striae .
Onycholysis, increased fragility, longitudinal ridging and crumbling may occur in amyloidosis.
Systemic medications such as cancer chemotherapeutic agents, retinoids or antiretrovirals may be responsible for lamellar onychoschizia.
A severe deficiency of vitamins, trace elements and aminoacids from daily food intake may result in NF and thinning .
Traumas and Alteration of Nail Hydration
Traumas significantly contribute to damage of the NP and alteration of the NP surface. NF can be caused by mechanical micro-traumas secondary to routine professional activities, as in homemakers, housemaids, shoemakers, ironworkers, and carpenters. Occupational exposure to solvents and solutions (chemical/medical personnel, photographers or painters, for example) can dissolve intercellular lipids and damage intercellular cohesion. Onychotillomania and onychophagia are another two causes of traumatic BN (Fig. 11).
The methods used to prepare nails for cosmesis and all methods of removing the applied preparations damage the healthy nail plates. However, the application of nail polish itself does not cause an alteration of the pH of the nail lamina, being approximately 5.8, which is very close to the physiological value . Water is stored above all in the ventral portion of the NP and the alteration of water content of the NP could cause BN (Fig. 12). Dehydration is more rapid if nails are not kept short . Moreover, hydration and desiccation may play a significant role in household employees, hairdressers, and nurses where repeated wetting and drying of the hands leads to fractures between NP onychocytes.
Finally, occlusive gloves applied over wet hands while working may be a cause of BN, leading to splintered, fractured nails and onychoschisis .