Coffin–Siris Syndrome is a rare genetic disorder that causes developmental delays and absent fifth finger and toe nails.

There had been 31 reported cases by 1991. The numbers of occurrence since then has grown and is reported to be around 80.

The differential includes Nicolaides–Baraitser syndrome.

It is characterized by a nearly symmetrical presence of a spoon hand (classical type) or, more frequently, an oligodactylous hand. Individuals with this syndrome present the following symptoms: carpal, metacarpal and digital synostoses, disorganization of carpal bones, numeric reduction of digital rays and toe syndactyly. Additionally, other symptoms may include radioulnar synostosis, brachymesomelia, radius head dislocation, metatarsal synostoses and numeric reduction of rays.

Tooth and nail syndrome (also known as "Hypodontia with nail dysgenesis," and "Witkop syndrome") is a rare disorder, first described in 1965, characterized by nails that are thin, small, and friable, and which may show koilonychia at birth.

It is associated with "MSX1".

Oral-facial-digital syndrome is a group of at least 13 related conditions that affect the development of the mouth, facial features, and digits in between 1 in 50,000 to 250,000 newborns with the majority of cases being type I (Papillon-League-Psaume syndrome).

ANOTHER syndrome consists of alopecia, nail dystrophy, ophthalmic complications, thyroid dysfunction, hypohidrosis, ephelides and enteropathy, and respiratory tract infections. This is an autosomal recessive variant of ectodermal dysplasia.

Sugarman syndrome is the common name of autosomal recessive oral-facial-digital syndrome type III, one of ten distinct genetic disorders that involve developmental defects to the mouth.

Alternative names for this condition include: Brachydactyly of the hands and feet with duplication of the first toes, Sugarman brachydactyly and Brachydactyly with major proximal phalangeal shortening.

Ulnar–mammary syndrome or Schinzel syndrome is a cutaneous condition characterized by nipple and breast hypoplasia or aplasia.

It has been associated with TBX3.

Limb–mammary syndrome is a cutaneous condition characterized by p63 mutations.

The skeletal structures of individuals who have this disorder may have pronounced deformities. As reported by several medical doctors, the following features are commonly found in people who suffer from nail–patella syndrome:

Bones and joints

- Patellar involvement is present in approximately 90% of patients; however, patellar aplasia occurs in only 20%.

- In instances in which the patellae are smaller or luxated, the knees may be unstable.

- The elbows may have limited motion (e.g., limited pronation, supination, extension).

- Subluxation of the radial head may occur.

- Arthrodysplasia of the elbows is reported in approximately 90% of patients.

- General hyperextension of the joints can be present.

- Exostoses arising from the posterior aspect of the iliac bones ("iliac horns") are present in as many as 80% of patients; this finding is considered pathognomonic for the syndrome.

- Other reported bone changes include scoliosis, scapular hypoplasia, and the presence of cervical ribs.

Kidney issues may arise such as proteinuria and nephritis. Proteinuria is usually the first sign of renal involvement and either rapidly or years after suffering from asymptomatic proteinuria, renal failure occurs in around 5% of NPS patients. Hypothyroidism, irritable bowel syndrome, attention deficit hyperactivity disorder (ADHD), and thin tooth enamel are associated with NPS, but whether these are related or simply coincidences are unclear.

Cenani–Lenz syndactylism, also known as Cenani–Lenz syndrome or Cenani–syndactylism, is an autosomal recessive congenital malformation syndrome involving both upper and lower extremities.

Nakajo syndrome, also called nodular erythema with digital changes, is a rare autosomal recessive congenital disorder first reported in 1939 by A. Nakajo in the offspring of consanguineous (blood relative) parents. The syndrome can be characterized by erythema (reddened skin), loss of body fat in the upper part of the body, and disproportionately large eyes, ears, nose, lips, and fingers.

The diagnosis of constriction ring syndrome can be confirmed with an ultrasonography. The clinical manifestations can be extremely variable. It could be a single or multiple manifestation. This can be confirmed at the end of the first trimester or at the beginning of the second trimester. But not every patient will be diagnosed at that moment, most will get this diagnosis at birth.

The nails are markedly thickened with yellow to yellow-green discoloration of the nails. They grow slowly, at a rate of 0.25 mm/week or less. The nails may have ridges and increased side-to-side curvature, reduction of the white crescent and detachment of the nail from the nailbed. The nail changes may change over time.

Most people with yellow nail syndrome (four fifths) have lymphedema; it is symmetrical and typically affects both legs. It is the first symptom of the condition in about a third. Involvement of the arms and face is more unusual, as is lymphedema of the abdomen with ascites (fluid collection in the abdominal cavity) and fluid collection around the heart.

Various lung problems can occur in people with yellow nail syndrome. Many experience cough and shortness of breath. Forty percent of cases develop pleural effusions, which are collections of fluid in the pleural cavity (the space that contains the lungs and normally only has a minimal amount of fluid in it). About half of all people with yellow nail syndrome have either recurrent chest infections or a chronic lung condition known as bronchiectasis which causes chronic production of sputum with episodes of worsening. Forty percent of people with yellow nail syndrome have chronic sinusitis.

Yellow nail syndrome has been associated with some drugs, "e.g." penicillamine, bucillamine and gold sodium thiomalate.

The differential diagnosis includes;

- Symbrachydactyly

- Chorionic villus sampling

- Congenital amputations

- Hypoplasias of hand, digit, thumb

- Adams-Oliver syndrome

- ADAM complex

ADAM Complex; CRS is sometimes mislabeled as ADAM complex. ADAM is an abbreviation for Amniotic Deformity, Adhesions Mutilations. CRS is the malformation due to a constriction ring around mostly a limb. ADAM-complex is the association of limb defects (caused by constriction rings) and certain craniofacial clefts

“Adams-Oliver syndrome is often mislabeled as CRS and consists of cutis aplasia of the scalp in which a longitudinal defect can vary in size and can often be associated with full-thickness skullcap loss. The distal digital or toe hypoplasia-aplasia is often confused with CRS. Constriction rings with or without edema are not present. The digital or toe hypoplasia-aplasia usually contains diminutive nails or nail folds”.

Ectrodactyly involves the deficiency or absence of one or more central digits of the hand or foot and is also known as split hand–split foot malformation (SHFM). The hands and feet of people with ectrodactyly are often described as "claw-like" and may include only the thumb and one finger (usually either the little finger, ring finger, or a syndactyly of the two) with similar abnormalities of the feet.

Ectodermal dysplasia describes abnormalities of structures derived from the embryonic ectoderm. These abnormalities affect both the superficial ectodermal layer, as well as the mesectodermal layer constituted by the neural crest.

Ectodermal dysplasia is characterized by absent sweat glands resulting in dry (hypohydrotic), often scale-like skin, sparse and usually coarse scalp hair that is often blonde, sparse eyebrows and eyelashes, and small brittle nails. In addition, abnormalities of ectodermal derivatives, neuroectodermal derivatives, and mesectodermal derivatives are often found. The ectodermal derivative abnormalities can affect the epidermis including mammary, pituitary and sweat glands, as well as hairs, dental enamel, nails, lens, and the internal ear. Neuroectodermal derivatives that can be affected include sensory placodes, cutaneous pigmental cells, and hair buds. Mesectodermal derivatives affected can include the dermis, hypodermis, dentin, head muscles and conjunctival cells, cervicofacial vascular endothelial cells, and part of the maxillofacial skeleton.

The hypohydrotic symptoms of ectodermal dysplasia described above are evidenced not only in the skin of affected individuals, but also in their phonation and voice production. Because the vocal folds may not be as hydrated as is necessary during the adduction phase of vocal fold vibration (due to lack of lubrication), a complete seal may not be accomplished between the folds and mucosal wave movement may be disrupted. This results in air escapement between the folds and the production of breathy voice, which often accompanies the skin abnormalities of ectodermal dysplasia.

Lelis syndrome it is a genetic disorder, a rare condition with dermatological and dental findings characterized by the association of ectodermal dysplasia (hypotrichosis and hypohidrosis) with acanthosis nigricans. Other clinical features may include palmoplantar hyperkeratosis, nail dystrophy, intellectual deficit, disturbances of skin pigmentation (perioral and periorbital hyperpigmentation, vitiligo, and perinevic leukoderma) and hypodontia. Transmission is autosomal recessive.

Nail–patella syndrome (NPS) (also known as "HOOD syndrome") is a genetic disorder that results in small, poorly developed nails and kneecaps, but can also affect many other areas of the body, such as the elbows, chest, and hips. The name "nail–patella" can be very misleading because the syndrome often affects many other areas of the body, including even the production of certain proteins. Those affected by NPS may have one or more affected areas of the body, and its severity varies depending on the individual. It is also referred to as iliac horn syndrome, hereditary onychoosteodysplasia (HOOD syndrome), Fong disease or Turner–Kieser syndrome.

Diagnosis of NPS can be made at birth, but is common for it to remain undiagnosed for several generations. While there is no cure available for NPS, treatment is available and recommended.

Muehrcke's nails, Muehrcke's lines, or leukonychia striata are changes in the fingernail that may be a sign of an underlying medical disorder or condition. Muehrcke's lines are white lines (leukonychia) that extend all the way across the nail and lie parallel to the lunula (half moon). In contrast to Beau's lines, they are not grooved. The lines are actually in the vascular nail bed underneath the nail plate, and as such, they do not move with nail growth. Meuhrcke's lines disappear when pressure is placed over the nail, blanching the underlying nail bed.

The appearance of Muehrcke's lines is nonspecific, but they are often associated with decreased protein synthesis, which may occur during periods of metabolic stress (e.g., after chemotherapy) and in hypoalbuminemic states such as the nephrotic syndrome.

Muehrcke's lines have also been described in infants with chronic hypolbuminemia secondary to various etiologies.

An example of Muehrcke's lines in the context of chemotherapy was published in the "New England Journal of Medicine" in 2007.

Muehrcke's lines were described by Robert C. Muehrcke (August 4, 1921 – September 7, 2003), an American physician in 1956.

Autosomal recessive inheritance is the most likely, but sporadic mutations and autosomal dominant cases may also occur.

This syndrome has been associated with mutations in the ARID1B gene.

Mutations in SOX11 are associated to this syndrome.

The diagnosis is generally based on the presence of major and at least one minor clinical sign and can be confirmed by molecular genetic testing of the causative genes. Recent studies revealed that fifth finger nail/distal phalanx hypoplasia or aplasia is not a mandatory finding.

Dysplastic nails are a cutaneous condition, and may be a subtle finding of ridging, flaking, or poor growth of the nails, or more diffuse with nearly complete loss of nails. This condition may be seen in a number of syndromes, including Dyskeratosis congenita and Nail–patella syndrome.

Odonto–tricho-ungual–digital–palmar syndrome is an autosomal dominant skin condition with salient clinical features of natal teeth, trichodystrophy, prominent interdigital folds, simian-like hands with transverse palmar creases, and ungual digital dystrophy.

Yellow nail syndrome, also known as "primary lymphedema associated with yellow nails and pleural effusion", is a very rare medical syndrome that includes pleural effusions, lymphedema (due to under development of the lymphatic vessels) and yellow nails. Approximately 40% will also have bronchiectasis. It is also associated with chronic sinusitis and persistent coughing. It usually affects adults.

PDP has a number of visible symptoms. Most important clinical features are: pachydermia (thickening and wrinkling of the skin), furrowing of the face and scalp, periostosis (swelling of periarticular tissue and shaggy periosteal new bone formation of long bones) and digital clubbing (enlargement of fingertips). Other features include excessive sweating, arthralgia and gastrointestinal abnormalities. An overview of all symptoms is provided in table 2.

Table 2. Overview of symptoms

Beau's lines are deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate. This condition of the nail was named by a French physician, Joseph Honoré Simon Beau (1806–1865), who first described it in 1846.

Beau's lines are horizontal, going across the nail, and should not be confused with vertical ridges going from the bottom (cuticle) of the nail out to the fingertip. These vertical lines are usually a natural consequence of aging and are harmless. Beau's lines should also be distinguished from Muehrcke's lines of the fingernails. While Beau's lines are actual ridges and indentations in the nail plate, Muehrcke lines are areas of hypopigmentation without palpable ridges; they affect the underlying nail bed, and not the nail itself. Beau's lines should also be distinguished from Mees' lines of the fingernails, which are areas of discoloration in the nail plate.

There are several causes of Beau's lines. It is believed that there is a temporary cessation of cell division in the nail matrix. This may be caused by an infection or problem in the nail fold, where the nail begins to form, or it may be caused by an injury to that area. Some other reasons for these lines include trauma, coronary occlusion, hypocalcaemia, and skin disease. They may be a sign of systemic disease, or may also be caused by an illness of the body, as well as drugs used in chemotherapy or malnutrition. Beau's lines can also be seen one to two months after the onset of fever in children with Kawasaki disease.

Human nails grow at a rate which varies with many factors: age, and the finger or toe in question as well as nutrition. However, typically in healthy populations fingernails grow at about 0.1mm/day and toenails at about 0.05mm/day. With this in mind the date of the stress causing Beau's lines and other identifiable marks on nails can be estimated. As the nail grows out, the ridge visibly moves upwards toward the nail edge. When the ridge reaches the nail edge, the fingertips can become quite sore due to the mis-shapen nail pressing into the flesh deeper than usual, exposing the sensitive nail bed (the quick) at the nail edge.

A researcher found Beau's lines in the fingernails of two of six divers following a deep saturation dive to a pressure equal to 305 meters of sea water, and in six of six divers following a similar dive to 335 meters. They have also been seen in Ötzi the Iceman.