Pachyonychia congenita
What is pachyonychia congenita?
Pachyonychia congenita is the name given to a group of rare, inherited disorders of keratinisation. Keratinisation is the process by which keratin (scale) is formed and deposited in the outer most layer of the skin. Pachyonychia congenita occurs when there is a mutation in the genes encoding keratin, K6a, K16, K17, K6b and, possibly, K6c (listed in decreasing frequency). Nearly 100 mutations have been described by the International Pachyonychia Congenita Research Registry (IPCRR, February 2014 [1]). The mutations cause the skin to be more fragile than usual.
Prior to genetic testing, some patients were diagnosed with pachyonychia congenita that actually did not have this disorder. Mutations in other keratin genes can lead to similar skin conditions.
How is pachyonychia congenita inherited?
Pachyonychia congenita is autosomal dominantly inherited. That means the defective gene comes from one parent. An affected person with an autosomal dominant disorder has a 50% chance of passing on the disease to his or her offspring at each pregnancy.
No family history is present in many patients with pachyonychia congenita, due to sporadic mutations during conception.
Pachyonychia congenita is found in all ethnic groups and in equal numbers in males and females.
What are the features of pachyonychia congenita?
Pachyonychia congenita affects the skin (especially palms and soles), nails and mucous membranes. Specific features depend on which keratin gene is involved.
Pachyonychia congenita
| Clinical features of pachyonychia congenita | |
|---|---|
| Palms and soles |
|
| Nails |
|
| Cysts |
|
| Dry skin |
|
| Mouth |
|
| Larynx |
|
Classification of pachyonychia congenita
Pachyonychia congenita was traditionally classified into Type 1 and Type 2, according to the clinical features. Classification now depends on which keratin gene has the specific mutation.[2]
| Clinical features | Percent affected with each genetic mutation | ||||
|---|---|---|---|---|---|
| K6a | K6b | K6c | K16 | K17 | |
| Thick toenails | 99% | 98% | 56% | 95% | 99% |
| Thick fingernails | 99% | 51% | 0 | 61% | 87% |
| Plantar keratoderma | 89% | 98% | 94% | 100% | 80% |
| Plantar pain | 96% | 100% | 100% | 97% | 89% |
| Palmar keratoderma | 54% | 41% | 19% | 81% | 51% |
| Cysts | 68% | 71% | 25% | 26% | 92% |
| Follicular hyperkeratosis | 62% | 47% | 0 | 14% | 68% |
| Natal or prenatal teeth | 2% | 0 | 0 | 0 | 77% |
| Oral leukokeratosis | 88% | 31% | 19% | 41% | 27% |
*Data from 218, 49, 16, 150 and 90 patients with mutations of K6a, K6b, K6c, K16 and K17 respectively [2].
How is pachyonychia congenita diagnosed?
Pachyonychia congenita is usually diagnosed by its clinical appearance.
Skin biopsy of the affected tissues will only show nonspecific changes.
Molecular genetic studies can be done by specialist laboratories to detect mutations in the affected keratin genes. A genetic counsellor can advise whether the test is available in your area. Testing is available free of cost via Pachyonychia Congenita Project.
Prenatal testing is sometimes offered to pregnant women when they or a partner is affected by pachyonychia congenita. Pre-implantation diagnosis of ‘test-tube’ embryos has also been reported.
What is the treatment for pachyonychia congenita?
Unfortunately there is as yet no cure for pachyonychia congenita. The effectiveness of treatment depends on the severity of the specific problem.
Treatment of keratoderma might include:
- Emollients
- Keratolytics eg salicylic acid ointment, urea cream or other forms of heel balm
- Mechanical debridement
- Bleach baths to reduce infection
- Pain relief
- Oral retinoids eg acitretin
Thickened nails should be regularly trimmed.
Experimental treatments are being offered to some patients with pachyonychia congenita as part of clinical trials of novel agents such as mTOR inhibitors (eg sirolimus).
