Congenital Skin Diseases

Congenital cutaneous disease accounts for only a small percentage of veterinary surgical cases; nonetheless, a few congenital skin disorders are noteworthy in that they may benefit from surgical intervention or may drastically affect an animal's response to surgical treatment. The congenital dermatopathies of greatest importance to the veterinary surgeon are cutaneous asthenia, dermoid sinus, nasal dermoid sinus cyst, and aplasia cutis. Other congenital skin diseases of minor surgical importance that are briefly discussed in this chapter include Chediak-Higashi syndrome, cutaneous mucinosis, acral mutilation syndrome, and inherited disorders of keratinization. Congenital dermatoses are skin conditions present at birth. Although the exact cause of all congenital dermatoses is not known, most are thought to be the result of genetic defects. True congenital dermatoses are uncommon; more common are hereditary defects that manifest later in life.

Cutaneous Asthenia

Definitions and Mechanisms

Cutaneous asthenia has also been called Ehlers-Danlos syndrome," rubber puppy" disease, dermal fragility syndrome, dominant collagen dysplasia, and dermatosparaxis [1,2]. Cutaneous asthenia is a group of inherited congenital connective tissue diseases that affect man and a number of animal species including dogs, cats, horses, cattle, sheep, rabbits, and mink [1-5]. The disease complex resembles Ehlers-Danlos syndrome in humans, which has at least eleven different variations that may be distinguished clinically, genetically, and biochemically [1,3]. In general, the condition is inherited as a simple autosomal dominant trait in dogs and cats [1,2,4-7], although it has been reported to be a recessive trait in Himalayan cats [1,2,8-10].

The condition is characterized by skin fragility, hyperextensibility (Fig. 63.1), and laxity, with fragility being the primary feature of the condition [1-5,11-13]. The lack of tensile strength and decreased stiffness of the skin may be related to the organization or cross-linking of collagen molecules in the dermis [3,13]. The tensile strength of affected skin has been reported to be anywhere from one third to one fortieth that of normal skin [1,11,12,14]. In cats, a nine-fold reduction in tensile strength has been reported [1,12]. The collagen abnormality in Himalayan cats is due to a deficiency of procollagen peptidase [8,10].

Figure 63-1. This cat with cutaneous asthenia has typical hyperextensibility of the skin.

Histopathologically, the dermis of affected dogs has been reported to be normal in thickness [15], or thinner than that of normal dogs (1.21 vs. 1.71 mm, respectively) [12]. No significant difference in dermal thickness was noted between affected and normal cats in one study (1.41 and 1.44 mm, respectively) [12]; another study reported significantly thinner dermis between affected and normal cats (0.25 vs. 1.71 mm, respectively) [15]. Epidermal thicknesses of both affected and unaffected dogs and cats ranged between 0.02 and 0.03 mm [1,3,12].

Histopathologic abnormalities in the dermis are related to collagen, which gives tensile strength to the skin. In general, in affected animals, dermal collagen bundles are irregular in size and fibrillar in nature, and they lack the normal characteristic interweaving pattern of collagen bundles [3,12]. In addition, collagen bundles appear to be fine, often fragmented, with a lack of orientation and have a whorled appearance [3,5,12,14]. Mature collagen bundles are also lacking, and in some areas collagen fibers are surrounded by mucinous degeneration. [3,14].

The dermis of affected cats has fine, fibrillar collagen with collagen bundles of varying sizes, some of which appear to be unraveled. Large, short, whorled collagen bundles appear in the deep dermis [12]. The wrapping of collagen fibers around blood vessels and a coiled arrangement of fibers around sweat glands has also been reported in cats [16]. Electron microscopy reveals disorganization of collagen fibrils, with variation in fibril size and shape and irregular orientation resulting in a "moth-eaten" appearance [1,5].

Surgical Significance

The fragility of skin presents clinically significant factors for the veterinary surgeon. The incidence of lacerations is increased in affected animals, owing to the ease with which skin tears. Lacerations should be sutured promptly [1]. When manipulating skin for wound closure, excessive forceps tension on the wound edges should be avoided to prevent tearing the tissue, and careful undermining of the skin may be necessary to allow closure without tension [8,12,17].

Mattress sutures, specifically a horizontal mattress pattern with or without stents of rubber tubing, has been recommended to help prevent the sutures from tearing through the skin [1,8]. However, cruciate sutures placed 5 mm from the wound edge have been found to be less likely to tear through the skin than simple interrupted, vertical, or horizontal mattress sutures [12,13]. Small tapered-point needles with swaged-on suture material have been advocated to help prevent skin tearing [12,17]. Because of the thin dermis, the skin tends to evert when it is sutured [17].

Wound Healing

Delayed wound healing has been reported as a complication in Ehlers-Danlos syndrome in humans [13,17], but using clinical and histologic criteria, wound healing in dogs and cats with cutaneous asthenia appears to be similar to that in unaffected animals [17]. In dogs with cutaneous asthenia, healed scar tissue has greater tensile strength than adjacent skin; in affected cats, however, the tensile strength of scar tissue is no different from adjacent skin [13]. The increased tensile strength reported in dogs was speculated to be caused by (1) a greater degree of intra- and intermolecular cross-linking of collagen fibrils in the scar, (2) amplified production of type I collagen in the scar, or (3) reduced collagenolysis, or a greater degree of collagen ground substance interaction in the scar [13].

Dermoid Sinus and Nasal Dermoid Sinus Cyst

Definition and Mechanism

A dermoid sinus (pilonidal sinus) is a neural tube defect that results from incomplete separation of the skin and neural tube during embryonic development [1,2,18-24]. The sinus is a tubular skin indentation that extends as a blind sac from the dorsal midline to varying depths in the underlying tissues [1]. Dermoid sinuses have been classified by their depth of penetration (Fig. 63-2) [20,21]. Classes I through IV have an opening to the skin surface; in addition, a fifth type (Class V) has been reported and is a true cyst with no skin opening [21,23,24]. The sinuses occur in the cervical, craniothoracic, and sacrococcygeal regions (i.e., cranial and caudal to the area of the ridge of hair on Rhodesian ridgebacks). The sinuses occur most frequently in the cervical area and less often in the sacrococcygeal area [20]. One or more sinuses may be present on the same animal [18-20,23]. In the cervical area, the sinus is generally attached in the area of the dorsal spinous process of the second cervical vertebra and seldom extends any deeper. Class IV sinuses, which communicate with the dura mater, are seen mainly in the sacrococcygeal region, extending cranioventral to the last sacral vertebra [20]. Dermoid sinuses occur by far most commonly in the Rhodesian ridgeback (Fig. 63.3) [18-21,24]. Cases have also been reported in the golden retriever [25], shih tzu, boxer [26], Yorkshire terrier [22], springer spaniel [27], chow-chow [23], and Siberian husky [28]. In Rhodesian ridgebacks the condition may be caused by a gene complex [1,21]. Most historical data on inheritance of dermoid sinus suggests that the condition may be inherited as a simple recessive trait [18-20]; a more recent study suggested a more complex dihybrid mode of inheritance with differential penetrance [21].

Figure 63-2. Classes of dermoid sinus. Class I, sinus extends to the supraspinous ligament where it is attached. Class II, sinus extends into muscular or subcutaneous tissue and is connected to the supraspinous ligament by a fibrous band. Class III, sinus extends into muscular or subcutaneous tissue but is not connected to the supraspinous ligament. Class IV, sinus extends to the spinal canal and is attached to the dura mater. Class V, (not illustrated) is a true cyst with no connection to the skin. (Adapted from Mann GE, Stratton J: Dermoid sinus in the Rhodesian ridgeback. J Small Anim Pract 7:631, 1966).)

Figure 63-3. A. fistulogram of a cervical dermoid sinus in a Rhodesian ridgeback. Note the superficial cyst-like structure and extension of the contrast (delineating the sinus) to the spinous process of the third cervical vertebra. B. Surgical resection of the sinus. C. The excised sinus tract. Note the slightly darker area in the center of the ellipse of excised skin; this was the location of the sinus communication with the skin surface.

Nasal dermoid sinus cyst is unique from dermoid sinus and has been reported in the Golden retriever [29], English bull terrier [30], and Brittany, springer, and cocker spaniels [29,31]. In this condition, a sinus tract of invaginated keratinized epithelium with adnexal structures (hair follicles and glands) extends from a dorsal midline opening just caudal to the nasal planum (the so-called "nasal pit"), caudally into the nasal septum [29]. The condition is congenital and in man is caused by a neuroectodermal remnant in the prenasal space located between the nasal and frontal bones and the cartilaginous nasal capsule in the embryo [29].

Surgical Significance

Surgical excision is the treatment of choice for dermoid sinus and nasal dermoid sinus cyst [1,18,20,29], however, observation without treatment has been described if the lesion is quiescent. When drainage or neurologic signs are present, surgery should be performed [1]. Careful dissection of all sinus structures is important; introduction of a small (3.5 Fr) urinary catheter into the sinus may help with identification of the sinus as dissection progresses. In dermoid sinus, deep fibrous attachments, possibly to the dura mater, may make complete excision impossible [1,18]. Incomplete sinus removal may result in recurrence of clinical signs [18]. Meningitis often complicates cases in which the dermoid sinus is attached to the dura mater [1]; therefore, extreme care and aseptic technique are indicated [1,19]. When dermoid sinuses communicate with the dura mater, hemilaminectomy or craniotomy with removal of part of the dura mater and associated abnormal tissue may be necessary to ensure complete removal of all sinus tissue [20,26].

Aplasia Cutis

Definition and Mechanism

Aplasia cutis is a rare congenital disorder characterized by focal discontinuity of the epidermis. The condition has also been referred to as epitheliogenesis imperfecta. Most of the reports of this disease in the veterinary literature have been in cattle, horses, sheep, and pigs. In these species aplasia cutis is thought to be an autosomal recessive disorder [32]. There have been only isolated case reports in puppies and kittens [33-35]. The cause of the disease in these species is unknown.

Surgical Significance

Small lesions may be corrected by surgical repair or may heal by contraction and epithelialization. It has been suggested that in cattle the disease is not only an inherited defect in epidermal development but a more complex disorder of fibroblast metabolism [36]. Cultured fibroblasts from affected cattle have several metabolic disturbances, including a decrease in biosynthesis of both collagen and lipids. If the same is true in other species, the probability of normal wound healing following surgical repair is decreased.

Chediak-Higashi Syndrome

Definition and Mechanism

The Chediak-Higashi syndrome is a rare autosomal recessive genetic defect that has been seen in several species, including humans and Persian cats [37,38]. Affected individuals have a structural defect that results in abnormally large granules in neutrophils, monocytes, and eosinophils. Melanin granules are likewise enlarged. The cellular abnormalities of affected animals include increased fragility of leukocyte granules, defective granular fusion, and enzyme deficiencies. These abnormalities may result in cellular rupture and tissue damage, defective chemotactic responsiveness, and reduced intracellular killing [38,39]. Affected animals may succumb to recurrent infections or lymphoid neoplasia [38].

Surgical Significance

The distinct coat and eye color of affected Persian cats should alert the veterinary surgeon to the possibility of Chediak-Higashi syndrome. Knowledge of this syndrome may prevent unnecessary surgery, which could be complicated by excessive postoperative bleeding and infections. Recombinant canine granulocyte colony-stimulating factor has been shown to enhance neutrophil function in affected cats and may be a useful adjunct to reduce risk of infection when surgery is required [39].

Cutaneous Mucinosis

Definition and Mechanism

In dogs both discoid lupus erythematosus [40] and hypothyroidism [41,42] have been shown to increase dermal mucin deposition. The Chinese Shar pei appears to be unique in its propensity to have cutaneous mucinosis, and this dermal abnormality appears to be inherited, and "normal" to some degree, in all Shar peis. In extreme cases, a Shar pei may develop severe myxedema and mucoid vesicles [43,44]. Cutaneous mucinosis is manifested histologically by wide separation of dermal collagen bundles by mucin. Even the "normal" Shar pei has more mucin deposition between collagen bundles than other breeds.

The cause of this increase in dermal mucin is unclear, but the source is thought to be the dermal fibroblast [40]. Myxedema may be the result of an increase in the synthesis or a decrease in the degranulation of mucopolysaccharides [45]. The mucopolysaccharides that accumulate in the dermis are chondroitin sulfate and hyaluronic acid.

Surgical Significance

Incision into the skin of the Shar pei will reveal a gelatinous material that tends to form beads or threads. It has been suggested that in severely affected Shar peis maturation of dermal fibroblasts may be delayed [43] which could result in poor wound healing.

Acral Mutilation Syndrome

Definition and Mechanism

The acral mutilation syndrome is not a congenital anomaly but rather an inherited disease. This disease of self-mutilation has been reported in English pointers and French spaniels in North America, German shorthaired pointers in Europe, and English springer spaniels in Australia [46-48]. Pedigree examination in each affected breed indicates that the condition is probably inherited as an autosomal recessive sensory neuropathy. Biochemical studies of affected pointers have demonstrated a reduction in substance P-like immunoreactivity [49], while histologic examination has shown a reduction in the size of spinal ganglia as well as a deficiency of ganglionic neurons [47].

Surgical Significance

Affected animals are difficult to maintain because of their persistent determination to chew their paws. Chronic wound management with topical medications and bandages, and constant use of muzzles or other forms of restraint are required. Even after healing, many dogs relapse as soon as restraints are removed; faced with this discouraging situation, many owners elect euthanasia [46-48].

Inherited Defects of Keratinization

Definition and Mechanism

This category of rare diseases is grouped together because all share the feature of abnormal keratinization. Footpad hyperkeratosis has been described as a familial condition in the Irish terrier [50] and Dogue de Bordeaux [51]; related individuals of several other breeds (Labrador and golden retrievers, Kerry blue terriers, and mixed breed dogs) have also been reported [1]. A heritable defect of keratin filament formation causing increased epidermolysis was reported in 7 related Norfolk terriers; autosomal recessive inheritance is suspected [52]. Epidermolytic hyperkeratosis, a congenital condition in man characterized by collapse of the keratin filament cytoskeleton, has been reported in a single canine patient, a 6 month-old Labrador-retriever mix [53].

Surgical Significance

The aforementioned keratinization defects appear to be rare in dogs, and although specific information regarding surgical complications is lacking, several reasonable inferences can be made. In each of these conditions, the cytologic and biochemical abnormalities appear to be limited to the superficial layers of the epidermis (superficial to the stratum basale) [50,52,53];. therefore, primary healing should not be impacted to any great extent. However, the greatly thickened and cracked skin predisposes to bacterial and mycotic infections [52], so superficial wound infection may be expected to be a problem. Appropriate therapy should be instituted prior to elective procedures to improve the condition of the epidermis and, thereby, minimize infection risk. In addition, while not confirmed, second intention healing might be impacted owing to possible effects of abnormal keratinization on wound epithelialization. A moist wound environment and judicious use of mild desquamating agents may prove beneficial.

Conclusion

Although the exact mechanism for the development of all of the congenital skin diseases is unknown, most have been found to have a genetic basis. In many instances, the condition is inherited as an autosomal recessive trait. With this in mind, it must be recommended that not only should affected animals be withdrawn from the breeding pool, but in most instances, the parents, and perhaps siblings, should not be used for breeding purposes. In some cases, test mating to affected animals may provide evidence that a particular animal does not carry the abnormal gene [1,2,54].