ALBINISM

 ALBINISM 

Introduction 

• Albinism is a genetically determined, heterogeneous group of disorders of melanin synthesis  in which either the eyes alone (ocular albinism) or the eyes, skin and hair (oculocutaneous  albinism) may be affected. 
• The latter may be either tyrosinase-positive or tyrosinase-negative. 
• The different mutations are thought to act through a common pathway involving reduced  melanin synthesis in the eye during development. 
• Tyrosinase activity is assessed by using the hair bulb incubation test, which is reliable only  after 5 years of age. 
• Patients with albinism have an increased risk of cutaneous basal cell and squamous cell  carcinoma that usually occurs before the fourth decade. 

Tyrosinase-negative oculocutaneous albinism 

• Tyrosinase-negative (complete) albinos are incapable of synthesizing melanin and have white  hair and very pale skin throughout life with lack of melanin pigment in all ocular structures. 
• These individuals are deficient in the enzyme tyrosinase and are incapable of synthesizing melanin.  
• The clinical features are:  
• Blonde hair and fair skin. 
• Diaphanous blue irides which are responsible for photophobia. 
• Pendular nystagmus and grossly reduced visual acuity due to lack of differentiation of  the fovea and hypopigmentation of the fundus 

1. Inheritance is usually AR; the condition is genetically heterogeneous. 

2. Signs 

• (a) VA is usually <6/60 due to foveal hypoplasia. 
• (b) Nystagmus is typically pendular and horizontal. It usually increases in bright illumination and  tends to lessen in severity with age. 
• (c) The iris is diaphanous and translucent, giving rise to a ‘pink-eyed’ appearance. 
• (d) The fundus lacks pigment and shows conspicuously large choroidal vessels. There is also  foveal hypoplasia with absence of the foveal pit and lack of vessels forming the perimacular  arcades. 
• (e) The optic chiasm has fewer uncrossed nerve fibres than normal so that the majority of fibres  from each eye cross to the contralateral hemisphere. This can be demonstrated by visual evoked  potential which shows predominance in the response to monocular stimulation. 
• (f) Other features commonly seen include high refractive errors of various types, positive angle  kappa, squint and absence of stereopsis. 

Tyrosinase-positive oculocutaneous albinism 

• Tyrosinase-positive (incomplete) albinos synthesize variable amounts of melanin. The hair  may be white, yellow or red and darkens with age. 
• Skin colour is very pale at birth but usually darkens by 2 years of age. 

Ocular features 

1. Inheritance is usually AR with at least two gene loci. 

2. Signs 

• (a) VA is usually impaired due to foveal hypoplasia. 
• (b) Iris may be blue or dark-brown with variable translucency. 
• (c) Fundus shows variable hypopigmentation.

Associated systemic syndromes  

1. Chediak–Higashi syndrome 

• ∙ Inheritance is AR with the gene locus on 1q42. 
• ∙ Mild oculocutaneous albinism. 
• ∙ Leucocytic abnormalities resulting in recurrent pyogenic infections. 
• ∙ The vast majority of patients eventually develop a lymphoproliferative syndrome  (accelerated phase) characterized by fever, jaundice, hepatosplenomegaly, pancytopenia and  bleeding that requires bone marrow transplantation. 
• ∙ Prognosis for life is generally poor with demise in the 2nd decade. 

2. Hermansky–Pudlak syndrome 

• ∙ It is a lysosomal storage disease of the reticuloendothelial system. 
• ∙ Inheritance is AR. 
• ∙ Mild oculocutaneous albinism. 
• ∙ Platelet dysfunction resulting in early bruising. 
• ∙ Pulmonary fibrosis, granulomatous colitis and renal failure in some cases. 

3. Waardenburg syndrome 

• ∙ It is an AD condition of which there are four types. 
• ∙ The main systemic features are white forelock, cutaneous hypopigmentation, poliosis,  sensorineural deafness (particularly in type 2), and synophrys or an unusual hair  distribution.  
• ∙ Upper limb defects, flexion contractures and syndactyly in type 3 and neurological  anomalies in type 4. 
• ∙ Ocular features include lateral displacement of the medial canthi (not present in type 2), broad nasal bridge, hypochromic irides with segmental or total heterochromia, and  segmental or total choroidal depigmentation. 

Ocular albinism 

• Involvement is predominantly ocular with normal skin and hair although occasionally  hypopigmented skin macules may be seen. 

  

1. Inheritance is usually XL and occasionally AR with multiple gene loci identified. 

2. Female carriers are asymptomatic although they may show partial iris translucency,  macular stippling and mid-peripheral scattered areas of depigmentation and granularity. 

3. Affected males manifest hypopigmented irides and fundi.