27.1 Introduction

Myogenic ptosis, also known as myopathic ptosis, refers to a process intrinsic to the retractor muscle of the upper eyelid, the levator palpebrae superioris (LPS), as opposed to its innervation or connective tissue (aponeurosis). There are both static and progressive forms of myogenic ptosis, which generally correlate with congenital and acquired, respectively (Table 27.1). In each form, the palpebral fissure and margin reflex distance 1 (MRD1) are diminished. The upper eyelid crease may be absent, but when present, the margin crease distance (from upper eyelid margin to crease in downgaze) is generally normal and the margin fold distance (from eyelid margin to fold in primary gaze [MFD]) is increased. The palpebral fissure on downgaze is increased in congenital ptosis, and the Bell phenomenon (upward movement of the eye with eyelid closure) is affected in progressive myopathic ptosis. This chapter highlights the more common specific etiologies of myogenic ptosis in each of these two groups and briefly reviews their presentation and management.

27.2 Static Myogenic Ptosis

Simple congenital ptosis, an isolated developmental abnormality in the levator muscle, is by far the most common presentation of static myogenic ptosis, affecting up to 0.18% of the population. ¹ It may be bilateral, but most cases are unilateral, with a slight left-sided predominance (55%). ² Most often congenital ptosis is sporadic, but it may also be familial. ^{1 ,​ 2} Histopathology shows fibrosis of the LPS with decreased muscle fibers, although the exact pathogenesis remains unclear. Theories include disordered muscle development and/or innervation or even birth trauma–induced levator dehiscence. ^{1 ,​ 3}

Congenital ptosis may also be associated with congenital abnormalities in other extraocular muscles, for example, in monocular elevation deficiency (previously known as double elevator palsy), and congenital fibrosis of the extraocular muscles (CFEOM).

Congenital ptosis also forms one component of blepharophimosis ptosis epicanthus inversus syndrome and may be present in several other developmental craniofacial syndromes such as Noonan syndrome, Saethre–Chotzen syndrome, and lymphedema–distichiasis syndrome (see Chapter 32). ⁴

The management of congenital ptosis depends on many factors, including the severity and laterality of the ptosis, the function of the levator muscle (levator excursion), and the presence or risk of amblyopia. Unilateral ptosis poses a great risk for deprivational or anisometropic amblyopia, but bilateral ptosis may still cause significant astigmatism and amblyopia. Surgical repair is guided primarily by levator excursion and includes conjunctivomullerectomy for levator excursion greater than 10 mm, external levator advancement/resection for levator excursion greater than 4 mm, and frontalis suspension procedures for levator excursion less than 4 mm (Table 27.1). If the Bell phenomenon is compromised, such as in CFEOM, more conservative surgery is indicated.