A working classification of ptosis is critical because subdividing the disorders can optimize patient management, distinguish progressive from static myopathy, and help in deciding when interdisciplinary care is needed (Table 10.1). It also helps the clinician decide when genetic testing or laboratory investigations may be required, factors that could ultimately maximize patient surgical outcomes.⁵ More than 30 years ago, Beard warned that the terms congenital and acquired ptosis should be abandoned.⁴ The temporal classification of ptosis into congenital and acquired provides little clinically useful information and is not helpful for management purposes. Nevertheless, unfortunately, it is still commonly used today, long after it has lost true clinical relevance.⁶,⁷,⁸,⁹ This textbook uses a modification of a mechanistic or etiologic classification that was originally proposed by Frueh in 1980.¹⁰ That etiological approach classified ptosis into myogenic, aponeurotic, mechanical, and neurogenic subtypes, as well as simulating conditions causing pseudoptosis or apparent ptosis.¹⁰,¹¹ In this system, simple or dysgenetic congenital ptosis is classified under the rubric of myopathic ptosis because it is of myogenic origin. However, it shares little if any clinical features with other types of myogenic ptosis that are discussed in a separate chapter (Chapter 11). An alternative classification has recently been proposed,⁵ which is also a modification of the mechanistic classification system and has a particular emphasis on myopathic ptosis. These authors further subclassified myogenic ptosis into two categories (static vs progressive) according to the disease tempo. The first category, which they dubbed static myopathic ptosis, includes the classic congenital myopathic ptosis (discussed in this chapter), in addition to Duane retraction syndrome, Marcus Gunn jaw-winking ptosis (Chapter 38), congenital fibrosis of the extraocular muscles (CFEOM 1, 2, 3A-C), monocular elevation deficiency (formerly double elevator palsy), as well as blepharophimosis-ptosis-epicanthus inversus syndrome (Chapter 28). The progressive variety consists of oculopharyngeal dystrophy, chronic progressive external ophthalmoplegia, and myotonic dystrophy, which are discussed separately in Chapter 11.⁵

Simple or dysgenetic myopathic blepharoptosis is an idiopathic, persistent, nonprogressive type of ptosis, which results from a primary developmental defect in the levator muscle and replacement of muscle tissue by fibrous and fatty tissue.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰ Currently, the consensus is that it is of a muscular dysgenetic etiology (nonprogressive developmental defect) and should not be classified as a dystrophy (inherited, progressive muscle weakness) as was previously thought.²¹,²²,²³,²⁴,²⁵ Indeed, histologic studies have shown the destruction of muscle fibers and their replacement with fibrous tissue, changes that are usually proportional to the severity of the ptosis.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰ The lack of subsequent regeneration of muscle
fibers, as well as the lack of inflammatory cells, lends further support to the theory of dysgenesis of the levator muscle rather than progressive dystrophy.¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰ Fatty infiltration of the muscle has also been observed in some studies, but this is probably a degenerative response.¹⁴,¹⁷ Of note is that a recent study has demonstrated fatty infiltration with or without fibrotic changes (fibrous bands or sheets) in the levator aponeurosis. Whether these findings represent a true developmental abnormality of the aponeurosis and its surrounding fibrous frame that may be partially involved in the genesis of ptosis, or whether these changes are simply a secondary degenerative or inflammatory response, is unknown at present.¹⁵

Although most cases are sporadic, Mendelian autosomal dominant (AD) or x-linked dominant (XD) inheritance periodically have been suggested.²⁶,²⁷,²⁸ However, it is highly unlikely that there is a single genetic defect responsible for simple congenital ptosis, and despite a heritability index of 0.75 in identical twins, local in utero environmental factors cannot be excluded.¹²,²⁸ Possible candidate genes are the 1p32 (PTOS1), Xq24 (PTOS2), 12q24.3, 14q22.3, 8q21.1 (ZFH-4 gene), and 4q25 (COL25A1 gene).²⁹,³⁰,³¹,³²,³³,³⁴

The incidence rate of simple myopathic dysgenetic ptosis is difficult to establish because of the scarcity of population-based studies in the literature.³⁵,³⁶,³⁷,³⁸,³⁹,⁴⁰ Unfortunately, most of these studies discuss the prevalence of pediatric ptosis in general and make no reference to dysgenetic congenital ptosis in particular, except one study which estimated the prevalence to be 1 in 842 live births.³⁵ Another population-based study from China, which was conducted on 247,000 live births, showed a slightly higher prevalence of 1:552, or 0.18%.⁴⁰ In general, unilateral poor function simple dysgenetic myopathic ptosis is the most common form of ptosis, and for unknown reasons, it more frequently involves the left side (Figure 10.1).³⁵,⁴¹,⁴² Bilateral cases are less frequently observed (Figure 10.2).

Thorough history taking is of paramount importance in all forms of ptosis, especially dysgenetic myopathic ptosis. Parents should specifically be questioned if the ptosis was immediately noticed at birth or shortly afterward, and whether it was stable since birth or improved.⁴³ Occasionally,
parents may comment that ptosis has slightly improved by the end of the first year of life and later stabilized. This variability in eyelid level or palpebral fissure height in the first year of life is a normal physiologic phenomenon.⁴⁴ If parents cannot recall these details, photographic documentation should be sought.⁴³ Acquiring the details of the child’s delivery is important in assessing the possibility of a traumatic injury during the birth process.⁴³ Infants and toddlers may be unwilling to be examined and the physician must start observing the inattentive child from a nonthreatening distance while taking history long before a formal examination has started.⁴³ Several characteristics such as the degree of the ptosis, recruitment of the frontalis muscle, the adoption of a chin-up head position, and the presence of strabismus can all be assessed without even touching the patient.⁴³ When formal examination later begins, the child is usually more at ease with the examiner.

It usually is not easy to obtain adequate ptosis measurements in pediatric patients, but if possible, the most important parameters to measure are the levator excursion and the margin reflex distance 1 (MRD₁). When assessing the severity of ptosis it is the MRD₁ and not the vertical palpebral fissure height that is relevant because this measurement is not confounded by the position of the lower eyelid.⁴³ MRD₁ measures the distance between the upper eyelid margin and the center of the pupil. A value of zero indicates a ptotic eyelid that bisects the pupil. Positive and negative numbers are given if the eyelid position is above or below that level, respectively.⁴³ Levator function is the most important single measure to retrieve, although it may be difficult to assess in young infants. Ideally, the brow should be fixed firmly with the thumb; the child is instructed to look downward in extreme downgaze and then to raise their eyelids slowly toward the ceiling. The amount of lid lift or “levator excursion” is measured with a millimeter ruler.⁴⁵

The position of the upper eyelid crease is noted. It may be absent if levator function is poor, but still may be observed at its normal position in patients with good to moderate levator function.⁴⁵ The position of the brows should also be marked. Compensatory frontalis muscle overaction may raise a ptotic eyelid to a deceptively normal position. This may be encountered in the less ptotic eyelid in patients with bilateral asymmetric ptosis and could give the impression that the contralateral upper eyelid is normal and that the ptosis is unilateral (Figure 10.2C).⁴³ Another factor that may confuse inexperienced clinicians is that overstimulation of the levator muscle in the ptotic eyelid rarely may translate into overstimulation of the contralateral “normal” levator with subsequent pseudoretraction.⁴³,⁴⁶,⁴⁷ Both factors are variations of the Hering law of equal innervation, which admittedly plays a far less significant role in simple dysgenetic myopathic ptosis than in aponeurotic ptosis because in the congenital variety the levator is fibrotic.⁴³,⁴⁶,⁴⁷

Because amblyopia is a devastating complication of ptosis in young children, ophthalmic and eyelid examinations should both go hand in hand.¹¹,⁴³ Visual acuity and refraction should be a routine part of the assessment, and amblyopia, as well as anisometropia, should be ruled out.¹¹,⁴³,⁴⁵ Amblyopia in children with ptosis is multifactorial. It could be deprivational from occlusion of the visual axis, anisometropic, or strabismic.⁴⁸,⁴⁹ Estimates vary from as low as 14.8% to as high as 71%,³⁵,⁴⁸,⁴⁹,⁵⁰,⁵¹,⁵² but even with the lowest estimates, the incidence of amblyopia is still several times higher than among the general population (3%).⁵¹ As expected, amblyopia mostly develops in unilateral or in bilateral but asymmetric ptosis.⁵⁰ Why stimulus deprivation amblyopia develops in some and not all children with moderate to severe ptosis remains an enigma, but a compensatory head posture may nullify the effect of ptosis through clearing the
visual axis.⁵³ Refractive errors in the form of anisometropia or astigmatism are also quite prevalent both before⁴⁷,⁵⁴,⁵⁵ and after ptosis surgery.⁵⁶,⁵⁷ More than 70% ptosis cases show a transient change toward with-the-rule astigmatism after ptosis surgery. This effect appears to regress gradually during the first postoperative year.⁵⁶